GEOLOGY OF THE CENTRAL JURA AND THE MOLASSE BASIN: NEW INSIGHT INTO AN EVAPORITE-BASED FORELAND FOLD AND THRUST BELT ANNA SOMMARUGA Dr ès Sciences II ©1997 Anna Sommaruga, Université de Neuchâtel, Institut de Géologie, Rue Emile Argand 115 2007 Neuchâtel, Suisse. Tél. 032 718 26 00 Fax 032 718 26 01 Editeur: Société neuchâteloise des Sciences naturelles, p.a. Bibliothèque publique et universitaire de la Ville, 2000 Neuchâtel, Suisse. Rédacteur: Willy Matthey Rédacteur technique: Jacques Ayer ISBN 2-88347-001-4 Imprimé en Suisse Photo de couverture: Eric Leuba (Photo-Report, La Chaux-de-Fonds) Photo panoramique à 140 degrés, 2000 m au-dessus du sol. Vue du Jura neuchâtelois en direction du Nord-Est. A gauche, les villes du Locle et de la Chaux-de-Fonds, la Vallée de La Sagne et des Ponts; au centre l'anticlinal du Mont-Racine et le synclinal losangique du Val de Ruz; à droite les Rochers-des-Miroirs et la Montagne de Boudry, le lac de Neuchâtel; à l'horizon le lac de Morat et le Plateau suisse (Bassin molassique). GEOLOGY OF THE CENTRAL JURA AND THE MOLASSE BASIN: NEW INSIGHT INTO AN EVAPORITE-BASED FORELAND FOLD AND THRUST BELT IMPRIMATUR POUR LA THÈSE Géologie du Jura central et du bassin molasslque: nouveaux aspects sur une chaîne d'avant-pays plissée et décollée sur des couches d'évaporites de Mme Anna Sommaruga Mosar UNIVERSITÉ DE NEUCHÂTEL FACULTÉ DES SCIENCES La Faculté des sciences de l'Université de Neuchâtel sur le rapport des membres du jury, MM. M. Burkhard (directeur de thèse), J.-P. Schaer, G. Gorin (Genève), D. Roberts (Londres) et A.W. BaIIy (Houston, USA) autorise l'impression de la présente thèse. Neuchâtel, le 2 octobre 1997 Le doyen: R. Dändliker GEOLOGY OF THE CENTRAL JURA AND THE MOLASSE BASIN: NEW INSIGHT INTO AN EVAPORITE-BASED FORELAND FOLD AND THRUST BELT GÉOLOGIE DU JURA CENTRAL ET DU BASSIN MOLASSIQUE: NOUVEAUX ASPECTS D 'UNE CHAÎNE D 'AVANT-PAYS PLISSÉE ET DÉCOLLÉE SUR DES COUCHES D ÉVAPORJTES ANNA SOMMARUGA Dr es Sciences Lauréate 1996 du Prix de la Conférence AIH-AISH de Lausanne 1990 Schweizerische Akademie der Naturwissenschaften SANW Académie suisse des sciences naturelles ASSN Swiss Academy of Sciences SAS MÉMOIRE DE LA SOCIÉTÉ NEUCHÂTELOISE DES SCIENCES NATURELLES TOME XII remerciements: Le présent travail a été soutenu par le Fonds National Suisse de Ia Recherche Scientifique (requêtes N° 4020-031533; 21-37366.93) et par l'Institut de Géologie de l'Université de Neuchâtel. Je tiens à remercier les compagnies pétrolières et les institutions, propriétaires des profils sismiques et des logs de forages, d'avoir autorisé l'accès, l'utilisation, ainsi que la publication d'une partie de leurs données dans ce mémoire: - British Petroleum Exploration Operating Company (Angleterre), en la personne du Dr David G. Roberts - Forces Motrices Neuchâteloises S.A. (Canton de Neuchâtel), en la personne de J. Rossât - Musée géologique du Canton de Vaud à Lausanne, en la personne de son directeur Dr Aymon Baud - Office de l'économie hydraulique et énergétique du Canton de Berne - SEAG (A.G. für schweizerisches Erdöl) - Shell International Exploration and Production B.V. (Pays-Bas), en la personne du Dr Harry Doust. La publication de ce mémoire a pu être réalisée grâce au soutien financier de nombreuses institutions, fondations et sociétés, auxquelles je témoigne toute ma reconnaissance: - Académie Suisse des Sciences Naturelles (Berne) - British Petroleum Exploration Operating Company (Angleterre) - CEDRA, Société coopérative nationale pour l'entreposage de déchets radioactifs (Canton d'Argovie) - Conférence de !'Assoc, internat, des hydrogéol. et des sci. hydrolog, (AIH-AISH) de Lausanne 1990 (Prix 1996) - Département de l'Instruction publique de la République et du Canton de Neuchâtel - Fondation Dr Joachim de Giacomi (Canton de Bâle) - Forces Motrices Neuchâteloises S.A. (Canton de Neuchâtel) - Schweizer Rheinsalinen (Canton de Bâle) - Société Neuchâteloise des Sciences Naturelles. 4 AVANT-PROPOS "En traitant de la géologie du Jura neuchâtelois, nous ne saurions avoir la prétention d'intéresser par la nouveauté du sujet. Tl est peu de chaînes de mon- tagnes qui soient aujourd'hui aussi bien connues que le Jura ...". C'est en ces termes que Desor & Gressly, en 1859, introduisent leurs "Etudes géologiques sur le Jura Neuchâtelois", publiées dans le tome IV de cette même série des Mémoires de la Société Neuchâteloise des Sciences Naturelles. Cent trente- huit ans plus tard. Ie Jura n'a pas encore cessé d'inté- resser les géologues du monde entier et fait toujours partie des chaînes de montagnes les plus étudiées. En effet, il n'est guère d'ouvrages, traitant de la tec- tonique ou de la géologie structurale, où ne figurent soit les plis du Jura, soit sa structure arquée. Durant toutes ces années, de nouveaux concepts et méthodes géologiques ont été développés, appliqués et testés dans cette petite chaîne modèle, localisée au coeur de l'Europe. Aujourd'hui, tout comme au temps de Desor et de Gressly, de nouvelles données géologiques ont fourni une "occasion ... de vérifier la justesse de nos théories" (op. cit. p. V). A l'époque, il s'agissait du creusement du tunnel ferroviaire sous la Vue des Alpes, tandis qu'aujourd'hui, ce sont les résultats de sondages par la méthode de "sismique réflexion", qui permettent de compléter nos connaissances et nos visions de la structure profonde du Jura. Depuis la Deuxième Guerre mondiale, le Bassin molassique et le Jura ont été examinés sous l'aspect de leur potentiel pétrolier, d'abord par des études classiques de géologie de surface et ensuite à partir des années 70, à l'aide d'une nouvelle technologie, telle que la "sismique réflexion". Cette méthode, comparable à une radiographie du sous-sol, permet d'identifier la nature des couches et leurs structures, notamment celles qui pourraient piéger du pétrole. A présent, tous ces résultats accompagnés d'une quarantaine de forages n'ont pas fourni de succès tangibles du point de vue pétrolier. Une des dernières campagnes d'exploration pétro- lière en Suisse a été entreprise en 1988 par la com- pagnie British Petroleum associée aux Forces Motrices Neuchâteloises. Elle a permis d'acquérir environ 300 km de profils de "sismique réflexion", dans le seul canton de Neuchâtel, sondé pour la pre- mière fois par cette méthode. L'Institut de Géologie de l'Université de Neuchâtel a eu la possibilité d'examiner les résultats de cette campagne et les premières interprétations réalisées par British Petroleum. Celles-ci ont d'ailleurs conduit à l'aban- don du projet d'une exploration pétrolière dans le canton. A notre demande, les propriétaires de ces données ont eu la générosité de donner accès à l'en- semble des profils sismiques à des fins non-com- merciales. C'est ainsi, que Anna Sommaruga alors doctorante en géologie, a pu disposer d'un matériel scientifique de grande valeur pour l'étude de la structure profonde et de l'évolution tectoniqe du Jura central. Pour se perfectionner dans l'interpréta- tion des profils sismiques, un domaine encore peu enseigné dans les universités suisses, A. Sommaruga a effectué deux stages à l'Université de Rice à Houston chez le Prof. A.W. Bally. Excellent connaisseur de Ia géologie de la Suisse et du Monde, ce dernier fut aussitôt intéressé par ce projet et accepta de parrainer la thèse d'Anna Sommaruga. Vu depuis le Texas, où "everything is big", le canton de Neuchâtel apparaissait cependant comme assez petit et des efforts supplémentaires furent entrepris pour étendre le réseau des profils sismiques au Bassin molassique suisse ainsi que du coté du Jura français, deux régions où la compagnie Shell, avec divers partenaires locaux, avait entrepris préalable- ment des campagnes de prospection pétrolière. Finalement, grâce à Shell et à Swisspetrol (SEAG), qui ont ouvert leurs archives jusqu'alors confiden- tielles, un vaste réseau de plus de 1500 km de pro- fils sismiques a été mis à disposition. Il a permis d'analyser la géométrie des structures profondes s'étendant du Jura externe, à travers la Haute Chaîne du Jura plissé, du Bassin molassique jusqu'à la bor- dure des Alpes. Anna Sommaruga ne s'est pas per- due dans les montagnes de papiers que représentent les originaux de ces profils, elle a réussi à en condenser l'information et à en tirer les points essentiels pour la tectonique de nos régions. De nouveaux éléments de réponse ont pu être apportés aux questions classiques concernant la for- mation de la chaîne jurassienne. Parmi les plus importantes, la théorie du "Fernschub", trouve une très belle confirmation dans le présent mémoire. Proposée en 1907 par Buxtorf, elle explique le plis- sement du Jura par une poussée lointaine horizon- tale depuis les Alpes, avec un décollement de la couverture sédimentaire au sein des roches évapori- tiques du Trias. Bien que cette théorie soit large- ment acceptée aujourd'hui, il restait bien des points de détail à éclaircir quant aux relations géomé- triques entre le socle et Ia couverture. C'est pour la première fois que la profondeur du socle et sa 5 géométrie lisse sous le Jura et le Bassin molassiqué ont pu être cartographiées. Ces données excluent une implication considérable du socle dans la for- mation des plis du Jura. Ces mêmes données sis- miques ont permis de mettre en évidence deux types de plis, interprétés comme des stades d'évolution successifs du plissement de la couverture. Les pro- fils sismiques de British Petroleum dans le canton de Neuchâtel fournissent pour la première fois une image complète et très claire des structures pro- fondes, notamment sous le Creux du Van. Enfin, la Société Neuchâteloise des Sciences Naturelles a accepté de publier ce travail dans la série de ses Mémoires, par lesquels, traditionnelle- ment, les nouvelles données géologiques sur le Jura ont été diffusées. La parution coïncide avec la réunion annuelle de l'Académie Suisse des Sciences Naturelles à La Chaux-de-Fonds en automne 1997; son thème général "Paysage calcaires de l'Arc juras- sien: du minéral au vivant" s'accorde parfaitement avec celui de ce nouveau mémoire. Prof. Martin Burkhard Institut de Géologie, Université de Neuchâtel Avertissement au lecteur Ce mémoire s'articule en deux parties: un volume texte avec de nombreuses figures et une série de planches et de panneaux hors-texte. Ces documents, de différents formats, sont cités dans le texte et leurs légendes se situent à la fin du mémoire. 6 TABLE OF CONTENTS Acknowledgments............................................................................................................................9 Résumé étendu en français............................................................................................................11 1. Introduction 1.1. Aim and scope of the present study...........................................................................................17 1.2. Geological setting......................................................................................................................17 1.2.1. The Jura 1.2.2. The Molasse Basin .1.3. Regional boundary conditions to the formation of the Jura fold and thrust belt........................22 1.4. Formation of the Jura: short review and open questions............................................................23 1.5. Sources of data...........................................................................................................................29 1.5.1. Surface geological data 1.5.2. Subsurface data 1.6. Methodology..............................................................................................................................29 2. Stratigraphy 2.1. Introduction................................................................................................................................33 2.2. Regional overview.....................................................................................................................33 2.2.1. Introduction 2.2.2. The basement: Paleozoic or older rocks 2.2.3. Mesozoic 2.2.4. Cenozoic: Tertiary 2.3. Outcrop and subsurface stratigraphy..........................................................................................44 2.3.1. Introduction 2.3.2. A stratigraphie column for the Neuchâtel Jura 2.3.3. Surface and well log correlation 2.3.4. Conclusions 2.4. Seismic units..............................................................................................................................52 2.4.1. Neuchâtel (Val de Ruz) seismic line 2.4.2. Seismic lines and well log data 2.4.3. Correlation across the seismic grid: "jump correlation" 2.5. Isopachmaps..............................................................................................................................63 2.5.1. Introduction 2.5.2. Isopachs of the upper Malm unit 2.5.3. Isopachs of the "Argovian" unit (lower Malm) 2.5.4. Isopachs of the Dogger unit 2.5.5. Isopachs of the Liassic unit 2.5.6. Isopachs of the Triassic Unit 1 2.5.7. Isopachs of the Triassic Unit 2 2.5.8. Summary 2.6. Rheological Stratigraphy............................................................................................................70 2.6.1. General comments 2.6.2. Rheological behavior of rocks from the Jura 2.6.3. Rheological profiles for the central and eastern Jura 3. Structures: overview, examples and interpretation 3.1. Introduction................................................................................................................................75 3.2. Folds and thrusts.........................................................................................................................75 3.2.1. Geometry and mechanisms: definitions 3.2.2. Evaporite-related folds (low amplitude) 3.2.3. Thrust-related folds (high amplitude) 3.3. Tear faults................................................................................................................................102 3.3.1. Definitions 3.3.2. Geomorphological evidence 3.3.3. Geophysical evidence from seismic profiles 3.3.4. Examples illustrated by seismic profiles 3.3.5. Description from outcrops 3.3.6. Interpretation of the central Jura and Molasse Basin tear faults 3.4. "Reef-like" features......:...........................................................................................................111 4. Regional geology 4.1. Introduction..............................................................................................................................113 4.2. Neuchâtel Jura..........................................................................................................................113 4.2.1. Previous studies 4.2.2. Eastern part 4.2.3. Western part 4.3. Risoux Jura...............................................................................................................................126 4.3.1. Previous studies 4.3.2. Interpretation 4.4. The Champagnole-Mouthe region...........................................................................................127 4.4.1. General comments 4.4.2. Previous studies 4.4.3. Interpretation 4.5. The western Molasse Basin......................................................................................................131 4.5.1. Previous studies 4.5.2. Interpretations and contour maps 4.5.3. The Yverdon-Treycovagnes area 5. Synthesis and discussion..........................................................................................................141 6. Conclusions...............................................................................................................................153 Abstract, Résumé, Riassunto, Zusammenfassung....................................................................154 Appendices....................................................................................................................................157 References.....................................................................................................................................166 Panel and Plate captions........................................................................................................................175 ACKNOWLEDGMENTS This research is the result of a collaboration and cooperation with many individuals and institutions. I would like to express my gratitude to all those who have helped or supported me. Firstly, I would like to thank Prof. Jean-Paul Schaer, who asked me, four and half years ago, to participate in a one year PNR20 project interpreting four seismic reflection lines (30 km) crossing the Val de Ruz. At the time we would not have imagi- ned that this research was to continue in a thesis focusing on the interpretation of more than 1500 km of seismic lines! I also thank him for introducing me to seismic interpretation, advising me to go to Rice University for a short stay and again for having sha- red all his precious knowledge on the historical and regional geology of the Jura Mountains. To Prof. Martin Burkhard, my advisor, to whom I owe countless discussions about structural geology, regional geology of the Jura and some ideas presen- ted in this work. I thank him for his general enthu- siasm, his generosity providing me with the neces- sary means for developing this research and for his encouragement and confidence in my work. All my gratitude goes to Prof. Albert Bally who has kindly invited me twice to Rice University in Houston as visiting student (January-August 1993, Summer 1994) and has introduced me to the fasci- nating world of seismic interpretation. During his lectures he taught me the regional geology of many areas around the world as well as Occam's Razor philosophical maxime. I thank him for the nume- rous discussions on foreland fold belts and salt tec- tonics, which gave me a new insight on the Jura. Furthermore, he helped me to obtain most of the seismic data which I have been working on during my thesis. I am grateful to Dr David Roberts from British Petroleum, who kindly provided the data from the Neuchâtel Jura which made it possible to broaden considerably the initial scope of my thesis. My thanks go to Prof. Georges Gorin who gave me the first advice on seismic interpretation and correlation with well logs. I am indebted to Dr Peter Lehner and Dr Hans Andreas Jordi for stimulating discussions on the interpretation of seismic lines from the Jura and Molasse Basin. To the whole staff of Professors, assistants, stu- dents and personnel from the Institut de Géologie de Neuchâtel who has contributed to a familial atmos- phere at the Department. I particularly remember the same year PhD students (Pierre Lambert, Essaïd Zeroual, Marie-Caroline Blanc-Aletru, Marc Rolli, Philippe Mouchet), with whom I have shared the same preoccupation and amount of stress during the last months of our theses. I thank Gregor Schönborn for sharing with me his knowledge about the regio- nal geology of the eastern Jura and about thrust tec- tonics and for his precious advice. I am indebted to Xavier Tschanz who, during the first year of my thesis, took me to many field trips in the Neuchâtel Jura. To my officemate and friend Thierry Baudin, many thanks for countless "philosophical" discus- sions about geology and life and also for his encou- ragement. I do not forget all my friends from the "barrio" (Gabor Tari, Joan Flinch, Norbert Pralle, CJ Liu, Sebastian Galeazzi. ...) or from the Geology Department of Rice University (Laura De Santis. Phil Bart, Olivier Aubert, Klaus Holliger, ...) who have helped me during my stay in Houston. I also owe my gratitude to Julie Schaer and David Hindle, who kindly helped me improve my know- ledge in english. I would like to thank A.W. Bally, M. Burkhard, G. Gorin, D. Roberts and J.-R Schaer for critical review of this manuscript. Many thanks to Jacques Ayer, who considerably helped with the final editing of this work. I am grateful to my parents and my brothers and sisters, who encouraged me to do my studies and helped me put things back into perspective during difficult moments. I also thank all my friends for their support and I would especially like to thank Jon for his understan- ding during the last months of my thesis. I apprecia- ted his numerous and humorous encouragement during my "up side down" moments. 9 RESUME ETENDU Géologie du Jura central et du Bassin molassique: nouveaux aspects d'une chaîne d'avant-paysplissée et décollée sur des couches d'évaporites. INTRODUCTION But du travail La chaîne du Jura ainsi que le Bassin molassique sont généralement considérés comme une chaîne plis- sée d'avant-pays, représentant la zone de déformation la plus externe d'âge tardi-Miocène des Alpes du Nord-Ouest. La formation de cette chaîne est un sujet classique, discuté depuis le début de ce siècle par de nombreux auteurs (Fig. 1.7). Dans ce travail, plus de 1500 km de profils sismiques réflexion ont été inter- prétés. Ils sont localisés dans les Jura suisses neuchâ- telois et vaudois, dans le Jura français et dans le Bassin molassique occidental, et ont été mis à dispo- sition par l'industrie pétrolière (British Petroleum, Shell Switzerland, Société anonyme des Hydrocarbures, Shellrex, Fig. 1.4) et le Musée de Géologie de Lausanne. De plus, la grille sismique est contrainte par les résultats d'une vingtaine de forages. Cette analyse a permis d'approfondir nos connais- sances sur la stratigraphie et la géométrie des couches enfouies et par la suite de tester différents modèles concernant la formation de la chaîne du Jura et du Bassin molassique. La géométrie en profondeur des structures du Jura et du Bassin molassique (plis, chevauchements, décrochements) procure des informations importantes sur leur développement et sur la formation de l'avant- pays alpin. Les sujets principaux discutés dans cette thèse sont: les corrélations entre les observations géo- logiques de surface et les données de la subsurface, l'épaisseur stratigraphique des couches enfouies, la géométrie des plis et des chevauchements et leur développement, les relations socle-couverture et la structure du socle. L'interprétation des profils sismiques démontre que la couverture mésozoïque et cénozoïque du Jura et du Bassin molassique a été déformée au-dessus d'un niveau de décollement basai et déplacée de quelques 20 à 25 km vers le NW. Le socle sous la région étu- diée montre une surface relativement plane et plonge de quelques degrés (Io à 3°) vers le S-SE. Situation géologique Le Jura Le Jura représente une chaîne d'avant-pays locali- sée au front de la partie Ouest de l'arc alpin (Fig. 1.1). Cette chaîne a donné son nom aux couches du Jurassique, qui affleurent essentiellement dans ces montagnes. La chaîne du Jura est entourée par des bassins ter- tiaires de différents types; au Nord, le Graben (ou fossé) du Rhin, à l'Ouest le Graben de la Bresse et au SSE le Bassin molassique (Fig. 1.2). Les Grabens du Rhin et de la Bresse sont associés au rifting ouest- européen d'âge Oligocène, tandis que le Bassin molassique correspond à un bassin d'avant-pays d'âge Oligo-Miocène, qui s'est développé au front des Alpes. Le Jura et le Bassin molassique représen- tent la zone de déformation la plus externe et la plus jeune des Alpes occidentales. Dans toute la Suisse, le socle est composé de roches métamorphiques et de roches plutoniques associées à l'orogenèse hercynienne. A la fin de celle-ci, de nombreux fossés (grabens) allongés se sont formés et ont été par la suite remplis par des séries lacustres et fluviátiles d'âge Carbonifère et Permien. Bien que quelques uns de ces fossés, orien- tés E-W3 soient bien localisés grâce aux résultats des forages ou aux affleurements de massifs voisins, ils en restent encore beaucoup à découvrir sous le Jura et le Bassin molassique où les sondages sont peu nom- breux. Le socle cristallin n'affleure jamais dans le Jura et le Bassin molassique. Cependant, il a été pénétré par plusieurs forages et latéralement il affleure: au Nord, dans les Vosges et la Forêt Noire et au Sud, dans les massifs cristallins externes. Le cycle alpin commence au début du Trias avec la formation d'une pénéplaine et Ia transgression de la mer, lorsque les bords du Jura et le Bassin molassique commencent à faire partie de la marge passive de la mer téthysienne alpine. Au cours du Trias, plus d'un kilomètre d'évaporites et d'argiles sont déposées dans un bassin à forme légèrement elliptique avec un dépôt-centre sous le futur arc jurassien. Les calcaires 11 des couches du MaIm forment l'ossature des plis et affleurent dans le Jura oriental et central. Cependant, les calcaires des séries du Crétacé inférieur forment les crêtes des anticlinaux du Jura occidental. Au cours de l'Oligocène et du Miocène, des sédiments fluviá- tiles, lacustres et marins des séries de la Molasse se sont déposés dans l'avant-pays alpin ("foredeep basin"). Ces couches transgressent progressivement vers le Nord les couches du Mésozoïque sous-jacent. L'épaisseur du prisme tertiaire décroît du Sud (plus de 4 km) vers le Nord (quelques centaines de mètres). Ces séries affleurent essentiellement dans le Bassin molassique, mais sont aussi préservées dans les syn- clinaux du Jura. La limite entre le Bassin molassique et la chaîne du Jura est donc une limite naturelle d'érosion. Le Jura est divisé en deux parties dont le style structural est très différent: a) le Jura externe et b) le Jura interne (Fig. 1.2). a) Le Jura externe consiste en des régions légèrement faillées et sans grand relief, appelées les Plateaux. Hs sont séparés les uns des autres par les Faisceaux, qui représentent des zones très étroites et fortement déformées, caractérisées par des décrochements et des imbrications. b) Le Jura interne, aussi nommé la Haute Chaîne, consiste en une succession de plis bien développés. A grande échelle, la déformation est caractérisée par des plis, des chevauchements et des décrochements. Les plis sont en relation avec des chevauchements et leur orientation varie de 90° au sein de la chaîne. Leur continuation latérale est limitée soit progressive- ment par un plongement axial, soit plus brusquement par un décrochement. Des décrochements sénestres recoupent d'un fort angle les axes de plis; leur orien- tation dessine un éventail le long de la chaîne. Les parties septentrionales de la Haute Chaîne chevau- chent les Plateaux du Jura externe, tandis que le bord méridional plonge progressivement sous les sédi- ments tertiaires de la Molasse. Une troisième zone, le Jura tabulaire est souvent asso- ciée à la chaîne du Jura. Cette zone est localisée à l'exté- rieur de l'arc jurassien et représente la couverture méso- zoïque du socle cristallin de la Forêt Noire et des Vosges. Le Jura tabulaire représente la transition des grabens du Rhin et de Ia Bresse au Bassin de Paris. Le Bassin molassique Le Bassin molassique (sensu lato) représente un bassin d'avant-pays, qui s'étend sur plus de 700 km de la Savoie (France) à l'Ouest, à la région de Linz (Autriche) à l'Est (Figs. 1.1 et 1.2). Il s'étend parallè- lement au front alpin et s'ouvre progressivement vers l'Est. La sédimentation dans le Bassin a été continue depuis le début de l'Oligocène jusqu'au Serravallien. Le Bassin molassique est subdivisé en trois unités tectoniques: la Molasse du Jura, la Molasse du Plateau et la Molasse subalpine. La Molasse du Jura représente la partie septentrio- nale du Bassin qui a été passivement impliquée dans le plissement du Jura. Seuls quelques affleurements de Molasse sont préservés au sein des synclinaux du Jura interne. La Molasse du Plateau, qui constitue la partie essentielle du Bassin molassique, présente un style structural bien différent entre la région occidentale et orientale. Les plis de faible amplitude, orientés NW- SE, et les décrochements soulignent les structures principales à l'Ouest, tandis qu'à l'Est, on observe quelques failles normales orientées WSW-ENE paral- lèles au Basin et affectant les couches du Mésozoïque et du Cénozoïque. La Molasse subalpine représente une zone étroite le long du bord méridional du Bassin molassique (Fig. 1.2). Cette zone est caractérisée par une succes- sion d'écaillés composées de sédiments tertiaires et décollées à la base des couches tertiaires. La limite méridionale de cette zone correspond au front alpin d'âge Oligocène, représenté par les nappes alpines (Préalpes, Helvétique....). Le changement de style structural le long du Bassin molassique est influencé par la présence d'un niveau de décollement au sein des évaporites du Trias. La partie occidentale du Bassin molassique est détachée de son substratum cristallin ou Permo-Carbonifère; la déformation s'est développée du Sud en direction de la chaîne du Jura. Cependant, dans la partie orientale du Bassin molassique, il n'y a aucune évidence pour admettre un niveau de décollement, car les niveaux d'évaporites du Trias sont absents. Conditions pour ta formation de la chaîne plissée et chevauchée du Jura La formation d'une chaîne plissée et la nature du plissement dépendent considérablement des condi- tions aux limites. Pour le Jura, les conditions aux limites régionales les plus importantes sont les sui- vantes: la présence de couches évaporitiques du Trias agissant comme zone de décollement basai; la pré- sence d'un socle rigide plongeant de Io à 3° vers le Sud et situé sous une zone de décollement tendre; la rhéologie des couches de la couverture: l'épaisseur des couches compétentes croît du NE vers le SE, tan- dis que les couches incompétentes décroissent vers le SE; la structure en forme de prisme de l'avant-pays jurassien: une faible pente plongeant vers le NW à la surface tandis qu'à la base, elle plonge vers le SE (socle); la structure en forme de prisme du Bassin molassique: l'épaisseur des couches augmente consi- dérablement du Nord au Sud; les structures héritées d'âge Oligocène au sein de la couverture. 12 Formation du Jura: questions ouvertes Le développement de la chaîne du Jura reste tou- jours un sujet de discussion avec deux points de vue fondamentalement différents (Fig. 1.7). Plusieurs auteurs considèrent la chaîne du Jura comme autoch- tone. i.e. une couverture plissée et solidaire à un socle déformé par des grands décrochements et des chevau- chements intra-cristallins ou recoupant la couverture. D'autres auteurs stipulent que les plis de la couver- ture du Jura et les chevauchements associés se sont développés au-dessus d'un niveau de décollement et ont été déplacés sur de longues distances, grâce à la poussée des Alpes transmise à travers le Bassin molassique ("Hypothèse du Fernschub"). Bien que des arguments d'équilibrage favorisent la deuxième théorie, certains auteurs ont quand même utilisé des données sismiques pour relancer le débat et proposer un décollement au sein du socle plutôt qu'à la base de la couverture. Données et méthodologie de travail Les données de surface consistent en un nombre considérable de cartes géologiques publiées et non publiées (Tab. 1.1), de coupes géologiques de faible profondeur et de logs lithostratigraphiques. Les don- nées de subsurface comportent des profils de sis- mique réflexion et des forages provenant de l'indus- trie pétrolière. Elles ont été fournies soit directement par les compagnies pétrolières, soit par le Musée de Géologie de Lausanne où sont déposés la plupart des documents concernant les activités de sismique réflexion effectuées sur le territoire du Canton de Vaud. L'élaboration de ce travail a nécessité plusieurs étapes, dont voici les principales: compilation des données préexistantes, travaux de terrain, interpréta- tion sismique, conversion en profondeur (mètres) des profils sismiques (en temps), cartes, coupes géolo- giques semi-équilibrées et équilibrées. STRATIGRAPHIE Unités sismiques L'interprétation géologique d'une grille sismique est basée sur la corrélation de réflecteurs sismiques. Dans les roches sédimentaires, ces réflecteurs corres- pondent à des discontinuités lithologiques, qui peu- vent être calibrées stratigraphiquement grâce aux informations contenues dans les logs de forages. En général, il est préférable de sélectionner des unités de stratigraphie sismique limitées par de forts réflec- teurs, plutôt que de définir et de corréler chaque réflecteur sismique. Dans ce travail, huit unités sis- miques comprises entre deux intenses réflecteurs (ces derniers sont numérotés par les lettres de A à I, voir Tab. 2.1) ont été définies; chacune représente un intervalle, caractérisé par une lithologie prépondé- rante. La discordance tertiaire basale de l'avant-pays ("basal foredeep unconformity") est la seule discor- dance évidente visible sur nos profils sismiques. Les profils sismiques ont permis d'éclaircir la stra- tigraphie en profondeur tout particulièrement pour les couches d'âge triasique, qui n'affleurent pas dans la région étudiée. Deux unités ont été définies pour le Trias sur la base des interprétations des profils sis- miques: (de haut en bas) l'Unité 1 qui surmonte l'Unité 2. L'Unité 1 est bien litée avec des réflecteurs réguliers et continus; elle montre une épaisseur à peu près constante autour de 200 m. L'Unité 2 contraste fortement avec l'Unité sus-jacente: elle présente des réflecteurs discontinus et obliques. L'épaisseur de cette Unité varie fortement d'une ligne à l'autre en fonction de la position structurale: les épaississe- ments sont visibles sous les anticlinaux, tandis que les amincissements se localisent sous les synclinaux. Le toit du socle correspond aux derniers réflecteurs intenses, qui contrastent fortement avec le socle cris- tallin d'apparence transparent (sans réflecteur). Au sein du socle, il est toutefois possible par endroit de reconnaître quelques réflecteurs, interprétés soit comme multiples, soit comme des sédiments permo- carbonifères. Les profils sismiques longitudinaux, c'est-à-dire parallèles aux synclinaux, sont d'une grande utilité, puisqu'ils montrent des réflecteurs sub-horizontaux, nécessaires pour l'interprétation stratigraphique. Les profils sismiques transversaux, c'est-à-dire perpendicu- laires aux structures, sont par contre de moins bonne qualité et montrent des réflecteurs discontinus, probable- ment à cause du fort pendage des couches et des struc- tures complexes. Cartes des isopaques Des cartes d'isopaques (courbe de même épaisseur) ont été construites pour chaque unité du Mésozoïque au sein du Bassin molassique (Figs. 2.24 à 2.29). Les cartes d'isopaques sont basées sur la conversion en profondeur (en mètres) des profils sismiques (en temps) et calibrées sur les forages. Une vitesse sis- mique a été attribuée à chaque intervalle sismique (voir Appendix 3). Les cartes d'isopaques permettent de déterminer des changements, même mineurs, des épaisseurs des couches. Ces changements sont inté- ressants pour l'analyse des mécanismes de subsi- dence, ainsi que pour une réflexion sur la rhéologie des couches. Bien que ces cartes ne soient pas pré- cises quant au taux de subsidence, elles ont l'avantage d'être basées sur une large grille sismique donnant 13 une haute résolution spatiale et elles permettent de révéler les tendances des variations de la subsidence et l'existence de failles syn-sédimentaires au cours du Mésozoïque. Durant le Trias, plus de 1000 m de sédiments se sont accumulés dans Ia région de la future chaîne du Jura, tandis que 60 km plus au Sud, au sein des par- ties septentrionales du domaine Helvétique, moins de 50 m de sédiments se sont déposés au cours du même laps de temps. Malgré cette considérable variation latérale, aucune évidence pour des failles syn-sédi- mentaires n'a été trouvée. D'importantes variations latérales d'épaisseur dans les couches du Trias sont en relation avec la déformation d'âge Miocène et l'importante redistribution des évaporites dans ces couches. D'aspect tendre, elles ont agit comme zone de décollement durant la formation du Jura. Pendant la période du Jurassique, les variations latérales de subsidence sont significatives, mais moins impor- tantes que celles durant le Trias; aucune tendance régionale persistante n'a été identifiée. Quelques petits changements d'épaisseurs dans la région d'Yverdon peuvent être le résultat de failles syn-sédi- mentaires, mais une évidence directe est obscurcie par la présence de décrochements d'âge Miocène. En guise de synthèse, durant tout le Mésozoïque, la région étudiée était un domaine avec une subsidence très lente, située entre le Bassin de Paris au Nord- Ouest et la mer alpine téthysienne au Sud. Stratigraphie rhéologique L'épaisseur stratigraphique et la continuité latérale des formations compétentes et incompétentes au sein d'une colonne stratigraphique joue un rôle essentiel dans l'évolution tectonique d'une chaîne plissée. La rhéologie de chaque couche dépend principalement de la composition minéralogique de la roche, de la texture et de la température de déformation. Aux conditions de déformation du Jura, la compétence pour les sédiments impliqués dans la colonne strati- graphique augmente de la manière suivante: sel, gypse, anhydrite, argile, marne, grès de la Molasse, calcaire, dolomie. Trois degrés de compétence ont été définis au sein des unités strati graphique s étudiées (Fig. 2.30): 1) dur (forte viscosité); 2) tendre (faible viscosité); 3) très tendre (très faible viscosité). Les calcaires du Crétacé et du MaIm supérieur représen- tent principalement les niveaux durs, tandis que les argiles et les marnes du MaIm inférieur (Argovien) et du Lias, ainsi que l'Unité 1 du Trias, représentent les niveaux tendres. Enfin, les évaporites de l'Unité 2 du Trias caractérisent les niveaux les plus tendres. C'est d'ailleurs dans cette dernière Unité que se localise la zone du décollement du Jura. STRUCTURES L'interprétation des profils sismiques a permis de mettre en évidence deux types de plis différents: d'une part, des plis larges, de grande longueur d'onde et de faible amplitude localisés dans le Bassin molas- sique et les Plateaux jurassiens; d'autre part, des plis de forte amplitude, situés dans la Haute Chaîne juras- sienne. Plis associés à des évaporites: les Plateaux jurassiens et le Bassin molassique Les plis des Plateaux jurassiens et du Bassin molassique sont remarquablement bien visibles sur les profils sismiques transversaux grâce aux change- ments de l'épaisseur de l'Unité 2 du Trias. Ces plis sont contrôlés par des épaississements, dus à des empilements d'évaporites appelés coussins d'évapo- rites ou anticlinaux d'évaporites, localisés à l'inté- rieur de l'Unité 2 des couches du Trias. Ces empile- ments d'évaporites montrent, sur certains exemples des structures en forme de duplex. Le pli de Laveron, dans les Plateaux jurassiens, présente une épaisseur de plus de 1000 m, confirmée par forage, pour les couches de l'Unité 2 du Trias et correspond au plus grand épaississement reconnu dans la région étudiée. Il faut souligner qu'aucun chevauchement et qu'au- cune répétition au sein des couches du Mésozoïque, en association avec les plis, n'ont été observés sur les profils sismiques provenant du Bassin molassique et des Plateaux du Jura. Les cartes d'isopaques du Bassin molassique occidental montrent que les cous- sins d'évaporites sont alignés NE-SW, parallèlement à la direction des structures majeures du Jura. Les anti- clinaux décrits ci-dessus sont tout à fait comparables aux plis de faible amplitude, en relation avec des épaississements de sel de Melville Island dans les ter- ritoires arctiques du Canada. Plis associés à un chevauchement: la Haute Chaîne jurassienne (Jura neuchâtelois et vaudois, Jura français) Les plis de grande amplitude de la Haute Chaîne jurassienne sont en relation avec des chevauchements à vergence vers le NW ou le SE. Ils montrent un déplacement d'ordre kilométrique qui se traduit par le redoublement de toute la séquence jurassique. Ces chevauchements montent depuis Ia zone de décolle- ment principale (Unité 2 du Trias) à travers toute la série mésozoïque et cénozoïque. La répétition des unités de stratigraphie sismique est la principale évi- dence sismique pour déterminer ce type de plis. 14 Les Jura neuchâtelois et vaudois (région du Risoux) de la Haute Chaîne jurassienne montrent les plus beaux exemples de plis associés à des chevau- chements. Dans le Jura neuchâtelois, la continuité latérale des anticlinaux est de 10 km pour la région de l'Est ou 15 km à 20 km pour la région de l'Ouest. Les anticlinaux sont orientés soit ENE-WSW ou NNE- SSW et se terminent principalement contre des failles décrochantes. La combinaison des deux orientations résulte en des structures de forme losangique, comme par exemple le Val de Ruz ou le synclinal du Locle. Au contraire, le Jura vaudois et la région du Risoux (France) présentent une structure très régulière avec des anticlinaux orientés NE-SW (Mt-Tendre, Mt- Risoux), qui s'étendent latéralement sans disconti- nuité majeure sur plus de 30 km. Les décrochements Les décrochements sont bien définis dans le Jura, car la plupart s'expriment par une dépression mor- phologique bien visible sur les cartes topographiques. Les décrochements les plus importants montrent un mouvement sénestre et sont orientés NW-SE dans la partie méridionale du Jura, NNW-SSE à N-S dans le Jura central (par exemple Pontarlier, La Tourne, La Ferrière-Vue des Alpes) et NNE-SSW dans le Jura oriental. Des décrochements conjugués, avec un mou- vement dextre, leur sont très souvent associés. Les cartes géologiques présentent aussi des évidences pour déceler les décrochements: les axes de plis ont tendance à se terminer brusquement contre les failles décrochantes, sans présenter une corrélation évidente de part et d'autre de la faille. Les évidences géophy- siques pour déterminer les décrochements sur les pro- fils sismiques consistent en une large zone transpa- rente (sans réflecteur), en une succession stratigra- phique différente de part et d'autre de la faille et en un décalage des unités sismiques. Les principaux décrochements observés (Pontarlier, Morez, Mt Chamblon-Treycovagnes, La Ferrière-Vue des Alpes) affectent l'ensemble de la couverture mésozoïque et ne décalent pas le toit du socle d'un côté à l'autre de la faille. Aucun élément valable n'a été observé pour étendre ces failles dans le socle. Ces failles décro- chantes sont alors, soit des décrochements limités à la couverture ("tear faults"), soit des rampes latérales. SYNTHÈSE ET CONCLUSIONS L'interprétation d'un réseau de profils sismiques a permis de réfléchir sur la formation des plis du Jura et du Bassin molassique et de présenter une évolution dans le temps. Les plis des Plateaux jurassiens, locali- sés dans l'avant-pays, sont interprétés comme des plis de flambage correspondant à un stade précoce de la déformation. En augmentant la déformation, une rampe se génère et ce type de pli se développe en association avec un chevauchement, comme ceux observés dans la Haute Chaîne. La déformation aug- mente, donc, vers l'arrière-pays au sein de la chaîne plissée et chevauchée du Jura. Dans le Bassin molas- sique, les plis de faible amplitude représentent, cependant, un stade précoce, qui n'a pas pu évoluer à cause de la surcharge importante des sédiments ter- tiaires. Afin de synthétiser toutes les données, une carte du toit du socle (Fig. 5.1) de la région étudiée a été réali- sée à l'aide des résultats de la conversion en profon- deur des profils sismiques. Cette carte montre un socle plat et lisse plongeant de Io à 3° vers le S-SE. Aucun changement important de profondeur et de direction, qui affecterait les couches de la couverture sédimentaire, n'a pu être décelé sous les décroche- ments principaux. Aucune relation structurale n'a pu être déduite entre la carte d'isopaques de l'Unité 2 du Trias et celle des contours du toit du socle. Le socle n'est donc pas impliqué dans la formation des plis, des chevauchements et des décrochements du Jura central et du Bassin molassique. A l'échelle de la chaîne de montagne, il existe une très bonne corréla- tion entre la distribution des couches du Trias et l'étendue de la couverture déformée du Jura. Il semble donc que la forme arquée de la chaîne du Jura est étroitement dépendante de la présence d'évapo- rites du Trias à la base de la couverture. En conclusion, l'interprétation des profils sismiques démontre, que la couverture mésozoïque et céno- zoïque du Jura et du Bassin molassique a été défor- mée, au-dessus d'un niveau de décollement basai, et déplacée de quelques 20 à 25 km vers le NW. 15 !.INTRODUCTION 1.1 AIM AND SCOPE OF THE PRESENT STUDY The Jura, together with the Molasse Basin, is generally considered a foreland fold and thrust belt representing the most external, late Miocene defor- mation zone of the northwestern Alps. The deve- lopment of this belt is still a matter of debate. The availability of more than 3500 km of seismic lines has provided the opportunity to extend our know- ledge about the subsurface stratigraphy and the geometry of the Jura and Molasse Basin folds in depth. The geometry in depth of the Jura and Molasse Basin structures (folds, thrust faults and tear faults) provides important constraints on their develop- ment and the formation of the Alpine foreland. The main subjects addressed in this thesis are: - the correlation between the surface geological observations and subsurface data - the stratigraphie thickness of the buried strata - the geometry of the folds and thrusts and their development - the cover-basement relationship - the structure of the basement. Few public seismic reflection data are available to date within the Jura arc. A small seismic survey has been carried out by the NAGRA (National Cooperative for the Storage of Radioactive Waste) in the eastern Jura, whereas in the western Jura one profile has been shot by the ECORS group. The availability of recent industry seismic lines (Fig. 1.4) in the Neuchâtel Jura and older lines in the Risoux Jura, the French Jura and the western Molasse Basin has allowed infilling an important data gap for the central part of the Alpine foreland. This manuscript is organized into six chapters, providing information and illustrations about strati- graphy and tectonics of the central Jura and the western Molasse Basin. Subsurface, as well as sur- face geology are presented to offer an integrated regional geologic interpretation. Chapter 1 presents a general introduction, which exposes the aim of the study, the geological and historical setting and the methodology. Chapter 2 concerns stratigraphy. Although this thesis is focu- sed on structure, stratigraphy is discussed because it is necessary to understand structural development for example: regional overview, outcrop and sub- surface stratigraphy from well logs, seismic strati- graphic units and their correlation through the whole grid, isopach maps and rheological stratigra- phy. Chapter 3 presents an overview of the struc- tures (evaporite-related anticlines, thrust-related anticlines and tear faults), illustrated by seismic profiles. The most remarkable structures are evapo- rite stacks within the Triassic layers, controlling the anticline formation of the Molasse Basin and the Plateau Jura. The Haute Chaîne Jura folds, on the other hand, are leading to duplication of Mesozoic strata beneath the Haute Chaîne Jura thrust-related folds. Chapter 4 presents the regional geology of the central Jura and the western Molasse Basin, highlighted by seismic interpretation (line dra- wings), tectonic maps, cross-sections of specific areas and structural contour maps within the Molasse Basin. The depth to the basement map of the central Jura and Molasse Basin, derived from depth conversion of the whole seismic grid, as well as the isopach map of the Triassic beds of the Molasse Basin, will be the key for the general dis- cussion on the formation of the Jura fold and thrust belt (Chapter 5). Chapter 6 will summarize the main conclusions of this work. 1.2. GEOLOGICAL SETTING 1.2.1. The Jura The Jura is a small arcuate fold belt located in front of the western Alpine arc (Fig. 1.1). This chain has given its name to the Jurassic layers, because they represent the major part of the out- cropping rocks in the Jura. 17 1. Introduction Figure 1.1: Location of the Jura arc with respect to the major Cenozoic sedimentary basins (Rhine-Bresse Grabens, Molasse Basin, Po Plain) and the Alps. Late alpine culminations and external crystalline massifs are highlighted in dark grey. AR = Aiguilles Rouges massif; MB = Mont-Blanc Massif. Situation de l'arc jurassien par rapport aux bassins sédimentaires tertiaires avoisinants (Grabens du Rhin et de la Bresse, Bassin molassique. Plaine du Pò) et aia Alpes. Les culminations alpines tardives et les massifs cristallins externes sont soulignés en gris foncé. AR ~ Massif des Aiguilles Rouges; MB = Massif du Mont-Blanc. sedimentary wedge decreases from the South (up to 4 km) to the North (a few hundred meters). These series crop out mainly within the Molasse Basin, but they are also preserved in many Jura synclines (Val de Ruz, La Chaux-de-Fonds - Le Lode, Val de Travers, Delémont Basin.....) (Fig. 1.3). The Jura and the Molasse Basin consist of folded Mesozoic and Cenozoic beds, which are detached from the pre-Triassic basement. The latter crystal- line basement is never exposed in the Jura and Molasse Basin. It has, however, been penetrated by a few drill holes and laterally it crops out in the Vosges and Black Forest to the North, in the alpine external crystalline massifs to the South and in some small isolated outcrops along the northwestern external border of the Jura Mountains (Serre Massif)- At its southern termination, the Jura belt merges with the Subalpine Chains, which were folded contemporaneously. Along its western border, the Jura over-thrusts the Bresse Graben, whereas in the The outer arc of the Jura is 400 km long and the inner arc 340 km. The width between both arcs varies from 0 km at the eastern end, to 65 km bet- ween Neuchâtel and Besançon (Switzerland- France). The Jura arc is surrounded by Tertiary basins of different types: to the N, the Rhine Graben, to the W the Bresse Graben and to the SSE the Molasse Basin (Fig. 1.2). The Rhine and Bresse Grabens are associated with the Oligocene, West- European rift system, whereas the Molasse Basin corresponds to an Oligo-Miocene foredeep, which developed in front of the Alpine orogen. During the Mesozoic, the Jura and Molasse Basin realm was part of the Alpine Tethys passive margin and com- prises a total thickness of up to 2 km of alternating limestones and marls. Limestones of Malm age crop out in the central Jura, whereas the Cretaceous limestones (Barremian) form the crests of the wes- tern Jura anticlines. During the Oligocene and Miocene, alternating fluvial, lacustrine and marine clastic Molasse sediments were deposited. They progressively onlap the underlying Mesozoic rocks towards the Northwest. The thickness of this 1. Introduction 60OO' 7°00' 8°00' Figure 1.2: Tectonic sketch of the Jura arc with main structural units. Legend: PHS = Plateau de Haute-Saône; IC = Ile Crémieu; AM = Avants-Monts; Fe = Ferrette; FA ¦ Faisceau d'Ambérieu; Fb = Faisceau bisontin; Fl = Faisceau lédonien; FO = Faisceau d'Orgelet; Fsal = Faisceau salinois; FSy = Faisceau de Syam; Lo = Lomont; PC = Plateau de Champagnole; PL ¦ Plateau de Levier; Pl = Plateau lédonien; PO = Plateau d'Ornans; AR = Aiguilles Rouges; MB = Mont Blanc. Modified from Sommaruga (1995). Carte tectonique de l'arc jurassien avec les unités structurales majeures. Légende: PHS = Plateau de Haute-Saône; IC = Ile Crémieu; AM = Avants-Monts; Fe = Ferrette; FA = Faisceau d'Ambérieu; Fb = Faisceau bisontin; Fl = Faisceau lédonien; FO = Faisceau d'Orgelet; Fsal = Faisceau salinois; FSy = Faisceau de Syam; Lo = Lomont; PC = Plateau de Champagnole: PL = Plateau de Levier; Pl = Plateau lédonien; PO = Plateau d'Ornans; AR = Aiguilles Rouges: MB - Mont Blanc. Modifiée de Sommaruga (1995). 19 1. Introduction North, it overrides the Tabular Jura. At its eastern end, the last Jura fold (Lägern) dies out within the Molasse Basin. The Jura itself is divided into the external Jura and the internal Jura (Chauve et al., 1980). a) The external Jura The external Jura (Fig. 1.2) consists of flat areas, Plateaus, limited to the North and separated from each other by the so called "Faisceaux" (from North to South: F. bisontin, F. salinois, F. lédonien, F. de Syam. F. d'Orgelet and F. d'Amberieu). These are narrow, strongly deformed fold bundles characterized by a succession of numerous small-scale imbricates and tear faults. The Plateaux (P. d'Ornans, P. lédo- nien, P. Champagnole and P. Levier) correspond to weakly faulted, horizontal or slightly SE dipping areas. b) The internal Jura The internal Jura, often called the folded Jura, "Haute Chaîne" or "Faisceau helvétique" consists of a well developed fold train representing a natural present-day northern limit to the Molasse Basin. On a large scale, deformation is characterized by major folds, thrusts and tear faults. The amplitude of the Jura folds depends on the cover thickness (800 m- 2000 m) and the degree of shortening, which is highest in the internal part of the central Jura and decreases outwards. Folds are thrust-related and end laterally either with plunging axes or abruptly against tear faults. Some major sinistral tear faults cutting the whole cover are recognized; their orien- tation is N-S in the eastern Jura and changes gra- dually along the chain to a WNW-ESE direction at the western end. The outer border of the folded Jura is thrust over the Plateau Jura. At the southern bor- der, the Mesozoic beds dip below the Oligo- Miocene sediments of the Molasse Basin. A third zone, the Tabular Jura, is often associated with the Jura. This zone is located outside of the Jura arc and represents the Mesozoic cover of the sou- thern Black Forest and Vosges basement. The Tabular Jura represents the transition from the Rhine-Bresse Graben to the Paris Basin. Strata are subhorizontal and cut by a N-S or NE-SW Oligocene fault system. The Avant-Monts and the Ferrette areas represent the deformed cover of this Tabular Jura. At the southern end of the arc, the Ile Crémieu is the southern counterpart of the Tabular Jura. 1.2.2. The Molasse Basin The whole Molasse Basin (sensu lato) represents a foreland basin which extends over more than 700 km, from the French Savoy in the West (Chambéry, Figs. 1.1 and 1.2) to Linz (Austria) in the East. It runs parallel to the Alpine front and widens progressively to the East, from 50 km in the Neuchâtel traverse to some 150 km in southern Germany. Sedimentation in the foreland basin was continuous from earliest Oligocene to Serravallian time and took place in alternating marine, fluvial and lacustrine environments. The thickness of this sedimentary wedge decreases from some 4 km in the South to a few hundred meters in the North. The clastic wedge of the Molasse Basin is subdi- vided into three geological units, the Jura Molasse, the Plateau Molasse and the Subalpine Molasse (HOMEWooDeítf/., 1989). a) The Jura Molasse The Jura Molasse represents the northern feather edge of the Molasse Basin that has been passively involved in Jura folding and thrusting. Only isolated patches of Molasse are preserved within major syn- clines of the internal Jura. b) The Plateau Molasse The Plateau Molasse, representing the major part of the Molasse Basin, shows contrasting structural styles between the western and eastern parts. In the western Swiss part, the structures consist of broad anticlines oriented NE-SW and tear faults trending N-S, NW-SE and WNW-ESE. The nor- thern limit of the Plateau Molasse corresponds to an erosion limit along the most internal, high ampli- tude folds of the Jura belt. In eastern Switzerland and Bavaria, surface geo- logical outcrops show the onlap towards the North of the Tertiary wedge over the Tabular Jura (Franconian Platform) (Bachmann et ai., 1987). The only known deformation is characterized by small normal faults oriented WSW-ENE, parallel to the basin and affecting the Mesozoic and Cenozoic strata (Bachmann et al, 1982). e) The Subalpine Molasse The Subalpine Molasse represents a narrow zone located along the southern border of the Molasse 20 I. Introduction Basin (Fig. 1.2). This zone is characterized by a stack of thrust sheets of Tertiary sediments, detached along a décollement zone within these Cenozoic layers ("Grisigen shales") (Trümpy, 1980). The southern limit of this zone corresponds to the Oligocene Alpine front represented by the Alpine nappe stack (Prealps, Helvetic nappes, ...) which overthrusts the Molasse sediments. The northern limit of this unit, correspon- ding to the transition between the Subalpine- and the Plateau Molasse, is structurally an important triangle zone (Bachmann et ai, 1982; Vollmayr & Wendt, 1987; Müller étal, 1988; Vollmayr, 1992). According to Allen et al. (1986), Homewood (1986) and Vann et al (1986), the change in structu- ral style along the strike of the Molasse Basin is rela- ted to the presence of a décollement zone within the Triassic evaporites. The entire Basin in the western and central parts is considered to be detached from its crystalline or Permo-Carboniferous substratum. 220- 200- 180- 160- 140- 120- <§>Humiliy Figure 1.3: General geographical index map of the investigated area (cities, rivers, mountains and wells). Legend: ChI = Chapelle; Cua = Cuamy; Ess = Essertines; Her = Hermrigen; Tre = Treycovagnes; Val = Valempoulières. Carte géographique générale de la région étudiée (villes, rivières, montagnes et forages). Légende: ChI = Chapelle; Cua = Cuarny; Ess = Essertines: Her = Hermrigen; Tre = Treycovagnes; Va! = Valempoulières. 21 1. Introduction 480 220- 200- 180- 160 140 120- v50 Seismic line ® Well Border 10km Figure 1.4: Seismic data used in this work. Location of the various sectors: A = Neuchâtel and Vaud Jura Mountains, Panels 1, 2, 3. B= Molasse Basin, Panels 4, 5, 6. C = Mt- Risoux Jura Mountains, Panels 8, 9. D = Champagnole-Mouthe Jura Mountains, Panel 10. Legend: ChI = Chapelle; Cua = Cuarny; Ess = Essertines; Her = Hermrigen; Tre = Treycovagnes; Val = Valempoulières. Profus sismiques interprétés dans ce travati. Localisation des différents secteurs: A - Jura neuchâtelois et vaudois, Panneaux 1, 2, 3. B= Basin molassique. Panneaux 4, 5,6.C = Région jurassienne du Mt- Risoux , Panneaux 8,9.D= Région jurassienne de Champagnole-Mouthe. Panneau 10. Légende: ChI = Chapelle: Cua = Cuarny; Ess = Essertines; Her = Hermrigen; Tre = Treycovagnes; Val = Valempoulières. The deformation propagated from the South into the Jura belt, whereas to the East there is as yet no evi- dence for a décollement zone. Thick evaporite hori- zons in Triassic strata are present beneath the Jura and the Swiss Molasse Basin (Rigassi, 1977) (see also Fig. 2.5). Triassic beds progressively onlap basement to the North-East and are entirely absent beneath the Molasse Basin in Germany, where Jurassic strata lie unconformably on the basement; here, even where the Triassic is preserved, evaporite series are absent (Bachmann et al, 1987). 1.3. REGIONAL BOUNDARY CONDITIONS TO THE FORMATION OF THE JURA FOLD AND THRUST BELT The formation of a fold and thrust belt and the nature of folding are intimately dependent on the existing boundary conditions. For the Jura belt, the major regional boundary conditions can be summa- rized as follows: 22 J. Introduction - the presence of Triassic evaporite beds serving as basal décollement level; - the presence of a rigid basement dipping Io to 3° towards the South and underlying a weak décolle- ment level (Buxtorf, 1907); - the rheological stratigraphy in the cover (§2.6. and Fig. 2.30): the thickness of strong competent rocks (Malm and Dogger limestones) increases from NE to SW, whereas the weak incompetent beds above the décollement decrease towards the SE; - the small overburden and hence the weak burial depth: depths ranging from few hundreds of meters to a maximum of 2500 m in the southern Jura create low temperature conditions for the deformation of the rocks; - the wedge shape of the whole Jura foreland: the shallow surface slope towards the NW and the basal dip towards the SE; - the wedge shape of the Tertiary Molasse Basin: the thickness of the sediments increases strongly from the North to the South; - the inherited Oligocene structures within the cover: the West-European rift system results in the formation of the Rhine-Bresse Grabens and also in N-S to NE-SW faults affecting the cover and the basement. These faults are especially located in the externa] and eastern regions of the Jura arc. 1.4. FORMATION OF THE JURA: SHORT REVIEW AND OPEN QUESTIONS The Jura Mountains have drawn the interest of structural geologists and paleontologists since the , beginning of the last century. In the early years, research was focused on stratigraphy and paleonto- logy. Among the few early geological cross-sections that were drawn, the one of Von Buch (1806) in Von Buch ( 1867) is remarkable for its time. He presented a geological section from the Aar Massif to the Serre Massif (Fig. 1.2), where the so called limestone cover rocks of the Prealps, the Molasse Basin and the Jura chain overly granite rocks. On his section, the latter crop out in the Aar and Serre Massifs, the southern and the northern borders, respectively. The major issues of the formation of the Jura are already included in this early 19th century cross-section: what are the cover-basement relationships within the Jura and the Molasse Basin and where is the Jura cover shortening compensated in the basement ? Since the beginning of this century, many authors have attempted to answer these questions. In the fol- lowing sections, we will give a short review of various interpretation of the Jura formation illustra- ted by some published cross-sections (Fig. 1.5) and some conceptual models (Fig. 1.7) modified from Burkhard (1990). For the Jura, two fundamentally different assumptions dominate the debate i.e. base- ment involved folding versus décollement of the sedimentary cover. Basement, here, includes all rocks older than the Triassic anhydrites. The viabi- lity of these models will be discussed later in the Chapter 5 together with the main results of this the- sis. For a more detailed review on the evolution of the ideas during the first 50 years of this century, the reader should refer to Caire (1963). Basement involvement beneath the Jura: proponents Many authors have argued in favor of basement involvement. They regard the Jura as an essentially autochthonous cover, which has been deformed in response to deformation in the underlying basement so that the shortening of the Jura folds is taken up in the underlying basement. Aubert (1945) proposed disharmonie folding of the Jura cover in the Muschelkalk anhydrites due to basement slices penetrating into the cover (Fig. 1.5, section 3). This idea was inspired by the cross-sections of Staub (1924) in the Alps. Fifteen years later, Aubert (1959) reconsidered his theory taking into account the work of Glangeaud (1949) and especially new field work results in the Pontarlier strike slip fault area (Figs. 1.2 and 3.19). The latter fault, like all Jura strike slip faults, is considered in this model as deep Oligocene age strike-slip faults rooting in the basement. The contraction of the strike-slip faults of the Jura has induced some major folding in the over- lying cover ("contraction du socle" theory. Fig. 1.7). Following the results of the Risoux well, with its large scale duplication of Mesozoic strata, Aubert (1971) changed his views to conclude that the Jura cover is allochthonous. The influence of deep seated strike-slip faults is also central to Pavoni's (1961) theory of folding related to basement involved wrench faulting (Fig. 1.7). Based on a series of experiments, Pavoni demonstrated a possible connection between the relative movement and strike of the wrench faults and the resulting fold structures. In this model, the Jura folding results from horizontal shortening oblique to the general trend of the fold axes. 23 7. Introduction UJ c/j 4) )¾ UI9J5U3SS/3M CO O CJi .c O CO x/j»u//e¿/ BSVtff SSeSjY1P U0/fB8 © o T- 07 Xl 24 1. introduction UJ AfSO f , in ËPZsLÏ fflfl.Hl < E s B »'is Q IP ÍM.Y/& z flftWtn w od« WtI i- Trt I In (I »- w B tiff H S KU ¦ s w ISlfl I' I f + + W7Í -t fl< íí 4 P/jmVrí -4- Mj/ fi * 4- + X J Flu ti + ^ HItHFl o [In[I 4 S fffij/í J- iff + Hf + WM' + Wm' + \fû\M< KVj tit ¦* x EfftÈ* 4 s m + *- • BÌÌ#ì 4 3 '»Mí 1 Sí ^f RtI ï il * 4 [hl< 4 III* t- + 4 ¦RCn_l< + -t t fl< % * t ¦ *i + ¦J 4- 1 + "Il t : ir *"A- - < + ; + +¦ . < 1 ' 4 < +- , 4 4- ' + i 4 f + < J < f 1 ; :< f 3 + 3 '¦ < 1 5 : T >?- 1 * t I t 4 Q < ¦t ^- , ¦a < r » ¦< 4 I t * I I, I' t * i t , i t ¦ t t- ' t- + U/ Vl < « + Ul s k. * ? ¦ :;¦ f#7^ * í Í f/ff z o .;jf /.^. 11 o < C J S I > O ID CD CD 0 .C Ü CO aï © tu a D < -J £ O '^ -s: O u -se .¾ Vl -^ Vl s; e 3 O §¦ -a C U tu S "a Cu Ij-. S O 3 C =i. O SP ^ S- U 5¾ m O vj •g BH 60 îû 3 §. -Sf rS ÏZ U 25 1. Introduction Stretching in this model is essentially subhorizontal as expressed by the presence of conjugate systems of strike-slip faults. A wrench fold concept was also discussed by Wegmann (1963), who suggested that the crystalline basement was broken by N-S trending horsts and grabens and by NE-SW trending antithetical faults. Accordingly, major tear-faults of the Jura resulted from the movement of the basement and concentrated along narrow zones. Folding resul- ted from more diffuse movements in the cover. The availability of seismic reflection and well log data induced some authors (ZiEGLER1 1982; Gorin et al, 1993) to further develop arguments in favor of an autochthonous cover. Ziegler (1982) argues that seismic reflection data show that the Molasse Basin is transected by a set of basement-involving normal and wrench-faults. These faults die out in the Oligocene or Miocene sediments and were active during the folding of the Jura fold and thrust belt. On the seismic lines, basement appears much deeper in the hinterland part of the Jura than below the Molasse Basin. Based on these data, Ziegler (1982, Fig. 26) suggests, that "... the southern margin of the Jura belt is associated with a major basement imbri- cation along which the bulk of the shortening evident in the Jura-thrust belt is taken up. ... The configura- tion of the Molasse Basin implies that the postulated basement imbrication is carried out by a gently sou- theast-dipping thrust fault that possibly soles out within the crust. ...". This thick-skinned allochtho- nous model suggests that the folding of the Jura belt was associated with crustal delaminations (Fig. 1.7). Recently Gorin et al. (1993) have published struc- tural interpretations of industry seismic lines from the western Swiss Molasse Basin. According to these authors the "...foothills of the Jura Mountains are marked by a considerable thickening of the Triassic evaporites, which has been related by BiTTERU (1972) to salt flowage, but also coincides with a deep-seated Paleozoic lineament. ...". These lineaments would have been active during that time and reactivated several times until the present day. Gorin et al. then argued that the geometry of these faults did not permit the translation of the cover and therefore the latter is autochthonous. Pfiffner et al. (1997a) have also suggested recently that the presence of inverted Permo- Carboniferous grabens argues in favor of a thick- skin deformation. A first detachment would be loca- ted within the Triassic evaporites and a second detachment horizon would be present within the basement. An inverted Permo-Carboniferous graben has been interpreted on one seismic reflection line in the central Molasse Basin (Hermrigen) (Fig. 1.3) and another one has been postulated in order to balance a geological cross-section in the central Jura (Chasserai anticline) (Kühni, 1993) (Fig. 1.7). This model agrees with that of Guellec et al. (1990) based on a ECORS deep seismic line as well as sur- face and subsurface data, who propose a shallow décollement in the Triassic evaporites layers and a deeper one within the basement (Fig. 1.5, section 5). According to them, "*.. basement shortening is often required to account for the late deformation of the overlying but allochthonous sedimentary cover" A similar interpretation of the Jura in terms of "thick-skinned" tectonics is given by Jouanne et al (1994) based on the comparisons of triangulations and present day vertical uplift rates (Jouanne & Menard, 1994). Basement involvement beneath the Jura: opponents Many other authors have explained the formation of the Jura belt as folded above a main décollement level within Triassic evaporite series. This hypothe- sis supposes an allochthonous cover and no base- ment involvement. For these authors, the discussion centres around the question of where the shortening of Jura and Molasse Basin cover is compensated in the basement ? The review presented here is based on a discussion by Burkhard (1990) with the addi- tion of some new references. Buxtorf (1907) was among the first to interpret the Jura fold and thrust belt as an allochthonous cover deformation, resulting from a distant Alpine push (= "Fernschub" in German) transmitted through Mesozoic and Cenozoic strata of the Molasse Basin. The cover would be detached from the basement in the Triassic anhydrite beds. Buxtorf (1907; 1916) based his hypothesis on the observation of Triassic rocks in railway tunnel cut through anticlines (Fig. 1.5, section 2). The earlier section of Schardt (1906), shows that the Jura as an allochthonous cover folded over a slightly war- ped basement, stating a décollement zone between the cover and the basement (Schardt, 1906, 1908). Gravity sliding for the whole Jura belt was propo- sed as early as 1892 by Reyer (1892) and later more clearly expressed by Lugeon (1941, p. 9) for whom, the Jura resulted from the lateral migration of the evaporite layers from the Molasse Basin, 26 1. Introduction co EEEEEEEE §0000000 o 80000 >. CM ^- 1- CNJ O 'í IO C « s l_ 03 ü CD ¦»— =3 cd I 03 Q) O W '(O Cß m CO CO CO CD 3 > 3 ^- co g CO Q- =3 CO CD 1 CO C 0 I ¢¢:: CU l— =3 ~3 CD C <— co .n Ü CD *—' CO X > ¢522;- c e 1 ! D ß 3 U I g 1 Q g E ¦J I — B fi. r. E C |2 P H ¡H w y) = o .2 ö I Ql S <* 2 S y o §1 « o X! U- -S ai C C J •- tf.fi — O U O vi - A B 1 — U I I I I 5 I I 'S r Q I) £ ft) .¾ ^ -2 i* ~ 3 .5 -¾ ft) v. ft) .O O Il •ft) ft» II et 27 1. Introduction BASEMENT INVOL VED beneath the Jura: autochthonous cover Jura belt Molasse Basin "Basement contraction" Aubert1945, 1959 BASEMENT UNINVOLVED beneath the Jura: allochthonous cover Jura belt Molasse Basin "Fernschub" Alps Buxtorf 1916 "Gravity gliding Lugeon 1941 "Wrench folding" Pavoni 1961 Wegmann 1963 "Thick-skin" Ziegler 1982 Laubscher 1961 "Jura-Helvetic nappes" Laubscher 1973 Boyer & Elliot 1982 Paimo-Cartxxiterou* graben» Conceptual models modified from Burkhard 1990 Guellecetal. 1990 Figure 1.7: Review of conceptual models of the formation of the Jura fold and thrust. Modified from Burkhard (1990). Présentation de plusieurs modèles conceptuels expliquant la formation de la chaîne plissée du Jura. Modifié de Burkhard (1990). 28 1. introduction under a vertical loading stress (Fig. 1.7). The first model of Laubscher (1961, 1965) also suggested that gravity sliding played an important role (Fig. 1.7). The Mesozoic cover together with the over- lying Tertiary Molasse is folded and thrusted towards the NW from 2 to 25 km and rotated 8° around its northern tip, above a main décollement horizon located within the Triassic evaporite beds (Fig. 1.5, section 4). This model assumed a smooth underlying basement dipping 2° to 3° towards the South. Later, Laubscher (1973b) presented a kine- matic model of the Jura-Helvetic system, where the main Jura décollement level was connected to the basal Helvetic thrust; as a result the uplift of the external crystalline massifs would postdate the for- mation of the Jura thrust and fold belt. Burkhard (1990) referring to fission track data from the Alps (Schaer et al, 1975; Hurford, 1986) argues that "... the uplift of the Centrai Alps must have started at the Early Miocene time, when the Molasse Basin was still subsiding..... This means that during the thrusting of the Jura, there existed already an Aar massif culmination that had about half its present amplitude ...". Consequently, the Jura basal thrust must root below or in the front of the external crys- talline massifs (Boyer & Elliott, 1982) and it can- not connect with the basal Helvetic thrusts (Fig. 1.7). Recently, several seismic reflection lines have been shot and a series of drill holes have been made in the northeastern Swiss Jura by the NAGRA (Cooperative for the Storage of Radioactive Waste) (DiEBOLD et ai., 1991). This work has led to the dis- covery of an important Permo-Carboniferous trough. In addition the basement top seems to be affected by E-W trending normal faults with throws of some tens of meters. However, these faults are not important enough to impede a basal detachment within the thick Triassic beds (Laubscher, 1992) (Fig. 1.5, section 6). 1.5. SOURCES OF DATA Surface geological information, reflection seismic data and wells provide the data sets for this research. 7.5. 7. Surface geological data The surface geology data consist of a large num- ber of published and unpublished geological maps, near-surface cross-sections and lithostratigraphic logs. In Switzerland, the studied area is more or less covered by 1:25'000 scale geological maps publi- shed by the Swiss Geological and Hydrological Survey and in France by 1:50'000 scale maps publi- shed by the French BRGM (Bureau de recherches géologiques et minières). Several unpublished origi- nal maps and cross-sections (diploma, PhD theses and private contracts) deposited at the Geology Department of Neuchâtel University, have been very useful for understanding structures. All maps used in this investigation are listed in Table 1.1. 7.5.2. Subsurface data Industry seismic reflection surveys were conduc- ted in the study area between 1970 and 1988 by dif- ferent companies: British Petroleum (sector A in Figure 1.4), Shell Switzerland (sector B et C), Société anonyme des Hydrocarbures (sector B) and Shellrex (sector D). This subsurface data set consists of more than 1500 km of migrated or unmi- grated seismic profiles and lithology- and velocity- logs of some twenty wells. The parameters (acquisi- tion and processing) of the seismic set are more or less consistent, depending on each survey and espe- cially on the date of exploration (Tab. 1.2). The Neuchâtel Jura and French Jura data sets were pro- vided directly by the oil companies. Most of the industry seismic lines and well data from the Canton Vaud area are deposited at the Musée de Géologie at Lausanne, who kindly gave access to this informa- tion. The data sets include a migrated or unmigrated stack copy on paper. Appendix 1 presents an inventory of seismic lines with the renumbering adopted in this thesis. In this new line numbering scheme (see Tab. 1.2 thesis numbering and sector), even numbers denote strike lines (NE-SW), whereas odd numbers denote dip lines (NW-SE). For a wider area, Swisspetrol (1992) and Bitterli (1972) present a general loca- tion map with a seismic grid and survey names from the Jura arc and the Molasse Basin. Data from some twenty drill holes could be used for calibration of the seismic lines. These data include lithology and velocity logs (Appendix 2) coming essentially from the literature or provided by the Musée de Géologie in Lausanne. 1.6. METHODOLOGY This subchapter present a short view of the important steps involved in the elaboration of this research. 29 J. Introduction Title Location Scale Authors Val de Ruz NE1CH 1 25'0OO Bourquinelal. (1968) Bielcr See BE, NE, CH 1 25'0OO Schär(1971) Ncuchàtel NE, CH 1 25'0OO Freietal. (1974) Le Locle- La Chaux-de-Fonds NE, CH 1 25'0OO RoUier& Favre (1910) Chaumont (East sector) NE1CH 1 5'000 Schaer(1956) Chaumont (West sector) NE1CH 1 5'000 Margot (1962) Le Pâquier NE1CH 1 25'000 Aragno(1994) Mont Amin Kette NE1CH 1 5'000 Baer(1959) Biaufond-...-Saint-Imier NE, JU1CH 1 25'00O Bourquin&Suter(1946) Les Verrières NE1CH 1 25'0OO Miihlethalcr(1930) Val de Travers NE1CH 1 25'0OO Rickenbach (1925) Travers NE1CH 1 25'0OO Thiébaud(l936) Solmonl NE1CH 1 25'0OO Frei (1942) Les Gorges de l'Areuse NE1CH 1 15'000 SchardtÄ Dubois (1903) Les Gorges de l'Areuse NE1CH 1 25'0OO Mcia(1986) Creux du Van NE1CH 1 10'0OO Müller (1958) Ponts-de-Martel NE. CH 1 5'000 De Pury (1963) La Tourne NE1CH 1 5'000 Schwaar(1959) Mt-Aubert VD. NE, CH 1 25'000 Meia (1969) Les Gorges de La Vaux VD1CH 1 5'0OO Dessoulavy(1952) Ste-Croix VD1CH 1 25'000 Rigassi & Jaccard (1995) Yverdon VD1CH 1 25'000 Jordi (1994) Orbe VD1CH 1 25'000 Aubert&Dreyfuss(1963) Cossonay VD1CH 1 25'000 Custer &Aubert (1935) Jorat VD1CH 1 25'000 Bcrsier(1952) Morges VD1CH 1 25'000 Vcrnct(1972) Lausanne VD1CH 1 25'000 Weidmann (1988) Châtel-St.-Denis VD, FR1 CH 1 25*000 Weidmann (1992) Marchairuz VD1CH 1 25'000 Falconnier(1950) Vallée de Joux VD1CH 1 25'000 Aubert(1941) Baumc-Les-Dames F 1 50'000 BRGM (1972) Chalon-sur-Sâone F 1 250'000 BRGM (1987a) Champagnole F 1 50'0OO BRGM (1965a) Dijon F 1 250'0OO BRGM (1989) Dôle F 1 50'000 BRGM (1979) Lons-Le-Saunier F 1 50'0OO BRGM (1966) Morez-Bois d'Amont F 1 50'000 BRGM (1968a) Morteau F 1 50'0OO BRGM (1968b) Mouthc F ] 50'000 BRGM (1964) Ornan s F 1 50'000 BRGM (1963) Poligny F 1 50'000 BRGM (1981) Pontarlier F I 50'000 BRGM (1969) Quingey F 1 50'000 BRGM (1975) Salins-Les-Bains F 1 50'0OO BRGM (1967) Thonon-Les-Bains F 1 250'0OO BRGM (1987b) Vercel F 1 50'000 BRGM (1965b) Table 1.1: List of geological maps from the studied area. Cantons: NE = Neuchâtel; BE = Bern; VD = Vaud; FR = Fribourg. Countries: CH = Switzerland; F = France. Liste des caries géologiques utilisées dans cette étude. Cantons: NE = Neuchâtel; BE = Bern; VD = Vaud; FR = Fribourg. Pays: CH = Suisse; F = France. 30 I. Introduction - Compilation of existing data A large amount of data on the Central Jura and Molasse Basin exists, but few of these are easily accessible. Tools for documenting this study include: geological maps, dip data, cross-sections, drill hole data and seismic reflection lines. Different petroleum companies have shot thousands of kilo- meters of seismic reflection lines and drilled many wells in the project area. Many of them are neither published nor in the public domain. However, upon request BP and SHELL oil companies kindly provi- ded many useful seismic lines and drill logs. - Field work Field work was conducted in order to collect addi- tional dip data for the cross-sections. - Seismic interpretation More than 1500 km of seismic lines were inter- preted for this work (Fig. 1.4). Surface data from geological maps of the central Jura and additional data collected in the field have been integrated with seismic profiles and well logs. Knowledge about stratigraphy is essential for seismic interpretation. As first step drill hole logs were therefore compiled. Based on wells, seismic stratigraphie units have been defined and correlated through the whole seis- mic grid, with jump correlations if necessary, because good data in synclines are often interrupted by bad data across anticlines and tear faults. Due to the good quality of strike lines, mainly shot along synclines, it was possible to constrain the stratigra- phie column at depth. This has been especially important for unexposed Triassic formations. Each intersection of the seismic profiles was checked to obtain an internally consistent interpretation. - Depth conversion and contour maps Seismic time lines (two way time in seconds) were converted to depth (meters), in order to construct contour and isopach maps. Seismic veloci- ties were constrained by nearby drill hole velocities. In the Jura itself, a simple velocity model was used for depth conversion, whereas in the Molasse Basin region, powerlaw or linear functions, dependent on depth were used (Appendices 3). - Cross-sections One line length balanced cross-section across the whole Jura and Molasse Basin (Fig. 1.6 and Plate 9) and two more regional cross-sections across the Neuchâtel Jura (Val de Ruz: Fig. 4.5; Val de Travers: Fig. 4.8) were drawn. These sections are based on the depth conversions of the nearest seis- mic lines and constructed using modern concepts of structural geology (Woodward et ai., 1989). Survey Year Migration Quantity Sector British Petroleum 1988 yes 17 A Shell Switzerland 1973-1978 11 B SADH *) 1972-1979 34 B Shell Switzerland 1972-1974 yes 14 C Shellrex 1970-1974 mostly 20 D Lenght Source D. P. 300km vibraseis 500m 450km vibraseis 500m 200km vibraseis 500m 100km vibraseis 500m 600km vibraseis 800m Thesis numbering 1-19 20- -71 80-95 100-125 Table 1.2: Seismic data parameters for each survey. For sectors, see Figure 1.4. D.P. = Datum Plane. *) SADH = Société anonyme des Hydrocarbures. Paramètres sismiques caractérisant chaque campagne sismique. Pour ¡es secteurs, voir Figure 1.4. D.P. = Niveau de référence sis- inique. *) SADH = Société anonyme des Hydrocarbures. 31 2. STRATIGRAPHY 2.1. INTRODUCTION The geological interpretation of a seismic grid is based on the correlation of major seismic reflectors throughout the whole area. In sedimentary rocks, reflectors correspond to major lithological disconti- nuities. The stratigraphie calibration of the reflec- tors is done with the information obtained from the interpretation of drill hole logs. This chapter will first provide a regional overview and then present a synthetic stratigraphie column for the Neuchâtel Jura. Selected reflectors and seismic units used for the interpretation of the lines will then be presented taking into account the lithostratigraphic considera- tions. This chapter will also offer a series of isopach maps of the major stratigraphie units below the Molasse Basin and a rheological framework, which are both crucial for the understanding of the struc- tures. 2.2. REGIONAL OVERVIEW 2.2.1. Introduction The evolution of the Molasse and Jura basins during Mesozoic and Cenozoic times is here discus- sed in the larger geographic and tectonic context of central Europe. The following summary is not based on seismic data, but mostly on surface geology and wells, as reported in published books, guides or papers, that discuss the setting and the stratigraphie evolution of the sedimentary cover. Key references are listed below: Diesler (1914), Heim (1921), Jeannet (Plate IV) in Favre & Jeannet (1934), Guillaume (1966), Debelmas (1974), Chauve (1975), Aubert (1975), Trümpy (1980), Debrand-Passard et al. (1984), Debrand-Passard & Courbouleix (1984), Homewood (1986), Gygi & Persoz (1986, 1987), Martin (1987), Ziegler (1982, 1988, 1990), Homewood et ai ( 1989), Keller ( 1989). Formation names in quotation marks are given in French or German to avoid confusion associated with translation. 2.2.2. The basement: Paleozoic or older rocks Paleozoic and older rocks underlie the Mesozoic strata of the Jura and the Molasse Basin. Crystalline basement crops out in the Vosges and Black Forest massifs (migmatitic gneisses), in the Serre massif and in the external Alpine massifs (Fig. 1.2). Permo-Carboniferous sediments are locally preser- ved in graben-like structures in all of these areas. Both kinds of rocks have been penetrated by wells underneath the Jura and the Molasse Basin. The crystalline basement (metamorphic rocks) was formed during the Variscan (or Hercynian) oro- geny. The latter is considered the product of the col- lision between the Gondwana and the Laurasia conti- nents during lower Carboniferous time. Syn-kinema- tic granitic intrusion is associated with Variscan structures; post-kinematic granitic intrusion is rela- ted to crustal extension beginning in the upper Carboniferous. The latter corresponds to the final stage of the Hercynian orogeny, which is characteri- zed by presumed crustal thinning along low angle normal faults and widespread strike-slip faults. Transtensional basins (pull-aparts, grabens), volca- nism and hydrothermal activity is obviously the result of extension tectonics. Detrital fluvial sedi- ments derived from high relief areas filled the basins and a luxuriant continental vegetation developed in warm climatic conditions. Coal seams result from the burial of this organic matter and Permian red sandstones and conglomerates ("Rotliegendes") overlie the coal-bearing strata. Important subsurface basins occur at the edge of the Jura: one example is the Lons-le-Saunier basin located at the western edge of the Jura (Fig. 1.2) and another basin located in the northern part of Switzerland was discovered in 1983 by the Swiss NAGRA (National Cooperative for the Storage of Radioactive Waste). Peneplanation followed the end of the Hercynian orogeny. The basement is therefore characterized by two major unconformities: one below the Carboniferous sediments and second below the Triassic rocks. 33 2. Stratigraphy 2,2.3. Mesozoic During the Mesozoic, an epicontinental sea deve- loped in the area of the future Jura Mountains. lnterlayered marl and limestone beds were deposited in a platform or lagoonal environment. An impor- tant facies limit (barrier or barrier reefs) appears to be located today approximately along the internal part of the Haute Chaîne Jura (Trümpy, 1980; Persoz, 1982; Mouchet, 1995). In the Molasse Basin, more basinal facies have been recognized (Persoz, 1982). Three typical stratigraphie columns of the Mesozoic formations, outcropping along the Swiss Jura, are presented on Figure 2.1. This figure shows the most important lateral changes in thickness and facies of the formations along the Swiss Jura. 2.2.3.1. Triassic The Triassic is not exposed in the study area, except in the "Faisceaux" Jura of France. Triassic also crops out in the eastern Jura of Switzerland and it also has been drilled (Figs. 2.11 to 2.13) in the French Jura, in the Molasse Basin and in the Bresse Graben. The stratigraphy of the Triassic layers is difficult to establish in the subsurface of the Jura and the Molasse Basin, because tectonic complications occur within these formations. As mentioned earlier (Chapter 1), the cover has been deformed over a main Triassic décollement level. Tectonic thicke- ning, thinning and doubling of beds make it difficult to establish a detailed stratigraphy. Figure 2.2 pre- sents a composite geological profile through the Triassic décollement level as interpreted from the well data of Schafisheim (Canton Aargau, Switzerland) (Matter et ai, 1988). This area, loca- ted in the eastern Swiss Molasse Basin and a few kilometers south of the easternmost fold of the Haute Chaîne Jura, is structurally less complicated than the French Jura Plateau or the Haute Chaîne Jura. Even if the classic German Triassic and its stratigraphie framework is presented here, the area remains difficult to correlate with the limited and tectonically complicated data provided by the Jura and the Molasse Basin drill holes. The terms Keuper and Muschelkalk are not used in this work in view of the lack of reliable biostratigraphy and the tecto- nic considerations discussed above. Instead, we have adopted Triassic Unit 1 and Triassic Unit 2 as main subdivisions, based on seismic correlation (§2.4.3.7. and §2.4.3.8.). At the beginning of the Mesozoic, the peneplaned basement was invaded by a marine transgression. An epicontinental shallow water sea developed with a depocenter in northern Germany (Fig. 2.3). The basin was almost completely landlocked. During times of limited connection with the Tethys, conti- nental elastics and evaporites were deposited. During marine ingression huge lowland areas were flooded and carbonates were precipitated. The Triassic deposits in the Jura are of Germanic type and traditionally subdivided into three lithostratigra- phic units (Fig. 2.4): the "Buntsandstein" consists of sandstones or conglomerates also called the "Grès bigarrés" formation; the "Muschelkalk" begins with the "Wellen-gebirge", marine sandstones, marls and dolomites and is followed by the "Anhydritgruppe" with its salt and evaporites suggesting only a restric- ted connection between the Germanic inland sea and the Tethys (Fig. 2.3) and terminates with the "Hauptmuschelkalk" and its marine shelly limes- tones; the last of the trilogy, the "Keuper", starts with the marker bed "Lettenkhole", containing lignite and dolomite and continues mainly with eva- porites ("Gipskeuper") and marls. The overlying Rhaetian is represented by sandstones and marls. The total thickness of the Triassic series (Fig. 2.5) is less than 500 m outside of the Jura fold and thrust belt and appears to increase progressively towards the center of the chain (internal Jura), reaching more than 1000 m. The formation of the Jura fold and thrust belt seems to be closely related to the thick- ness distribution of the Triassic layers and will be discussed later in Chapter 5. 2.2.3.2. Jurassic Early Jurassic: Liassic The lower Jurassic formations, like those of the Triassic, crop out in the Tabular Jura, along the nor- thern margin of the Folded Jura and in France along the western margin of the Jura. The Early Jurassic was characterized by a trans- gression. The environment was a platform or a low energy shallow basin (Fig. 2.6). The studied area is too small to be a distinct paleogeographic domain itself, although it shows influences from the Alpine (Tethys basin), Swabish and Paris basins. Several positive areas of low relief, represented by anoroge- nic highs (i.e. Massif Central, London-Brabant mas- sif, Rhenish massif, Bohemian massif, ...) surround these basins. The southern part of the Molasse Basin was occupied by one of these islands called 34 2. Stratigraphy i— Om ¦ unGomecc PIERRE .AUNt MARBRE BÂTARD GOLOBERC. FM -500 m MARNE CE FURCIl -1000 m ammonites SW VAUD NE JURA AARGAU Limit of the Jura fold and thrust belt 50 km Figure 2.1: Stratigraphie sections of the Mesozoic formations in the Swiss Jura. Vaud, Jura and Aargau correspond to three Swiss Cantons, their location is given on the small adjoining map. Modified from TrOmpy (1980). Coupes stratigraphiques des formations mèsozoïques du Jura suisse. Vaud, Jura et Aargau représentent trois cantons suisses. Modifié de Trümpy (¡980). :^ 2. Stratigraphy Schalisheim LHHfMic MhjmmrMm« untre ¡uHnmaiien MiXXM IMMhM »W>M*M Nagra, NTB 86-03 (Matter et al. 1988 Figure 2.2a): Composite geological profile through the Triassic décollement zone. This section, based on the Schafisheim drill hole data, highlights the technically disturbed stratigraphy in the Triassic layers. From Matter et al. ( 1988). Coupe géologique à travers la zone de décollement du Trias. Ce profil, basé sur les données du forage de Schafisheim, souligne les perturbations tectoniques qui affectent la succession des couches du Trias. Tiré de Matter et al. (1988). Figure 2.2b): Type of folds observed in the Triassic evaporites in the Riepel quarry from Aargau Jura (645'200 m / 253'900 m, Swiss geographical reference). Types de plis observés dans les niveaux évaporitiques du Trias de la carrière de Riepel, Jura argovien (645 '200 m / 253 '900 m, coordonnées géographiques suisses). 36 2. Stratigraphy Middle Triassiez Bgjggggi Carbonates, shallow-marine Carbonates, /¡\ I /^ I evaporites and sulfates /i\ ¦ A —I—1—1— I- -I. .I. i .i/.i.j ; • L .1. .1 . J Í Shales Evaporites, elastics and carbonates Clastics and carbonates, shallow marine Depositional edge of Middle Triassic salt ¦ i. ». i. i .¦ i ¦ » ¦ i ¦aaaasas j Continental, \ lacustrine clastics Non-deposition area, low relief area * Normal fault \ Transcurrent fault I j Study Tertiary thrust fronts Figure 2.3: Paleogeographic map of Central Europe (not restored) in Middle Triassic times ("Muschelkalk"). AM = Armorican mas- sif: BM = Bohemian massif; LBM = London-Brabant massif; MC = Massif Central, RM = Rhenish massif; VH = Vindelician High. Modified from Ziegler ( 1988, 1990). The square corresponds to the studied area. Carte palèogèographique de l'Europe centrale (non reconstruite) au Trias moyen ("Muschelkalk"). AM = Massif armoricain; BM = Massif de Bohême; LBM = Massif de Londres-Brabant; MC ¦ Massif Central. RM = Massif rhénan; VH = Seuil vindélicien. Modifiée de Ziegler (1988, ¡990). Le carré représente la région étudiée. 37 2. Stratigraphy Stratigraphie column of the Triassic beds in eastern Jura. (from Jordan, 1994). Name of the Formations 400 m '•/¿-r — -^ -' - SSS — /\ — s\ _/ íN — rrrrri^ /^ /\ /^ — • 300 m ^ / — / — / — •\ I^ /\ /V /S .-S S\ A-. .*. /s S\ /; y\ S\ /S S\ S\ S\ /\ S\ /S /^ S. S^ j*. /S . rrrrw 200 m 1^1* yy-v /">. A ^v ^s A /\ ^ ys ¿^ ^s s\ /\ /\ ùìq-n."-b <> M* ^ ."v ^ A A A ^V A A í", /-\ .^ A A TT 1¾^½¾¾¾- 100 m /\ /\ ^ /\ ^ /\ /\ l||Trrr wrrrr r\ --s /s <% / - / / " / / /. /_ -- Rhaethian Obere Bunte Mergel Gansinger Dolomit Stubensandstein Untere Bunte Mergel Schilfsandstein ______ Ob. Bunte Schichten Ob. Sulfatschichten Mittl., Sulfatarme Seh. Uni. Bunte Schichten Unt. Sulfatschichten Q. O) JE (D O) Q V a 3 O « ö GrtriidolomH E>th«ri*n(chj«t«r Lettenkohle Trigonodusdolomit Plattenkalk Haupt- muschel- Trochitenkalk kalk Anhydritdolomit Mergeldominierte Seh. Sulfatdominierte Seh. Salzlager Untere Sulfatschrchlen O a a s O) ¦o >- C < Orbicularismergel Wellenmergel Wellendolomtt > OJ oberer unterer Buntsandstein bc lì CD C ¦o CD C W O D" S.E (N UJ C f- (U u -a U ¦ P ITI Ü H lit O C O 0 C .2 0^ "S E o1 "« - Ä UJ Ç" "a T-(N- 1 ON ^ ^- § ..3_i «SI £sE 3 E g Mû« FT to -J1 1 O O Co U *— -tu ¦15 — c SJ tu $3 CQ -¾ ?! O -Si Sì 5 ^ Ë 3 Si § -I S? tl § lu ï Ç « ^ » -¾ (¡J lr¡ — ^S fe -00 55 ^ I" 15 I •S'è -5= •- §¦¦« .¾ ¦SP .^ S 'C ni 3 -tu tu t "3 a S n. W G" »3 I §1 lì 3 t!l ^ o -S ¦S- u o, u M a m -tu -J fc ¦ o tj 47 2. Stratigraphy E o £ UJOOL ipee uoisiAipqns § o ' ' ' T- C O or- IA H n r-^ ¦ ' V) OS x: *' "ó 7- o 3 (3 o -o (/] ___ Uí *Ü U O S -a «si 5 ë P CJ Oí -i= > — H ¿ u &- ¦° «i 'S ^ « o -o « .a o o. E .Sf c tí > O B O Ol ° c ~ E .° T> .— JS O « ¡g g Z *> -o ¿5 a ü .0 ^ ¦5 ^ il - G o ¦s > S 3 3 8 C a, C ,¦*¦ tu t-5 DO Si .2 SS 311 ìli sii Sil 8 ä 2 i) ¦S t> "j a c 6> -. ty "W o ^ S -Sr e ir, K <*> % i|| ty S Ç. ^ H) ° Si -2 b |-| DO - 5^ S3 g- ^ QS S - g ^ S I S E £ .E ìli S S "D ¦FI ali g 48 2. Stratigraphy o o ff C .2 o. ü oc S -^- Q. u E C P s* ¦£ O £| -"^ rjj Os D — O r K * E ?.. X : m 2 È C C O oí ûd LU UJ — c o S o « 3 ï g .5 ^ u 12 = U-U jz r -c "" U 2 C ¦=> -a (A C 5 5- O W, Uî C3 S QJ S ^ T3 E T3 £ 3 eli o > 5 B--3 « 5 2 t. CO Xi S U c ° 3 O S -V3 -~ O « O C Z ¢3 5 3t "« ¡- a I B- £ ^ Z §. 8-si §¦1 =- •tu ¡H so J C-J "S Sgl _ii „ « -ta 3¾ tü o 3 1Sl *" CQ Il Il ¡ I* s L--¦ -* . í~ 9~ O !U 'c: -¾ ¦S * ¦S- sì Í3 =s "S "^ '¦^ O Lu JU ? £ K •T\ \\\s.\'«s\\. /f/S/SJt Lithologies 53 limestone ? sandstone FFI crinoidal limestone 12D dolomite ["•1 oolitic limestone (£Tj anhydrite & clay E33 marl (vg anhydrite & salt E=I shale E3 crystalline basement ES3 Tertiary sediments Strike line 8 along the Val de Ruz syncline Figure 2.15: Correlation of seismic reflectors (line 8, vertical scale in seconds, TWT) with simplified stratigraphie column of the Val de Ruz. For oblique reflectors in Triassic Units, see discusions in §3.4. Legend for the letters A, B, C,... is explained in Table 2.1. Correlation entre les réflecteurs sismiques (¡igne S, échelle verticale en secondes, temps double) et une colonne stratigraphique sim- plifiée du Val de Ruz. Concernant les réflecteurs obliques dans les unités du Trias, voir §3.4. La signification des lettres A, B, C, ...est expliquée dans le Tableau 2.Ì. 2.4.1. Neuchâtei (Val de Ruz) seismic line Figure 2.15 illustrates the calibration of the seis- mic reflectors followed throughout the whole area. The calibration is based on stratigraphie thicknesses and lithologies known from the surface and from nearby wells. The stratigraphie column of the Val de Ruz shown in Figure 2.15 is a simplified version of that in Figure 2.10. The Tertiary Molasse is preserved within many synclines and often buried underneath a thick flu- vio-glacial Quaternary cover. The important uncon- formity between the top Cretaceous and the base of the Tertiary (A) is not easily recognized on the seis- mic section shown in Figure 2.15. The Early Cretaceous beds between reflectors A and B consist of interbedded limestones ("Pierre jaune d'Hauterive", "Marbre bâtard") and marls ("Marnes bleues d'Hauterive"). The underlying, 350 to 400 m thick, upper Malm limestones form a homogeneous, massive bed. The strong reflector B corresponds to the top of the upper Malm limestones, which are transparent on seismic profiles. The transition from pure, massive limestones to the underlying, increa- singly marly sediments of the "Argovian" (lower Malm), appears as weak reflector C. The strong reflector D is one of the most prominent marker horizons in the region and corresponds to the "Dalle 53 2. Stratigraphy Labelling of the major seismic reflectors This work Naef & Diebold (Nagra Bulletin 1990) © Top Cretaceous @ Base Tertiary (§) Top upper Malm © Top lower Malm, Argovian © within Malm © Top Dogger © Base Malm (E) Top Aalenian © within "Opalinus Ton" © Top Liassic ® Top Liassic © Top Triassic Unit 1 ® Top Triassic Unit 2 © Top Hauptmuschelkalk © Top Basement ® Top Permian @ Carboniferous Table 2.1: Labeling of major seismic reflec- tors used in this work and correlation with the NAGRA tops (Naef & Diebold, 1990). Numérotation par une lettre des réflecteurs sismiques importants et comparaison avec ¡a numérotation de la CEDRA (Naef & Diebold, 1990). nacrée" formation, the top of the Dogger series, comprising some 250 m of well layered coarse grai- ned limestones. The Aalenian black shales corres- ponding to the "Opalinus Ton" (shales), contrast strongly with the overlying Dogger limestones. Thus, strong reflector E corresponds to the top Aalenian (base Dogger), in contrast to the weaker top Liassic F reflector. Between 0.6 and 0.65 s TWT3 a series of layered reflectors is interpreted as Liassic limestones. The top of the Triassic Units is reflector G and corresponds to the transition from the early Liassic "Calcaire à gryphées" formation to the underlying anhydrite and shales. Below reflector G, a strong, continuous reflector H most probably corresponds to the top of Triassic Unit 2, which may be the "Hauptmuschelkalk" dolomite or the "Lettenkohle" formation. Below this formation, oblique reflectors, between 0.73 and 0.85 s TWT, constitute a most remarkable feature on several strike lines. Their interpretation will be discussed later. The top basement (I) is not as a strong conti- nuous reflector on this profile. Reflectors below I may represent Permo-Carboniferous; however, the deeper reflectors (below 1 s) are, almost certainly, multiples. 2.4.2. Seismic lines and well log data Figures 2.16 (Essertines) and 2.17 (Treyco- vagnes) present the correlation between the seismic line, the sonic log and the lithology. The availability of a scale in meters and in seconds for both wells allows direct correlation of the lithology log (in meters) and the seismic line (in seconds). At the Essertines well location on the seismic line, good reflectors correspond to contrasts of lithology that are also highlighted by the sonic log (Fig. 2.16). Five good reflectors are observed: 1) within the Cretaceous; 2) C, top Argovian; 3) D, top Dogger; 4) F, top Liassic and 5) H, top Triassic Unit 2. The Jurassic interval is represented by a well bed- ded sequence, whereas both Triassic units consist of discontinuous reflectors. The Treycovagnes well log is characterized by multiple duplication of the stratigraphy (Fig. 2.17) within the Aalenian and the Triassic. The Dogger and the Triassic Unit 1 intervals show the best reflectors, whereas the Cretaceous, the Malm and the Triassic Unit 2 intervals are transparent zones, due to uniform lithology. The figures of the Courtion (Fig. 2.18) and Risoux (Fig. 2.19) well logs present a correlation between the sonic log and the lithology data. These logs are useful in terms of lithology and especially seismic velocities, which were used to depth convert the lines and are presented in Appendices 3. 54 2. Stratigraphy Genève ESSERTINES Seismic line 39 near the drill hole Sonic log velocity (m/s) LlTHOLOGY O O O Cl O O o crrrrfirtrfrrfgfttrEttELgrfrrrtfttfrrr: s Dogger Ml.....¦»»"........miiliimml « Aalenian I 0.7 TO Liassic Uniti saStea « -WifFjllhi.....i'i.....i».......n " imct^»uffmniwitrt|rt|Kp 0.9 /LJUIUWW- Unit 2 km iiMiwil---0.11- >> . IHlIO^ Pernio - Carb. or crystalline - 500 - 1000 T~r Œ rr^J" 1500¾ ¡-i-:-:-: cd p,-1, i.; Ed" - 2000 ^ ¦ 2500 ' ' " '~n~l T.D. = 2936m pnletval velocity Lithologies limestone EI] anhydrite & clay oolite limestone 53 salt Z3 marl ¡7D dolomite clay E23 Tertiary sediments Figure 2.16: Correlation between the Essertines (Buchi et ai., 1965b) well lithology, the sonic log (Gorin et ai., 1993) and a nearby seismic line. Corrélation entre ie iitholog du forage d'Essertines (Buchi et al., 1965b), le log sonìque (GoRìN et ai., 1993) et un profil sismiqite voisin. 55 2. Stratigraphy TREYCOVAGNES Sonic log LlTHOLOGY Permo - Carb. SaSeS '¦ Uthologies Ei=I limestone (ZZI anhydrite & clay m oolitic limestone ES3 salt tZ2 marl CZl sandstone ^ shale [7_D dolomite F'V'jLi Permian sediments Figure 2.17: Correlation between the Trcycovagnes well Iithology (Schegg et al., 1997), the sonic log (report deposited in Lausanne at the Musée géologique du Canton de Vaud) and a nearby seismic line. Corrélation entre le litholog du forage de Trcycovagnes (SCHEGG et al., ¡997). le log Sonique (rapport déposé à Lausanne au Musée géologique du Canton de Vaud) et un profil sismique voisin. 56 ^XV Counion Basel Bern 80km COURTION Velocity Interval velocity (m/s) IO J*. O) O O O O O O O O O LlTHOLOGY , ... , . T.D. = 3083m !interval velodlyl LlthOlOgieS ESl Tertiary sediments m limestone 03 spath limestone l~*1 oolite limestone ZZ\ marl ^ clay EU] anhydrite & clay ? sandstone E3 anhydrite & salt [ZD dolomite 53 salt Figure 2.18: Correlation between the Courtion well lithology (Fischer & Luterbacher, 1963) and the sonic log Corrélation entre le litholog (Fischer & Luterbacher, 1963) et le logsonique du forage de Courtion. 2. Stratigraphy Dogger RISOUX Velocity Interval velocity (m/s) CO A Ul O) o o o o o o o o o o o o LlTHOLOGY !interval velocitvl 2000 T.D. = 1958m Lithologies Eu limestone HZ] spath limestone Tl oolite limestone \rs\ marl Figure 2.19: Correlation between the Risoux well lithology (Winnock. 1961) and the sonic log. Corrélation entre ¡e litholog du forage du Risoux (Winnock, 1961) et le log sonique. 58 2. Stratigraphy 2.4.3. Correlation across the seismic grid: "jump correlation " 2.4.3.1. Definition of "jump correlation " Seismic jump correlation methods have to be used in the interpretation of the profiles, because typi- cally good data in synclines are interrupted by poor data areas underneath anticlines. The stratigraphy of adjacent synclines could be confidently correlated by juxtaposing and matching the best reflectors bet- ween adjacent synclines. Thus the major reflectors A to I were all traced and tied to the better quality strike lines. In general thickness variations were negligible from one syncline to another, as is sup- ported by the outcrop data. Three figures (Figs. 2.20 to 2.22) illustrate seismic jump correlations. Note seismic segments, but reflectors can still be traced easily using neighboring lines. Figure 2.20 shows the West-East correlation of seismic units through the Haute Chaîne and Plateau Jura lines. Figure 2.21 shows the Southwest-Northeast correlation of seis- mic units through the Plateau Molasse and Figure 2.22 presents North-South dip comparisons from the Neuchâtel folded Jura to the Subalpine Molasse. 2.4.3.2. Tertiary unit The Tertiary unit is mainly present in the Molasse Basin and in some Jura synclines (Fig. 2.20, line 6, 4 and close to Tschugg). The onlap of Tertiary sedi- ments to the underlying Mesozoic layers (Fig. 2.22 line 45) corresponds to the basal foredeep unconfor- mity (Bally, 1989), which is best visible on lines 45 and 43. The age of the onlapping sediments becomes younger from the South to the North, as explained in §2.2.4. Unfortunately no Molasse- interval subdivision could be distinguished. 2.4.3.3. Cretaceous unit: from reflector A to B The base Tertiary or top Cretaceous layer is high- lighted by reflector A (white reflector). For more convenience, the black reflector has been chosen for the correlations. This missmatch may represent an offset of 0.04 s. The Cretaceous unit is a well laye- red sequence of marls and limestones, especially in the Molasse Basin. 2.4.3.4. Upper Malm unit: from reflector B to C The massive platform limestone of the upper Malm is typically devoid of reflectors. To the SW, the lower part of the unit is more reflective (Line 46), due to the appearance of well-bedded limestones. Reflector B, the top of the Malm, may correspond either to the Purbeckian brackish facies or to the Portlandian limestones. TWT isopachs increase considerably towards SW and S, as shown on Figure 2.22. 2.4.3.5. Lower Malm (Argovian): from reflector C to D Argovian and Rauracian units have been highligh- ted in gray in Figures 2.20 to 2.22. This sequence is bounded by two strong reflectors, C at the top and D at the base. In the Jura, the sequence is unreflective where it corresponds to marls, but is more reflective to the Southwest, where the marly facies becomes intercalated with limestones. TWT isopachs are constant on Northeast-Southwest profiles, but thin south south westward to disappear on section 45 and then 43; seismic line 45 clearly confirms this change. A strati graph i cai log (Trümpy, 1980) of the autochtonous cover of the Aiguilles Rouges unit (the adjacent unit paleogeographically) shows well deve- loped upper Malm layers and no lower Malm strata and is in agreement with the disappearance of the Argovian as seen on seismic lines. 2.4.3.6. Dogger-Aalenian-Liassic unit: from reflector D to G The strong reflector D characterized by conside- rable lateral continuity throughout the seismic grid, corresponds to the top of the Dogger series, the "Dalle nacrée" formation. The Dogger is represen- ted by two, three, or in places, four strong reflectors. The underlying transparent zone of the Aalenian contrasts with the Liassic reflective zone. This contrast is due to facies differences, between the Aalenian "Opalinus Ton" beds (shales) and the Liassic limestones. The top Liassic is a good reflec- tor only within the Molasse Basin. The time interval thickness of this unit increases towards the Southwest (Fig. 2.21) and decreases toward the South (Fig. 2.22). The southern part of the Molasse Basin corresponds to an island (Alemannic high) during the Lower Jurassic, which explains the decrease in sediment thickness. 2.4.3.7. Triassic Unit J: from reflector G to H The strength of reflector G, marking the top of the Triassic beds, is due to the contrast between the early Liassic "Calcaire à Gryphées" formation and the Triassic evaporites or Rhaetian marls. Triassic Unit 1, highlighted in gray in Figures 2.20 to 2.22, is well layered with regular and continuous reflec- tions and contrasts strongly with the underlying unit. The correlation of the base of this unit is based only on seismic interpretation. 59 2, Stratigraphy ¡S 5¾. — 5 C too î" ¿3 fc O O QJ spuooes io u| uO!S|A|pqns 'ewii Aew oui 5 o II a ta O Oi eau ra i) a, -J P 'li P. s a PH^S U Ci. 3 ^ -Si U ,W -o ira a 3 —1 s1 Si tea a, Cu U C -¾ nés 1 CJ C x ^s (U 3 P P Vl , , "i c« O ¦S" C O S- ? -a o ». S is '^ .¾ 3^ Uj 'Si a § 60 2. Stratigraphy Iwo way time, subdivision in 0.1 seconds , nasse W Tertiary Malm Dogger a V a 'Z ^ (M - 3 ç 1 rboniferou I Ï ce O SI spuosas io ui uoisiAjpqns 'Biun AeM omi 61 2. Stratigraphy w vi 11 2 .2 •S 'S t -a 8.S C O U *- ¿•S 5 -= P 4: ¦g « g.ï U u «'S 5 « a "" C .2 3 fa K E o -20 5 I 11 ¦O t w = Q y j2 ¦- .2 .2 a S 3 Ü c a> u J= J= - »J* e o. c a s t- .2P feh S Li. 'S "5 •* 2 o « u. S O O O C .2.2 U 3 ^. O O — C- « c 5 « cû Ln (a to ~ =¾ ^ •§.5 ^ V; ^ -¾) w 13 ^. s; '5 3 - 3 3 c Cl.-J 'Sí i s: ¦-ÏÏ .3 S E ¡s -2 3 ¦* I"5 -. -DJ ¦a aj Ö 9- a y c fN Ci. O" ni i>. tu -2 ? s '3 s 3 o s w .5; H 62 2. Stratigraphy 2.4.3.8. Triassìc Unit 2: from reflector H to I Reflector H defines the top of Triassic Unit 2. Tn the East, it may represent the "Hauptmuschelkalk dolomite" formation, while toward the West, it cor- responds to the "Lettenkohle" beds. This sequence shows discontinuous and oblique reflectors, espe- cially visible on lines 30, 39, 6, 119, Tschugg, 2 and 38. The origin of these reflections is not obvious and will be discussed later (Chapter 3). This unit is clearly differentiated from the Triassic Unit 1 and has led to the correlation (dashed lines on Figures 2.20 to 2.22) of the top of Triassic Unit 2. The time interval varies strongly from one line to another, depending on the structural position: thic- kening is observed in anticlines and thinning in syn- clines. 2.4.3.9. Basement: from I reflector to downward The top of the basement corresponds to the base of the last strong reflection, that contrasts with the unreflective crystalline basement below (Naef & DiEBOLD, 1990). Occasionally however, reflections are present below I. These are either multiples (line 103, 117) or Permo-Carboniferous sediments (Fig. 2.23). Identifiable Permo-Carboniferous sediments are characterized by discontinuous strong reflectors, that contrast with the unreflective crystalline basement. 2.5. ISOPACH MAPS 2.5.1. Introduction A series of isopach maps for individual Mesozoic units has been constructed for the western Molasse Basin (Figs. 2.24 to 2.29). Isopach maps are a powerful tool to depict even slight lateral thickness variations. Such changes are interesting for the ana- lysis of subsidence mechanisms as well as for Theo- logical considerations. The subsidence has been analyzed in detail (Loup, 1992a, 1992b) for the wes- tern Molasse Basin and Helvetic realm. His study was based on well logs and stratigraphie profiles which were corrected for compaction, depositional water depth and eustatic sea level changes. The maps presented here cannot compete with Loup's analysis as far as time resolution and accuracy in subsidence rates are concerned, but is complemen- tary, however. Based on a large seismic grid with continuous profiles, this analysis has the advantage of a wide spatial resolution with the potential to reveal subsidence trends, synsedimentary faults and also anomalies not penetrated by wells. Isopach maps presented herein are based on the depth conversion of the interpreted seismic lines and calibrated by wells. Only data below the datum plane (D.P.) of 500 m a.s.l. have been included in the contour maps, which do not extend to the folded and thrusted Jura. The velocities assigned to each seis- mic unit are described in Appendices 3. Contouring has been performed by computer, using the Uniras/ Unimap package. The chosen bi-linear interpolation (to a uniform grid 1 km by 1 km) between indivi- dual points (with a spacing of 1 to 2 km along seis- mic profiles) results in a conservative extrapolation in areas with no data. Major tear faults, as known from surface mapping, have been introduced as dis- continuities. Contouring is performed independently on either side of such faults. The advantage of this automatic contouring are no "observer-bias" and the capability to treat large data sets very rapidly. Disadvantages are interpolation artifacts in areas with little data coverage, e.g. close to faults or region borders, as well as crooked contour lines around isolated data points (an interpreter might have smoothed them out or else reviewed the basic data input). Blank holes appear where data density was insufficient for contouring. Actual data points are superimposed on the contours together with the tear faults. 2.5.2. Isopachs of the upper Malm unit These isopachs represent the depth converted interval from reflector B to C (see §2.4.3.4.). The isopach map of the upper Malm layer (Fig. 2.24) confirms a progressive increase in thickness from N to S. No abrupt changes are visible on this map. The thickness of the complete unit is around 350-400 m in the southern Jura, whereas in the Molasse Basin it increases from 400 m in the Northnortheast to 850 m in the Southsoutheast (Humilly). Some contour intervals are offset by tear faults, whose sense of offset is consistent with field observation of sinistral N-S tear faults and dextral WNW-ESE tear faults. 2.5.3. Isopachs of the "Argovian" unit (lower Malm) This isopach is the depth conversion of the "Argovian" seismic unit between reflector C and D (see §2.4.3.5.). The thickness ranges from 150 to 300 m and decreases gently from the Northwest to the Southeast and is close to zero near Savigny (southeastern edge) (Fig. 2.25). This decrease is well expressed also on the dip transect of Figure 2.22. No major changes across tear faults are noti- ceable. 63 2. Stratigraphy J^gg^Trr Section 46|~?^ 3a\ Permo-Carboniferous sediments Figure 2.23: Seismic section (intersection between lines 46 and 34) showing Permo-Carboniferous sediments under the Mesozoic cover. Letters A to I label the reflectors (see Tab. 2.1 ). Profil sismique (intersection entre les lignes 46 et 34) montrant des sédiments Permo-Carbonifires sous la couverture mésozoïque. Les lettres de A à I soulignent des réflecteurs importants (voir Tab. 2.1). 64 2. Stratigraphy 2.5.4. Isopachs of the Dogger unit The isopachs of the Dogger unit (including the Aalenian unit), between reflectors D and F, are shown in Figure 2.26. In the Molasse Basin, the Dogger thickness is maximum along the W-E tren- ding zone Treycovagnes - Essertines - Courtion (600 m) and decreases towards the SW (Humilly, 200 m). The same trends can also be seen on Dogger subsidence rate maps presented by Loup (1992a). Abrupt changes are observable on the Treycovagnes-Essertines transects (see also Figure 4.18). These changes may be explained by a facies change either related or not to a synsedimentary normal fault. The Treycovagnes-Yverdón area will be discussed and illustrated in Chapter 4 (Regional geology of the Yverdon area, Fig. 4.18). 2.5.5. Isopachs of the Liassic unit These isopachs represent the depth converted interval from reflectors F to G. The thickness increases from 200 m (Neuchâtel Jura) to 500 m (Essertines, Humilly) (Fig. 2.27). Changes obser- vable on the Treycovagnes-Essertines or Essavilly- Laveron transects (see Appendix 2 for well data) can be explained either by tectonic thickening of these ductile beds during the Miocene deformation (e.g. in the Laveron area), or by facies changes (Yverdon area). Isopach map of the upper Malm unit ¦¦ ABOVE 1100 1000- 1100 900- 1000 H 800- 900 700- 800 Wk 600- rao 500- 600 ¦ I 400- 500 LJ BELOW 400 185000- I7B0O0- 1 SS'IOJ - 150000- ¦| i i i i i i i i i i i i i i i i i i i i r i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i 505000 510000 515000 520000 525000 530000 535000 540000 545000 550000 555000 560000 565000 Figure 2.24: Isopach map of the upper Malm unit of the western Molasse Basin. Thickness is in meter. Black dots are shotpoints used for depth control of contour map. Note that blank areas and tight closed circles are computer-related artefacts in areas of sparse control. Numbers on the side and bottom are in meters, that relate to the national Swiss geographical coordinate system. Lakes Geneva and Neuchâtel are outlined for general reference. Carte des isopaches (en mètres) des couches du Malm supérieur du Bassin molassique occidental. Les points noirs représentent la position géographique des points de tir, utilisés pour le contrôle des profondeurs de la carte de contours. Les zones vides et les petits cercles fermés sont des artefacts, dus au programme de contourage, dans des régions où la densité des données est faible. Les coordonnées géographiques (en mètres) se réfèrent à la grille suisse. Les lacs Léman et de Neuchâtel permettent une localisation générale de la carte. 65 2. Stratigraphy lsopach map of the "Argovian" (lower Malm) unit I ABOVE 200 100- 200 0- 100 ] BELOW 0 185000- I HOOiVH 175000- 170000- 160000- 155000- 150000- 145000- I I I I I 1 I I I I I I I I I I I f I I I I I I I I I I I I I I I I I I I I I I I I 'I I M I I I I I I I I I 505000 510000 515000 520000 525000 530000 535000 540000 545000 550000 555000 560000 565000 Figure 2.25: lsopach map of the "Argovian" unit (lower Malm) of the western Molasse Basin. Thickness is in meter. For explana- tion see legend of Figure 2.24. Carte des isopaches (en mètres) des couches de V'Argovien " (Malm inférieur) du Bassin molassique occidental. Pour les explica- tions. voir la légende de la Figure 2.24. Figure 2.26 (page 67, top): lsopach map of the Dogger-Aalenian unit of the western Molasse Basin. Thickness is in meter. For explanation see legend of Figure 2.24. Carte des isopaches (en mètres) des couches du Dogger-Aalénien du Bassin molassique occidental. Pour les explications, voir la légende de la Figure 2.24. Figure 2.27 (page 67, bottom): lsopach map of the Liassic unit of the western Molasse Basin. Thickness is in meter. For explana- tion see legend of Figure 2.24. Carte des isopaches (en mètres) des couches du Lias du Bassin molassique occidental. Pour les explications, voir la légende de la Figure 2.24. 66 2. Stratigraphy 2.5.6. isopacks oftke Triassic Unit 1 These isopachs represent the depth converted interval from reflector G to H (see §2.4.3.7.). The thickness of Triassic Unit 1 ranges from 400 m to 200 m, decreasing progressively toward the ESE (Fig. 2.28). The weil thicknesses do not match with this map, because of the new correlation based on our seismic interpretation (for more details see §2.4.3.7. and §2.4.3.8.)- 2.5.7. Isopacks oftke Triassic Unit 2 These isopachs consist of the depth converted interval from reflector H to I (see §2.4.3.8). The thickness varies considerably and ranges from 300 m to 1200 m (Fig. 2.29). In addition to a gene- ral trend of thickening towards the NW. this isopach map shows a conspicuous series of lows and highs, which alternate on the 10 km scale with a weak NE- SW trend, parallel to the Jura folds further to the North. A hand contoured version of this map is pre- sented in Chapter 3 (Fig. 3.6). The highs are inter- preted in terms of stacks of evaporites, clays and salt and will be discussed in detail in Chapter 3. 2.5.8. Summary Thicknesses of individual stratigraphie intervals have been mapped from the interpretation of seis- mic lines and allow assessment of some gross regio- nal trends in the Mesozoic subsidence pattern. During the Triassic, more than 1000 meters of tectonically thickened sediments were accumula- ted in the area of the future Jura arc, whereas some 60 km further south, in the North Helvetic domain, less than 50 m of sediments were deposited in the same time span. Despite this considerable lateral variation, no evidence for synsedimentary faults has been detected. Important lateral thickness variations in the Triassic on the scale of about 10 km are rela- ted to Miocene deformation and material redistribu- tion within this weak layer, which served as the major décollement horizon for Jura tectonics. During Liassic and Jurassic times, lateral varia- tions in subsidence were significant, but less impor- tant than during the Triassic. Depocenters shifted in an irregular manner over the study area, but no per- sistent trends were found. Some small abrupt thick- ness changes in the Yverdon region may be the result of synsedimentary faults, but direct evidence is obscured by Miocene tear faults. Throughout the Mesozoic, the studied area was a slowly subsiding region intermediate between the Paris basin to the NW and the Alpine Tethys to the South. According to Loup (1992b), the subsidence was polyphase and cannot easily be described in terms of a single Mesozoic event. Modeling the sub- sidence curves yields stretching factors for Triassic, Liassic and Late Jurassic rifting events on the order of ] .05 to 1.20. Subsidence ceased during the upper Cretaceous. Figure 2.28 (page 69, top): Isopach map of the Triassic Unit I of the western Molasse Basin. Thickness is in meter. For explanation see legend of Figure 2.24. Carte des isopaches (en mètres) des couches de !'Unité 1 du Trias du Bassin molassique occidental. Pour les explications, voir la légende de la Figure 2.24. Figure 2.29 (page 69, bottom): Isopach map of the Triassic Unit 2 of the western Molasse Basin. Thickness is in meter. For expla- nation see legend of Figure 2.24. Carte des isopaches (en mètres) des couches de ¡'Unité 2 du Trias du Bassin molassique occidental. Pour les explications, voir la légende de la Figure 2.24. 68 2. Stratigraphy BOO 600 500 400 300 200 r:i lsopach map of the Triassic Unit 1 I i i i i I i i i i I i i i i I i i i i I i i i i I i i i i I i i i i Li i i i I i i i i.l i i i i I i i i i I, 165000- i i i i I i i i i I I i I I I i I I i I I i i i I i i i i I- i i i i I i i i i I I I I I I I I I I I i i i i I 505000 510000 515000 520000 525000 530000 535000 540000 545000 550000 555000 560000 565000 .28 1300 1300 1200 1100 1000 900 800 700 600 ',or, 400 lsopach map of the Triassic Unit 2 1111111111111111111111111.111111 ¦' » 11 185000- I i i i i I i i i i I i i i i I ' ' ' ' I ' ' ' M I ' ' > I ' ' ' ' I ' ' ' ' I.....I i i i i i i 505000 510000 515000 520000 525000 530000 535000 540000 545000 550000 555000 560000 565000 .29 69 2. Stratigraphy 2.6. RHEOLOGICAL STRATIGRAPHY 2.6.1. Generai comments The stratigraphie thickness and lateral continuity of competent and incompetent formations within the stratigraphie column plays a key role in the tectonic evolution of foreland fold and thrust belts. The rheology of individual layers is primarily dependent on the mineralogical composition of the rock, the texture and the deformation temperature. In the case of a sedimentary cover consisting of alternating competent and incompetent beds, the relative thick- ness of these layers also plays an important role in the style of deformation (Ramsay, 1967). Variations in any of these parameters across a foreland area will strongly influence the tectonic and structural style. Given a maximum total thickness of less than 1.5 km of Mesozoic and Cenozoic cover rocks in the northern Jura, increasing to 2 km in the central Jura and to more than 3 km at the southern rim of the Molasse Basin, maximum temperatures at the base of the cover series can be estimated to be bet- ween 6O0C and 13O0C respectively (Jordan, 1992). Under these very low temperature conditions, the relative competence of the sediments present in the stratigraphie column increases by orders of magni- tude in the following estimated order (from weakest to strongest): salt, gypsum, anhydrite, shale, marl, Molasse sandstone, limestone, dolomite (Nüesch, 1991; Twiss & Moores, 1992; Jordan, 1994). Although the entire area lies in the low temperature field, a slight southward temperature increase is important insofar, because the rheology of gypsum and anhydrite are very sensitive in this temperature range (Olgaard & Dell'Angelo, 1991). 2.6.2. ¡theological behavior of rocks from the Jura The following rheological behavior of individual layers in the Jura can be expected and has been confirmed by detailed structural observations from outcrops and experiments: 2.6.2.1.SaIt Laboratory data (Carter & Hansen, 1983) indi- cate that pure rock salt is by far the most ductile lithology at low temperature. For reasonable geolo- gical strain rates, salt will deform plastically by dis- location - glide and - climb, as well as pressure solu- tion mechanisms. This behavior has been confirmed by observation in samples of the Schafisheim drill cores (Matter et ai., 1988; Jordan & Nüesch, 1989a). Experimental work (Carter & Hansen, 1983) on halite, with a similar grain size to thé Jura halite, yields very low shear strengths, especially in the presence of pore fluids. In the Jura, pure salt thicknesses are generally thin and vary between 20 m and 300 m in the Laveron drill hole (Figs. 2.11 to 2.13; see also Jordan, 1992). In terms of rheology, salt behavior will be the same for a 10 m or a 100 m thick salt layer. There seems to be little or no justification for trying to establish some critical minimum thickness of salt (Jenyon, 1986), The difference will reside in the shape and the size of the features resulting from thinning or thickening. Thus a very thin salt layer will probably act as a lubricant between floor and roof sequences, once thrust or décollement move- ment has been initiated. 2.6.2.2. Gypsum /Anhydrite At the depth of the main Jura décollement, gyp- sum is expected to be transformed into anhydrite (Heard & Rubey, 1966). This transition is impor- tant for the rheological behavior, since anhydrite is somewhat stronger than gypsum. The transforma- tion of gypsum to anhydrite involves dewatering, a reaction which in turn, could have weakened the associated clay rich rocks by creating higher than hydrostatic fluid pressures. Outcrops of Triassic gypsum/anhydrite series show compelling evidence for the ductile behavior of anhydrite layers during Jura deformation (Fig. 2.2b). Deformation within the anhydrite shear zones from the eastern Jura has been studied by Jordan (1994). Even at moderate temperatures and shallow depths, anhydrite is in an intracrystalline glide deformation regime in combination with grain boundary migration. Fluids are responsible for the early onset of grain boundary migration and there- fore for the weakening of the anhydrite (fluid enhan- ced flow). Highly deformed anhydrite tectonites show evidence for diffusion and grain boundary gli- ding processes (Olgaard & Dell Angelo, 1991). Fluids seem to have been omnipresent in the Triassic evaporites during the deformation of the Jura belt. This appears to be reasonably correlated with the experimental results of Heard & Rubey (1966) on the dehydration of the gypsum transfor- med to anhydrite and water and other experiments 70 2. Stratigraphy on clays (Jordan & Nüesch, 1989b). Dehydration increases fluid pressure and decreases the rock strength. During an initial stage, strain was mainly accom- modated by ductile flow of anhydrite, while at a later subsurface stage, main shear deformation changed into new-grown gypsum and finally into fault breccias. Comparable behavior has been obser- ved in Triassic imbrications outcropping in the nor- theastern Jura (Jordan et ai, 1990). 2.6.2.3. Shales / shaly Marls It is commonly accepted, that shales are efficient décollement horizons. Their competence, however is strongly dependent on porosity, fluid pressure and anisotropy. Dry, compacted shales, at low tempera- ture. may be almost as strong as limestone, whereas wet, overpressured shales can be weaker than evapo- rites. Experimental results confirm the existence of a regime of low-strength pervasive cataclastic-type deformation in shales not known in other rock types (Jordan & Nüesch, 1989b). Such a deformation regime is explained by the ability of clay minerals to adsorb and to retain water. This interlayer water decreases the strength of clay and hence that of shales significantly. Observations on the naturally deformed shale layers of the eastern Jura show that deformation is generally friction controlled. Shales interbedded with evaporites on the other hand show plastic deformation behavior, interpreted as controlled by sulfate lubrication (Jordan & Nüesch, 1989b). Therefore, in the Jura, deformation of the Opalinus shales (Aalenian layers of the eastern Jura, Fig. 2.30) is predominantly brittle, due to the lack of lubricant, whereas in the Triassic evaporites the deformation is dominantly plastic. In the Jura, shale interlayers within the evaporites result in two types of deformation, depending on burial depth. The domains of deformation are sepa- rated by the inversion of relative competence of shale and anhydrite and the onset of gypsification (Jordan & Nüesch, 1989b). In deeper domains, strain is focused on slickensides, which are often coated with sulfate that acts as a lubricant. Anhydrite is ductily deformed and shale interlayers are boudinaged. In shallower domains, and espe- cially in domains with significant exposure to water, strain becomes pervasive and very low strength duc- tile shale-gypsum tectonites are formed. The defor- mation is characterized by boudinaged brittle anhy- drite layers within highly ductile shale-gypsum cata- clasites. Such tectonites are restricted to areas of little overburden and to domains of significant water content. 2.6.2.4. Limestones At low temperature, dry limestones are very strong compared to the above mentioned litholo- gies, because intracrystalline deformation mecha- nisms are not operative below temperatures of about 200 0C (Rutter, 1974). From this, an entirely brittle behavior would be expected for a limestone succession. In the presence of water, however, pres- sure solution and recrystallization play an important role (Gratier, 1984). This mechanism impairs some apparent macroscopic plasticity to the rock, although in terms of rheological behavior, limes- tones at low temperature must be regarded rather as brittle than ductile. 2.6.3. Rheological profiles for the central and eastern Jura The more theoretical considerations discussed above, together with outcrop observations and data from seismic lines, allow construction of a compo- site rheological stratigraphie profile (Fig. 2.30). The Jurassic and Cretaceous stratigraphie columns from Vaud, Jura and Aargau are taken from Trümpy (1980). The Neuchâtel stratigraphie log and the Triassic log are compiled from the literature and from subsurface data respectively. In the four sec- tions, three degrees of competence have been distin- guished : 1) strong (limestone dominated); 2) weak (shale and marl dominated); 3) very weak (evaporite dominated). The main incompetent formations are the Lower Cretaceous shales ("Marnes bleues d'Hauterive" in the Neuchâtel Jura), the "Purbeckian" formation at the transition of Malm to Cretaceous strata, the Lower Malm marls ("Argovian" formation in the northwestern part of the Swiss Jura), the Aalenian shales ("Opalinuston") and the evaporitic levels of the Triassic Unit 1 and Unit 2. Based on interpreta- tion of seismic lines and on well data, Triassic Unit 2 appears as the most ductile zone or weakest zone of the whole stratigraphie column in the central Jura and the western Swiss Molasse Basin. Field obser- vations (eastern Jura) and core sample observations 71 2. Stratigraphy S-So'''; 72 2. Stratigraphy show several shear zones and ductile deformation in the Triassic evaporites (Weber et ai, 1986; Jordan & Nüesch, 1989a; Jordan et al., 1990; Hauber, 1993; Jordan, 1994). The mineralogical composi- tion of this unit consists of anhydrite, gypsum, salt, clay, calcite and dolomite. Salt has been recognized in all wells, but pure rock salt thicknesses are diffi- cult to establish. There were variations of Theologi- cal behavior of the individual layers during com- pressional deformation, ranging from low strength viscous flow within pure halite and anhydrite layers, to rigid behavior or high-strength cataclastic flow in pure shales, marls and carbonates (Jordan & Nüesch, 1989a). The shear zones tend to be locali- zed in pure halite and anhydrite layers. The rigid layers are the upper Malm limestones (especially in the West) and the Dogger bioclastic limestones. The net thickness of these layers deter- mines the wavelength of the first order folds. The ratio between competent and incompetent layers in the Mesozoic cover decreases from SW toward NE i.e. from 2:1 in the southern Jura to 1:2 in the eastern Jura, passing through 1:1 in the central Jura. The total cover thickness has a major influence on the structu- ral relief which also decreases from SW to NE. Major and minor potential décollement levels are indicated in Figure 2.30 with thick and thin black lines respectively. Minor décollement horizons include Triassic Unit 1 evaporites, Aalenian black shales, Argovian maris and lower Cretaceous marls. Deformation and structures resulting from minor décollement along these levels is discussed in §3.2.3.4. Among these, the most important in the Neuchâtel and Vaud Jura is the lower Cretaceous horizon. A strong decoupling level within the Cretaceous is observed in many outcrops (Droxler, 1978; Martin et ai, 1991), where a strong dishar- mony in folding style is observed between the large folds of the Malm and small scale folds in the Cretaceous. The latter are too small to be resolved on seismic lines. The same may be true for the slightly disharmonie folding observed within the Argovian (Pfiffner, 1990) (see Figure 3.17). However, only one case has been found on the exa- mined seismic lines (Section 71, Fig. 3.2), where the Argovian acts as a significant décollement level for the overlying fold. The major décollement zone is located in the Triassic Unit 2. This appears clearly from the inter- pretation of seismic lines. This zone is bound both by a roof décollement and a basal décollement and the style of deformation within this zone is very dif- ferent from the underlying rigid basement and from the overlying Triassic Unit 1 and especially the Jurassic layers. Harrison & Bally (1988), in the salt-based Melville Island fold and thrust belt, pre- ferred to define the whole interval as a ductile zone, (although the processes are not truly plastic at microscopic scale), instead of a décollement zone, because the term décollement refers to a single weak plane. In this work we will interchangeably use ductile zone or décollement zone, keeping in mind that this décollement is represented by a thick zone and not by one single weak plane. The enve- lope of the ductile zone corresponds to the décolle- ment level. Previously Schardt (1908) expressed this very same idea: "... Il serait plus juste de parler d'une zone de glissement. Il est en effet peu probable que la poussée venant des Alpes ait produit un glis- sement sur un plan déterminé; mais ce sont certai- nement les couches marneuses dans leur ensemble qui ont servi de lits mobiles en se déformant dans toute leur masse. ...". The presence of a weak continuous basal décolle- ment horizon or zone is a primary condition for the development of any foreland fold and thrust belt. In the case of the Jura arc, the original paleogeographi- cal extent of Triassic evaporite series (Fig. 2.5), with salt, gypsum and/or anhydrite and clays, seems to be responsible for the shape of the external and lateral limits of the fold thrust belt. These outer limits are interpreted mainly as due to the disappearance of a suitable basal décollement horizon, i.e. the Triassic evaporites. Note that in Bavaria (Germany), where no Triassic evaporites exist, there is no lateral equi- valent to the Jura and the Alpine deformation front remains tens of kilometers further back at the sou- thern border of the Molasse Basin (Bachmann et al, 1987;Lemcke, 1988; Burkhard, 1990). 73 3. STRUCTURES: OVERVIEW, EXAMPLES AND INTERPRETATION 3.1. INTRODUCTION The Jura arc on a large scale shows all characte- ristic features of a foreland fold and thrust belt developed above a weak basal décollement (Davis & Engelder, 1985; Rodgers, 1990; Laubscher, 1992; Twiss & Moores, 1992). These features include the arcuate outward convex shape, folds, thrusts and tear faults, that are all kinematically compatible with a tectonic transport in a general NW direction. Seismic lines have allowed lateral correlation of the well known surface structures from the Haute Chaîne Jura to the less well known adjacent areas of the Molasse Basin hinterland and the Plateau Jura foreland (Fig. 1.2). Different types of folding styles have thus been identified: the Molasse Basin and the external Plateau Jura present broad, long wavelength, low amplitude folds cored by Triassic evaporites; by contrast, the Haute Chaîne Jura is characterized by high amplitude folds which formed above thrust faults stepping up from the basal Triassic décolle- ment. Despite the fact that seismic lines across such folds are of mediocre quality as compared to Melville Island (Harrison, 1995), they provide important geometric constraints which are most help- ful in the construction of viable kinematics model. The characteristics and particularities of the Jura fold thrust belt and its connection with the Molasse Basin are the result of a series of boundary condi- tions (see also Chapter 1); the most important are summarized below : 1) presence and thickness variations of a suitable basal décollement zone laid down in form of evapo- rites and shales during the Triassic (compare Fig. 2.5) 2) the rheological stratigraphy of the Mesozoic car- bonate cover with alternating competent limestones and incompetent marl series (Fig. 2.30) 3) the overall wedge shape of the Mesozoic and Cenozoic cover in the Alpine foreland - a result mainly of the Oligocene collision which produced a pronounced foreland basin filled with the clastic Tertiary Molasse wedge. In this Chapter, a brief theoretical introduction to folds will be given prior to presentation of the actual structures observed. The seismic expression of com- pressional structures developed in the Jura fold and thrust belt and in the Molasse Basin during the Miocene are discussed. The most prominent struc- tures are high amplitude, thrust-related folds of the Haute Chaîne Jura as well as low amplitude, broad buckle folds developed in the Plateau Jura and the Molasse Basin. Folds and thrusts are intimately lin- ked with the concomitant formation of tear faults. Seismic examples of these structures are compared with field observations on various scales. The late- ral continuity of the structures described below will be discussed further in a regional context in the next Chapter 4. 3.2. FOLDS AND THRUSTS 3.2.1. Geometry and mechanisms: definitions Apart from thrust faults, folds are the most promi- nent structures developed in the compressional tec- tonic regime. Folds are ubiquitous from the grain scale (e.g. kinked mica flakes) to kilometers (folded sedimentary series), to the scale of some hundred kilometers (lithosphère flexure). Reflecting this wide range, there is an overwhelming amount of literature which discusses the geometry, kinematics and mechanisms of folds and folding e.g. Ramsay (1967), Suppe (1985), Twiss & Moores (1992), Johnson & Fletcher (1994) and many others. Despite the apparent similarities of folds developed at various scales and in widely different materials, there seems to be no common classification bet- ween folds developed plastically, without failure (e.g. in high temperature deformed terranes) and folds developed brittly and often related to faults (e.g. low temperature terranes). Therefore, before describing the Jura and Molasse Basin folds, a sum- mary of definitions and concepts related with folds and folding is given. 75 3. Structures Jura fold belt I____) Tertiary I Jurassic & Cretaceous ¦ Triassic PreaJps Helvetic nappes External cristalline massifs nottnicale Figure 3.1: Schematic cross-section and bloc-diagram crossing the Jura fold and thrust belt from the foreland to the hinterland. The 3 styles of folds are represented: 1) evaporite-related anticline from the Plateau Jura, 2) thrust- related anticline from the Haute Chaîne Jura, 3) evaporites-related pillow from the Molasse Basin. From Sommaruga ( 1995). Bloc diagramme schématique recoupant la chaîne plissée du Jura depuis l'avant-pays aux parties internes. Trois styles de plis sont représentés: J) dans les Plateaux jurassiens, un anticlinal associé à un empilement d'évaporites, 2) dans la Haute Chaîne juras- sienne, un anticlinal associé à une rampe de chevauchement, 3) dans le Bassin molassique, un coussin d'évaporites. Tiré de Sommaruga (¡995). S. 2.1.1. Geometric classifications of folds Different geometric classifications of folds have been proposed. The most popular fold classification scheme was introduced by Ramsay (1967) and is based on the comparison of the two surfaces of one layer using any of the following parameters : ortho- gonal thickness, thickness parallel to the axial plane and the angle of dip isogons with respect to the axial plane. An alternative method by Twiss & Moores (1992, p.229) describes the style of a fold using its aspect ratio (ratio between the amplitude of a fold and the distance measured between the adjacent inflection points), tightness (interlimb angle) and bluntness (curvature of the fold). The latter and other purely mathematical descriptions (Fourier transform series) have not gained much attention among structural geologists. Although these classi- fications are purely descriptive and not genetic, they are clearly designed for the description of conti- nuous smoothly folded layers i.e. buckle folds and do not include in their description any discontinui- ties such as associated thrust faults. Fault-related folds on the other hand are classified in a totally different, genetic classification scheme. Three end members of fault-related folds, which result in distinct fold-thrust (ramp) interactions, are generally agreed (Jamison, 1987; Mitra, 1992) (Fig. 3.2): - fault-bend folds (Fig. 3.2a): folds generated in the hangingwall rocks by movement of a thrust sheet over a ramp (Rich, 1934; Suppe, 1983). The décolle- ment ramps from a lower structural level over a higher stratigraphie level. The fold develops as a result of the underlying flat-ramp geometry. Fault 76 3. Structures bend folds were described first by Rich, in the Pine Mountain thrust region of the Appalachians. He recognized that this fold style developed broad flat- topped symmetric anticlines. - fault-propagation folds (Fig. 3.2b): asymmetric folds, with one steep or overturned frontal limb associated with a thrust fault. These are generated at the tip of contemporaneously developing thrusts, which propagate into undeformed strata (Suppe, 1985). As long as the structure has not been faulted through (breakthrough), fault slip is consumed by folding of the overlying strata. - detachment folds (Fig. 3.2c, d): symmetric or asymmetric folds developed above the termination of a detachment or a bedding parallel thrust fault. The folding does not require a ramp and thus the detachment folds are not associated with ramp thrusts (Jamison, 1987; Mitra, 1992). Lift-off folds, chevron type (Fig. 3.2e) or box type folds (Fig. 3.2f) are a particular expression of fault-propagation folds and detachment folds. The beds and the detachment are isoclinally folded in the core of the anticline (Mitra &Namson, 1989). The important parameters used for the geometric description of these fault-related folds are the angu- lar relationships between the forelimb, the ramp and the backlimb dip. The emphasis in this classifica- tion scheme lies on the description of the geometry of the thrust fault and the overall shape of the fold above. The consideration of a temporal relationship bet- ween folding and faulting is implicit in the descrip- tion of these folds. Whereas the fault-bend folds develop subsequent to ramp formation, fault-propa- gation folds and detachment folds develop simulta- neously with the ramp or the décollement propaga- tion, respectively. Detachment folds like fault-pro- pagation folds develop at the termination of a thrust fault. In addition to the three basic end member fault- related fold types, a virtually endless and somewhat confusing terminology has been introduced for the geometric description of networks of thrust faults. The reader is referred to Boyer & Elliot (1982) and the glossary by McClay ( 1992). 3.2.1.2. Mechanisms and kinematics of folding Folds are the result of compression in a layered material where compression is applied in a direction subparallel to the anisotropy i.e. along the length of the layers. Two fundamentally different mechanisms lead thereby to the formation of folds : 1) Buckling 2) Fault-related folding Buckling and buckle folds Buckling results from the application of compres- sive stresses in layered materials with contrasting viscosities (rheologies), where both the strong and the weak materials are plastically deformed without failure. Above a certain threshold of compressive stress, the stiffest layers become unstable and buckle into a fold. Buckling is the dominant folding mecha- nism at relatively higher temperatures and confining pressures, where deformation is essentially plastic (flowing) and pervasive, although strongly partitio- ned into the weaker layers. Buckling is the most dis- cussed mechanism in the literature and was first proposed by Hall (1815) and later treated in many text books e.g. Biot (1957), Ramberg (1964), Ramsay (1967), Johnson & Fletcher (1994). Buckle folds (Fig. 3.2g) may be developed in a single layer or in a multilayer stack. Folding of a single layer means that the layer is embedded in vis- cous media. The layer has two interfaces and if the two interfaces deflect in the same direction the resulting structure is called a buckle fold. The deve- lopment of buckle folds in single-or multi-layers has been studied by mathematical and analog models (Johnson & Fletcher, 1994). The most important parameters which control the development of buckle folds are: the viscosity contrast of the materials, the layer thicknesses, as well as the cohesion between layers. The main results can be summarized as fol- lows: - competent (stiff) layers are less deformed inter- nally than the incompetent matrix - in stiff layers, deformation is concentrated in fold hinges - limbs of stifflayers show little internal deformation - hinges are formed early and remain fixed in the material - wavelength is determined by the viscosity contrast and the thickness of the stiff layer in a multilayer: - strong layers influence each other 77 3. Structures Fault-bend fold À y/////////////////////////, Fault-propagation fold Lift-off folds Fold first >¦ Detachment fold Fault-related folding V////////////////////////////////Ä Y////////////////////////, Buckling Buckle fold Figure 3.2: Geometry of folds (see text for discussion). Classification of fault-related folds: a) fault-bend fold (Rich, 1934; Suppe, 1983); b) fault-propagation fold (Suppe, 1985: Suppe & Medwedeff, 1990; Mosar & Suppe, 1992); c) detachment foíd from Mitra (1992); d) detachment fold from Jamison (1987); e) chevron type of lift-off fold (Mitra & Namson, 1989); f) box fold type of lift- off fold. Classification of buckle fold: g) buckle fold with a single layer. Géométrie des plis (voir texte pour discussion). Classification des plis associés à une faille: a) fault- bend fold (Rieti, ]934; Suppe, I9S3); h) fault-propagation fold (Suppe, 1985; Suppe & Medwedeff, 1990; Mosar & Suppe, 1992); c) detach metti'fold de Mitra (1992); d) detachment fold de Jamison (1987); e) type chevron du lift-off fold (Mitra & Namson, ¡989); J) type copè du lift-off fold. Classification des plis de flambage: g) pli deflamblage à une seule couche. 78 3. Structures - the most resistant (viscosity and thickness) layer dictates the dominant wavelength Multilayer sequences contain layers of widely dif- ferent strength and thickness. According to experi- ments, the form of the folds depends upon several parameters: the relative stiffness of the multilayer and its confinement; the relative thickness and stiff- ness of adjacent layers within a multilayer; the pro- perties of the contacts between layers; the degree of cohesion between layers. Experimental measure- ments (Johnson & Berger, 1989) in sedimentary rocks shows that if there are too few ductile layers in a multilayer stack, the stack will fail by faulting before it buckles into folds. Fault-related folding and fault-related folds The second mechanism of fault-related folding requires a fault to be active prior to and/or during fold formation. This mechanism does not require any viscosity contrast, nor does anisotropy have to be present in the material other than that of a locali- zed thrust fault. This mechanism develops in the low temperature regime, where deformation is pre- dominantly brittle. Folds related to a ramp active prior to folding (Fault-bend folds) were first described by Rich (1934). He proposed that the thrust surface followed some zone of easy gliding, such as shale until fric- tional resistance became too great and then sheared diagonally up across bedding forming a ramp to another shale, then following the shale for some dis- tance, to shear across the bedding at another ramp to the ground surface. This mechanism (ramp folding) was later discussed more precisely by Wiltschko (1979) and by Johnson & Berger (1989). A ramp fold is the result of duplication of strata at the ramp fault and along the detachment surface beyond the ramp fault. Folds are consequently formed passively by translation of a thrust sheet over a ramp. In this model, the thrust is clearly implied to develop first and the fold is a product of passive accommodation. The model has the advantage of being easy to ana- lyze in a rigorous geometrical way. A problem with this model is the location of the ramp. Another model is represented by folds which develop simultaneously with the ramp portion of a stepped thrust fault (fault-propagation folds). The mechanisms of those folds are not so well unders- tood as yet. Fault-related folds are developed in a multilayer sequence assuming bedding plane slip between the layers. Fault-bend and fault-propagation folds are formed over a discrete sole thrust and represent a folded multilayer sequence with a very low visco- sity contrast between layers. Detachment folds require a weak décollement layer (e.g. salt or shale layer), which can infill the space generated at the base of the fold. The latter have all the characteris- tics of a buckle folds. The nucieation of the faults is an interesting ques- tion. Models proposed by Dixon & Liu (1992) based on centrifuge modeling, suggests that, in a stratigraphie sequence with high contrast of visco- sity between layers, the thrust ramps are localized solely by earlier stage of low amplitude folds. Early buckling would be responsible for the localization of the ramps. Criteria to distinguish buckle and fault related folds ? The most obvious criterion for the distinction of the two folding mechanisms is the identification of a thrust fault which can genetically be related to the fold formation. In the case of folds observed in fore- land fold and thrust belts, this usually requires a detailed knowledge of the subsurface geometry, which may often not be available. The fold profile may provide some information about the relative competency of layers. However, there is no information regarding the underlying dominant folding mechanism (buckling vs. fault- related folding). A detailed knowledge of the internal deformation along the entire fold profile, in individual key layers, may provide other critical information about the folding mechanism: deformation can be expec- ted to be concentrated within fold hinges of compe- tent layers of buckle folds, whereas intricate but pre- dictable patterns of high and low strain zones (pos- sibly several superposed incremental strain "events") may be expected throughout in fault-rela- ted fold models, where material has to move through certain axial planes. Transitions between buckle folds and fault-related folds Transitions from buckling to fault-related folding mechanisms are commonplace and accordingly, there is no clear cut limit between the two end mem- 79 3. Structures ber mechanisms of folding. Folding may start by buckling of a competent layer to a certain amount of shortening, before deformation is localized into thrust faults (Dixon & Liu, 1992). Alternatively, a sedimentary wedge may start to deforme by thrust faulting and then be buckled at later stages. Moreover transitions between the two models of fault-related folding (fault-propagation folds to fault-bend folds) are also possible. 3.2.13. Kinematic sequences associated to salt flow The presence of weak layer in a multilayer sequence has a strong influence on the mechanisms of folding. Rock salt, one of the weakest material known in sedimentary sequences, is responsible for a particular set of deformation structures. Salt tectonics (syn. halotectonics) refers to any tectonic deformation involving salt or other evapo- rites as a substratum or a source layer (Jackson & Talbot, 1994). Halokinesis, on the other hand, desi- gnates the formation of salt structures which are the result of salt flow under the influence of gravity alone, without any significant lateral tectonic forces (Trusheim, 1957, 1960). ¦ , In the studied area, gravity forces are not unique and probably not the most important. A major late- ral push of the Alps towards the NW appears to be responsible for the formation of the Jura foreland fold and thrust belt. This stress is well known under the German term "Fernschub" (= distant push) defi- ned by Laubscher (1961). In sedimentary environments, the continuous deposition of layers during salt movements may record the timing and the character of the salt flow. Three kinematic sequences have been distinguished (Fig. 3.3). The prekinematic sequence is deposited before the salt starts to flow; the synkinematic sequence is deposited during the salt flow and shows internal onlaps or truncations; the postkine- matic sequence is deposited after the salt stopped flowing. The recognition of these sequences in a mountain belt, may give many information on the timing of deformation. In the field, evidence may also be furnished by thickness changes and trunca- tions or onlaps. High quality seismic lines may be required to reveal all the subtleties of such salt structures and their relation with the surrounding rocks. 3.2.2. Evaporite-relatedfolds (low amplitude) 3.2.2.1. General comments Low amplitude folds may be difficult to recognize on geological maps or in the field. The low limb dip and the low structural relief make these structures inconspicuous and difficult to observe at outcrops. Seismic lines are more useful in documenting the geometry of this fold type at depth. Interpretations of seismic lines across the Plateau Jura and the Molasse Basin, show a series of broad and gentle anticlines which are controlled by evapo- rite, salt and clay stacks within the ductile Unit 2 of the Triassic layers. In the scientific literature, this type of anticline is termed salt anticline or salt welt (Harrison & Bally, 1988) also defined by Jackson & Talbot (1994) as "an elongated upwelling of salt with concordant overburden". The term salt pillow has the same meaning, but is used for subcircular shapes (Fig. 3.4). In this work, we prefer to use the terms evaporite anticline and evaporile pillow, due to the uncertainty about the amount of pure salt in the Triassic layers. Conventional salt pillows, as first visualized by Trusheim (I960), are today often interpreted in an overall extensional context (Vendeville & Jackson, 1992). In a congressional context, ideally, the evaporites pinch out in the adja- cent synclines and flow into anticlinal evaporite ridges (Harrison, 1995). It is important not to confuse the term salt welt with the term salt weld, which describes a surface or zone of adjacent strata originally separated by autochthonous or allochthonous salt (Jackson & Talbot, 1989). Compressional salt welds occur when all ductile material has migrated from the syn- cline to the core of the anticline. 3.2.2.2. Geophysical evidence from seismic profiles Geophysical evidence for evaporite stacks include thickness variations of a seismic unit, which are spa- tially associated with broad folds in the overlying formations and velocity anomalies. The latter gene- rally consist of a positive deflection of the reflectors (velocity pull-up) beneath anticlines, caused by the thickening of the Triassic evaporite unit of suppo- sedly high velocity. This velocity pull-up is further enhanced by a velocity pull-down in the synclines, 80 3. Structures @ Prekinematic © umnrr> 2 @ Synkinematic 3 @ Postkinematic ;r ï < T i ï í i i * f * T * Figure 3.3: Sedimentary record of salt flow during shortening. Three kinematic sequences are inferred from the relation salt flow and sedimentary record. 1) Prekinematic sequence; 2) Synkinematic sequence; 3) Postkinematic sequence. Inspired from Jackson & Talbot (1994). Enregistrement syn-sédimentaire dußuage du sei durant un raccourcissement. Trois séquences cinématiques sont déduites des rela- tions entre ¡eßuage du sel et l'enregistrement sédimentaire. 1) Ante- cinématique; 2) Syn-cinémalique; 3) Post-cinématique. Inspiré de Jackson & Talbot (¡994). AO* Ideally salt is evacuated *Ä oiti NW SE IUI Triassic Unit 2 Figure 3.5: Northern part of the dip seismic Section 111 located in the Plateau Jura (external zone). The interpretation displays a broad anticline related to thickening of the Triassic Unit 2. Laveron drill hole (projection), which reaches the top of the Buntsandstein strata, confirms the seismic interpretation. Legend for the top of the layers: D = Dogger; G = Triassic Unit 1,H = Triassic Unit 2. Partie septentrionale du profil sismique Ili transversal localisé dans les Plateaux jurassiens (zone externe). L'interprétation montre un large anticlinal associé à un épaississement dans l'Unité 2 du Trias. Le forage de Laveron (projeté de 8.5 km) atteint le toit des couches du Buntsandstein et confirme l'interprétation sismique. Légende pour le toit des couches: D = Dogger; G = Unité 1 du Trias, H = Unité 2 du Trias. S3 3. Structures 190000- 185000- 160000¦ 175000- 170000 - 165000- 160000- 155000- 150000- 145000- 140000- 1111........ilii fé anticlines fé syncli tear fault !transparent zone thrust thickness in meters * t i i i » i i i i i i i i i » i i i i » i i » i i i i i i t i i i i i i i i i i i meters i i i i i i i 505000 510000 515000 520000 525000 530000 535000 540000 545000 550000 555000 560000 Figure 3.6: Isopach map of the Triassic Unit 2 beds from the western Molasse Basin (hand contouring). Compare with Figure 2.29. Anticline axes emphasize thick zones, syncline axes show thin zones. Location of six examples (Figs. 3.7 to 3.12) of evaporite stacks in the Triassic Unit 2. Coordinates in meters are according to the Swiss geographic reference grid. Modified from Sommaruga (1995). Carte des isopaches de I'Unité 2 des couches du Trias du Bassin molassique occidental. Méthode de contourage à la main, compa- rer avec la Figure 2.29. Les axes des anticlinaux indiquent les zones très épaisses, les axes des synclinaux soulignent les zones peu épaisses. Localisation de six exemples (Figs. 3.7 à 3.12) d'empilements d'évaporites dans l'Unité 2 du Trias. Les coordonnées en mètres correspondent à la grille de référence géographique de ¡a Suisse. Modifié de Sommaruga (¡995). 3.2.2.4. Interpretation of the evaporile anti- clines or pillows The kinematic sequences in the central Jura and the Molasse Basin lines can be recognized on a number of examples of evaporite anticlines (Fig. 3.5 and Figs. 3.7 to 3.12). Broad folds from the Plateau Jura and the Molasse Basin show in their core, thickening of eva- porites, salt and clays within the Triassic Unit 2 (Panel 10 and Panel 5), which result in folding of the overlying layers. However, the Cretaceous, Jurassic and Triassic Unit 1 intervals maintain an apparently more constant thickness. Maybe because the seismic data are not of high quality, especially in the Plateau Jura, no truncations are visible within these strata. In some dip lines, located in the sou- thern Molasse Basin e.g. Section 43 on Panel 5, Tertiary sediments onlap clearly the underlying strata. Onlaps have been observed mostly on south dipping limbs. The interpretation of these onlaps is not clear. On the one hand, these onlaps may be due to salt flow to the NW and may thus be interpreted as evidence of salt movements since the beginning of Cenozoic time. To confirm this hypothesis, however, there 84 3. Structures should be some onlaps onto North dipping limbs (there is only one case on strike Section 26, where onlaps are recognized to the SW). No onlap has been found on a north dipping limb, however. On the other hand, updip truncations toward the fore- land may be interpreted as the foredeep unconfor- mity. This major unconformity was induced by the subduction of the distal part of the European plate (Bally, 1989, see also Chapter 5). This second hypothesis appears the most likely. In the central Jura and Molasse Basin, the Mesozoic layers, with the Oligocene and early Miocene sediments repre- sent the prekinematic sequence. No synkinematic series has been observed on seismic lines. Evaporite flow was most likely contemporaneous to the main deformation, of late Miocene age in the Jura (Laubscher, 1961). No evidence for growth anti- clines, involving deposition of Tertiary Molasse sediments, has been found so far in the Jura or the Molasse Basin. The detailed internal structure of swells within Triassic Unit 2 is unknown, since few evaporite- related anticlines have been drilled (Laveron, Essertines) and most other wells do not reach the Triassic Unit 2. In the Jura and Molasse Basin, the evolution stage is similar for all broad anticlines and it is thus difficult to describe the evolution of the deformation. Salt flow and/or stacking of thrust sheets (duplexes) are possible explanations for the observed swells. True salt flow producing the so called salt pillows or salt anticlines is not really proven in the Jura fold and thrust belt, nor in the Molasse Basin. The Laveron stratigraphie well log, which shows more than 300 m of pure salt in Triassic layers, is the thic- kest pillow observed in the studied area and salt flow seems a reasonable assumption to explain this pillow (Fig. 3.5). The Triassic Unit 2 interval shows either discontinuous reflectors parallel to the over- lying strata or no reflectivity (transparent zone). The reflectors do not highlight any structural relation- ships. The thick amount of salt and the absence of structural features suggest that the thickening within the Triassic Unit 2 is indeed the result of an accu- mulation of salt and evaporite by lateral flow. This flow occurs in a compressional regime and is not to be confused with conventional salt diapirism, which often occurs in an extensional context. It is important to underline, however, that no salt diapir has been observed, to date, in the Jura belt and the Molasse Basin. This is probably due the scarcity and thinness of pure rock salt layers present in the Triassic. Low amplitude anticlines related to salt welts (see 3.2.2.1.) are well illustrated by Harrison & Bally (1988) and Harrison (1995) on high quality seis- mic data from the Parry Islands Fold Belt (Melville Island, Canadian Arctic, Fig. 3.13). The increasing intensity of deformation toward the hinterland can be viewed as representing progressive stages of deformation and, as result, gives insight into the evolution of deformation within large anticlines. The first stage illustrates a salt welt, i.e. a signifi- cant triangular disharmony in the salt layer over- lying the less disturbed unit. The triangular envelope of this salt welt is made of decoupling surfaces. In common language (Bally, oral communication), this structure has been called "Napoleon's hat", due to the strong similarity in shape. With increasing shortening, the competent layers overlying the duc- tile zone respond by brittle behavior and the incom- petent shales respond by flow (shale welt). The res- ponse to deformation varies, both laterally and verti- cally in the strata, as a function of the distribution of detachment levels and the relative thickness and competence of different formations. In the Melville Island case, thrust faults appear to be progressively younger from bottom to top. In comparison, the Jura Plateau and the Molasse Basin folds seem to be less evolved, since no obvious wedging is observed above the salt welts. In the Molasse Basin, many broad anticlines pre- sent structural features within the Triassic Unit 2 layer (best examples are in Fig. 3.8 and Fig. 3.12). Strong reflectors dipping toward the South crosscut the whole unit. These can be interpreted as small imbricate thrust faults linking the floor thrust to the roof thrust (below reflector H). According to the clas- sification of Boyer & Elliot (1982), these structures may correspond to hinterland dipping duplexes within an anticlinal core. These structures, confined to the lower unit just above the basement, result in folding of the mainly unfaulted overlying layers. The roof thrust of the duplex (just below reflector H) separates two different styles of deformation: the overlying layers are folded, whereas the underlying are faulted (thrusts) and/or ductily deformed. In the Appalachian Plateau (Pennsylvania), Mitra (1986) presents a seismic line example of a duplex in the core of a major anticline (Fig. 3.14). The Lower to Upper Devonian units are folded into a broad unfaulted anticlinal arch. The Middle Ordovician carbonates (Trenton Formation) are affected by a series of imbricate thrusts that consti- tute a duplex. Additional thickening occurs within 85 3. Structures the Upper Ordovician to Silurian units, but no reflectors are visible, due to the poor quality of the seismic lines. The basal Cambrian unit is not affec- ted by the deformation. Imbricate thrust systems of this sort, have attracted much interest among petro- leum geologists, since they constitute potential hydrocarbon traps. In the eastern Jura, seismic sections show also important thickening within Triassic strata, as shown in the strike line presented on Figure 3.15. This seismic line crosses a low amplitude anticline (Born anticline), which is surrounded by Tertiary Molasse sediments. Beneath the broad anticline, the layer thickness increases between the reflectors representing the base of the Mesozoic and the top of the Muschelkalk (in gray color on Figure 3.15). This interval corresponds to the Triassic Unit 2 of the central Jura. This anticline is located between the Haute Chaîne Jura (Folded Jura) and the Plateau Molasse unit. The structural style of this evaporite- related anticline may be compared to the examples of the Figures 3.10 and 3.11, located in the most internal part of the folded Jura. Essertines ^g^^tion 39 WNW 'M-V^WviLV^*^ Section Ji*m, 'In ÏHlufcfc I. Hi IMI n.iii,........................ Figure 3.7: Southern part of the dip seismic Section 39 from the western Swiss Molasse Basin. For location, see example 1 on Figure 3.6. The interpreta- tion, calibrated on the Essertines drill hole, shows a low amplitude fold related to a thickening within the Triassic Unit 2. Legend for the top of the layers: A = Lower Cretaceous or base Tertiary; B = upper Malm; D = Dogger; G = Triassic Unit I, H- Triassic Unit 2. Partie méridionale du profil sismique 39 transversal, situé dans le Bassin molas- sique suisse. Pour ¡a localisation, voir exemple 1 sur la Figure 3.6. L'interprétation, calibrée sur le forage d'Essertines, montre un pli de faible amplitude associé à un épaississement dans les couches de l'Unité 2 du Trias. Légende pour le toit des couches: A = Crétacé inférieur ou base du Tertiaire; B = MaIm supérieur; D = Dogger; G = Unité 1 du Trias. H = Unité 2 du Trias. Triassic Unit 2 86 3. Structures Triassic Unit 2 Figure 3.8: Northern part of the clip seismic Section 43 from the western Swiss Molasse Basin. For location, see example 2 on Figure 3.6. The interpretation shows a hinterland-vergent fold related to a thickening within the Triassic Unit 2. The thickening may be due to duplex structures. Legend for the top of the layers: A = Lower Cretaceous or base Tertiary; B = upper Malm; D = Dogger; G = Triassic Unit 1,H = Triassic Unit 2. Partie septentrionale du profil sismique 43 transversal, situé dans le Bassin molassique suisse. Pour la localisation, voir exemple 2 sur la Figure 3.6. L'interprétation montre un pli à vergence vers le Sud, associé à un épaississement dans les couches de l'Unité 2 du Trias. L'épaississement est probablement dû à des structures imbriquées en duplex. Légende pour le toit des couches: A = Crétacé inférieur ou base du Tertiaire; B = MaIm supérieur; D = Dogger; G = Unité l du Trias. H = Unité 2 du Trias. 87 3. Structures Triassic Unit 2 Figure 3.9: Southern part of the seismic Section 34 from the western Swiss Molasse Basin. For location, see example 3 on Figure 3.6. The interpretation shows a foreland-vergent fold related to a thickening within the Triassic Unit 2. A tear fault crosscuts the eva- porile anticline (compare map of Figure 3.6). Legend for the top of the layers: A = Lower Cretaceous or base Tertiary; B = upper Malm; D = Dogger; G = Triassic Unit I, H- Triassic Unit 2. Partie méridionale du profil si.smique 34, situé dans le Bassin molassique suisse. Pour la localisation, voir exemple 3 sur la Figure 3.6. L'interprétation montre un pli à vergence vers l'avant-pays, associé à un èpaississement dans les couches de l'Unité 2 du Trias. Un décrochement recoupe l'anticlinal (comparer avec la carte de la Figure 3.6). Légende pour le toit des couches: A = Crétacé inférieur ou base du Tertiaire; B = MaIm supérieur; D = Dogger; G = Unité I du Trias, H = Unité 2 du Trias. SS 3. Structures Section 47 Triassic Unit 2 Figure 3.10: Northern part of the dip seismic Section 47 from the western Swiss Molasse Basin. For location, see example 4 on Figure 3.6. The interpretation shows a foreland-vergent fold related to a thickening within the Triassic Unit 2. Legend for the top of the layers: A = Lower Cretaceous or base Tertiary; B = upper Malm; D = Dogger; G = Triassic Unit 1, H = Triassic Unit 2. Partie septentrionale du profil sismique 47 transversal, situé dans le Bassin molassique suisse. Pour la localisation, voir exemple 4 sur la Figure 3.6. L'interprétation montre un pli à vergence vers le NW, associé à un épaississement dans les couches de l'Unité 2 du Trias. Légende pour le toit des couches: A = Crétacé inférieur ou hase du Tertiaire; B = MaIm supérieur; D = Dogger; G = Unité 1 du Trias. H = Unité 2 du Trias. 89 3. Structures WNW ESE Triassic Unit 2 Figure 3.11: Northern part of the dip seismic Section 49 from the western Swiss Molasse Basin. For location, see example 5 on Figure 3.6. The interpretation shows a foreland-vergent fold related to a thickening within the Triassic Unit 2. The thickening may be due to duplex structures. Legend for the top of the layers: A = Lower Cretaceous or base Tertiary; B = upper Malm; D = Dogger; G = Triassic Unit 1,H = Triassic Unit 2. Partie septentrionale du profil sismique 49 transversal, situé dans le Bassin molassique suisse. Pour la localisation, voir exemple 5 sur la Figure 3.6. L 'interprétation montre un pli à vergence vers le Nord, associé à un épaississement dans les couches de l'Unité 2 du Trias. L'épaississement est peut-être dû à des structures imbriquées en duplex. Légende pour le toit des couches: A = Crétacé inférieur ou base du Tertiaire; B = MaIm supérieur; D = Dogger; G = Unité Ì du Trias, H = Unité 2 du Trias. 90 3. Structures WNW ESE WNW ESE Triassic Unit 2 Figure 3.12: Northern part of the dip seismic Section 45 from the western Swiss Molasse Basin. For location, see example 6 on Figure 3.6. The interpretation shows a foreland-vergent fold related to a thickening within the Triassic Unit 2. The thickening may be due to duplex structures. Legend for the top of the layers: A = Lower Cretaceous or base Tertiary; B = upper Malm; D = Dogger; G = Triassic Unit 1,H = Triassic Unit 2. Partie septentrionale du profil sismique 45 transversal, situé dans le Bassin molassique suisse. Pour la localisation, voir exemple 6 sur la Figure 3.6. L'interprétation montre un pli à vergence vers l'avant-pays, associé à un èpaississement dans les couches de l'Unité 2 du Trias. L 'èpaississement est peut-être dû à des structures imbriquées en duplex. Légende pour le toit des couches: A = Crétacé inférieur ou base du Tertiaire; B = MaIm supérieur; D = Dogger; G = Unité I du Trias, H = Unité 2 du Trias. 91 3. Structures From Harrison & Bally (1988) Figure 3.13: a) Stages from the evolution of an anticline in the salt-based Parry Islands Fold Belt (Melville Island). Modified from Harrison & Bally (1988). Evolution d'un anticlinal dans la chaîneplissée de l'ile de Melville (territoires arctiques). Modifié de Harrison & Bally (¡988). b) Example of an anticline from Melville Island. LRL = lower rigid layer; LDL = lower ductile layer; MRB = medial rigid beam; UDL = upper ductile layer; URL = upper rigid layer; 1 = footwall syncline; 2 = a compound anticlinal salt welt with a faulted and indistinct hinge saddle; 3 = dramatic local thinning of the lower Bay beam is encapsulated by evaporites; 5 = some anticlinal hinge thickening of mud rock is indicated for the Cape de Bray formation; 6 = the crest of structures at deeper level. From Harrison (1995, Figure 123). Exemple d'un anticlinal de l'île de Meville. LRL = couche rigide inférieure; LDL = couche ductile inférieure; MRB ¦ niveau rigide mown; UDL = couche ductile supérieure; URL = couche rigide supérieure; l = synclinal dans le mur; 2 = anticlinal composite avec une charnière fai liée en forme de selle; 3 = important amincissement du niveau "lower Bay " enveloppé par des èvaporites; 5 = un faible épaississement de la charnière par des roches argileuses est montré pour ¡a formation du Cape de Bray ; 6 = crête des structures localisées à un niveau inférieur. Tiré de Harrison (1995. Figure /23). 92 3. Structures DEVONIAN ORISKANY ORDOVICIAN TRENTON 2 BASAL CAMBRIAN *3 From Mitra (1986) Figure 3.14: Migrated seismic profile across an anticline in the Appalachian Plateau (Pennsylvania). Devonian units are folded into a broad anticlinal arch cored by duplexes in Ordovician carbonates. Interpretation and figure are from Mitra (©1986, Figure 27. reprinted by the permission of the "American Association of Petroleum Geologists"). Profil sismique migré d'un anticlinal du Plateau appalachien (Pennsylvanie). Les unités du Dévonien forment une large arche anti- clinale, remplie par des imbrications en duplex dans les carbonates de I 'Ordovicien. L'interprétation et la figure sont tirées de Mitra (&I9H6, Figure 27, reproduite avec la permission de !'"American Association <>/ Petroleum Geologists'). Concluding on the evaporite-related folds, it is suggested that the broad anticlines from the Plateau Jura are related to salt flow within the Triassic Unit 2, whereas Molasse Basin anticlines are related to well organized evaporite duplexes within the Triassic Unit 2. This difference is probably related to mineralogical composition and hence to the rheo- logy of the Triassic evaporites. In the northern parts, considerable amounts of pure salt seem to be pre- sent within this formation, whereas in the southern parts its presence is not yet proven. Duplexes seem to have formed within the slightly more competent Triassic Unit 2 of the Jura internal parts and of the Molasse Basin. In the preceding paragraphs, the geometry and the kinematics of evaporite pillows have been discus- sed. In terms of mechanism, buckling seems the most adequate for the observed structural style and rheology. The Triassic Unit 2, which consists of salt, evaporite and clay rocks, has a low viscosity in com- parison with the overlying alternating carbonate and shale layers. These rheological conditions favor fol- ding by flexural-flow. The weakest layer, Triassic Unit 2 (see Figure 2.30), flows into the core of the anticline presenting thickening of the unit, whereas the strong layers buckle without any thickness changes (concentric folds). 3.2.3. Thrust-related folds (high ampi i tu de) 3.2.3.1. General comments The sinusoidal shape of the Jura folds drawn by earlier geologists e.g. in De Margerie (1922), Heim (1921), Rickenbach (1925), Suter & Lüthi (1969) (see Chapter 4 Regional geology) has been shown to be an oversimplification, not only in the central Jura, but also in the eastern (Laubscher, 1977) and western Jura (Philippe, 1994). In most places, at the surface, a veneer of Quaternary sediments obscures the critical relationships between strata and thrust faults. Seismic data have, however, confirmed that folds are related to major thrust faults. This relation- ship has been already suggested by different authors in the eastern and western part of the Jura (Buxtorf, 1916; Laubscher, 1985; Diebold et al, 1991; Philippe, 1994). 9} 3. Structures Legend: TLi = Top Liassic TMk = Top Muschelkalk BMZ = Base Mesozoic MpI = Multiple WSW ENE Figure 3.15: Strike seismic line from the Nagra work located in the eastern Jura (southern side), a) Geological cross-section, b) Line drawing of the seismic section, c) Interpretation of the seismic line. "Muschelkalk" layers, highlighted in gray, present a thickening (evaporite pillow) beneath the broad anticline, d) Seismic profile. The location is shown on the map. The seismic line, the line dra- wing and the cross-section are from Diebold et al. ( 1991 ). Profil sismique longitudinal et coupe géologique localisés dans le Jura oriental (partie méridionale), a) Coupe géologique, b) Réflecteurs du profil sismique. c) Interprétation du profil sismique. Les couches du "Muschelkalk ", soulignées en gris, montrent un èpaississement (coussin d'évaporites) sous le large anticlinal, d) Profil sismique, voir localisation sur la carte. Le profil sismique, le dessin et ¡a coupe géologique sont lires de DlEBom et al. (1991). 94 3. Structures The observed scale of folding ranges from kilo- meters to meters. The first corresponds to large anti- clines (regional scale), whereas the second consists of disharmonie folds, with metric wavelength (out- crop or minor scale). The latter are developed contemporaneously with the large scale structures. Disharmonie folding is observed between the stiff limestone beds and the weak marl layers (Fig. 2.30). 3.2.3.2. Geophysical evidence from seismic profiles Generally the geophysical evidence for large sub- surface thrusts includes duplication of coherent suc- cessions of seismic stratigraphie reflectors and velo- city anomalies caused by tectonic duplication of strata higher in the section. On many profiles, velocity anomalies are obser- ved. The anomaly is usually a low positive deflec- tion of the reflectors beneath anticlines (velocity pull-up) and a negative deflection beneath synclines (velocity pull-down). The dip Section 1 (Panel 1 and Plate 1) and Section 3 (Panel 1 and Plate 2) and the strike Section 8 (Panel 3 and Plate 3) from the Neuchâtel Jura illustrate such anomalies. Recognition of repeated seismic stratigraphie reflections is the most common and evident form of identification of subsurface thrusts. Repetition, more obvious on lines parallel to the trend of the structures (strike lines), is one major reason for careful interpretation of seismic stratigraphy on strike lines. The lack of resolution often observed on dip profiles below the anticlines can be explai- ned by many structural complications, but also by the steep topography. Strike Section 14 (Panel 3 and Plate 5) and dip Section 11 (Panel 2 and Plate 4) from the Neuchâtel Jura and strike Sections 80, 82 and 84 (Panel 8) and dip Sections 81, 83, 85, 87, 93 from the Risoux Jura document clearly duplication of reflectors. The Mt-Risoux anticline on Section 93 also shows duplication of the Jurassic series, confir- med by a well (Winnock, 1961) (Fig. 2.19). 3.2.3.3. Geometry of large scale folds illustra- ted by seismic profiles Transverse lines crossing the Jura, oriented per- pendicularly to the fold axes (NW-SE), allow to constrain of the fold geometry at depth. Line dra- wings of dip sections on panels 1, 2, and 9 illustrate the type of folds from the Haute Chaîne Jura (see Figure 1.2, for location on geological map). These high amplitude folds, which are asymmetric and are clearly related to thrust faults, root in the basal décollement zone located within the evaporites of the Triassic Unit 2. These thrust-related anticlines cause duplication of these Mesozoic stratigraphie layers. Thrust-related anticlines are separated by broad or tight synclines, Val de Ruz (Panel 1) and Val de Travers (Panel 2) respectively, which display flat lying, parallel layers. Many thrust faults are NW (NNW) verging, like the main thrust system (foreland-vergent thrust). SE (SSE) vergent thrust faults are considered as back- thrusts (hinterland-vergent thrusts). Thrust faults include both flats and ramps, e.g. Section 1 on Panel 1 or Plate 1, Sections 11 (Nouvelle Censière anti- cline, Plate 4, see also Fig. 4.7b), 13, 17 on Panel 2 (Neuchâtel Jura) and Sections 85, 111 (Mt-Risoux anticline, Plate 8), 95 on Panel 9 (Vaud and France Jura). In the flats, thrust faults are parallel to the overlying layers or parallel to the main décollement plane, whereas, in the ramps, the thrust fault cuts across the layers. All seismically mapped thrust faults are throughgoing to the surface, breaking through the structures in the steep frontal limbs e.g. Sections 1, 3 on Panel 1 and Section 11 on Panel 2. The leading edge of the thrust sheet may also show some imbrications e.g. Sections 5 and 7 on Panel I. Many backthrusts are associated with foreland- vergent thrust faults. They seem to be localized in kink or steep dip data zones, identified on geologi- cal maps e.g. Section 3 on Panel 1, Sections 11, 13, 17 on Panel 2 and Section 111 on Panel 9, but also appear to be connected to a main thrust fault at the transition between a fìat and a ramp portion. Foreland-vergent thrusts have a kilometric dipslip displacement (e.g. Section 11 on Panel 2 and Section 111 -southern part- on Panel 9) and hinter- land-vergent thrusts generally have few tens or hun- dreds of meters of displacements. Sometimes, as in the backthrust of the Mt-Risoux anticline (Sections 85, 111 -southern part- on Panel 9), a kilometric displacement can be observed. The foreland-vergent thrust ramps have step-up angles between 20° and 30° (sometimes more), whereas backthrusts in the Neuchâtel Jura are much steeper (±60°) or else much shallower, as in the Vaud Jura (Mt-Risoux, Sections 85 and 111, sou- thern part). These angles are approximate, because they are deduced from the seismic interpretations, which are displayed in TWT (seconds). 95 3. Structures The Triassic Unit 2 evaporile layers are conside- red as the major regional décollement zone. The thrust ramps of the anticlines described above, root in this zone. Triassic Unit 2 layers appear very thick on the seismic lines (see line drawings on panels and Figure 2.30 of §2.6.). Formations beneath this very weak zone represent the basement. The term basement includes any rocks or sediments not invol- ved in the formation of the overlying folds. The top of the basement appears as a smooth and flat surface dipping Io to 3° to the S-SE (Fig. 5.1). Some smaller scale thrust-related folds, e.g. the example of Figure 3.16, have a main thrust fault that roots in the lower Malm layers ("Argovian facies"). This figure is a dip line, located at the southwestern edge of the studied area, that crosses the transition from the Molasse Basin to the Jura Haute Chaîne. The thrust fault on this seismic line (Section 71, Fig. 3.16) is clearly documented by numerous flat reflec- tors (between B and C reflectors) cut by south-dip- ping reflectors. Unfortunately, this is the only example documented on seismic lines of the study area. Such minor scale folds are, however, already well known from surface geology (Droxler, 1978; Pfiffner, 1990) and will be discussed later. In conclusion, the new seismic data confirm that large scale anticlines are formed above NNW ver- gent thrusts with kilometric dipslip displacement. Important thrusting results in duplication of the entire Jurassic stratigraphie sequence. These thrusts root in the basal décollement zone located in the evaporites of the Triassic Unit 2, which are surpri- singly thick and clearly involved in the thrusting. 3.2.3.4. Minor scale deformation and disharmo- nie folds illustrated by outcrops At the outcrop scale, deformation is brittle, cha- racterized by stylolites, veins and small faults bea- ring slickensides. Deformation is localized within fold hinges and narrow fractured zones separating seemingly undeformed regions (limbs). Construction of a pilot tunnel below La Vue des Alpes (between the cities of Neuchâtel and La Chaux de Fonds, eastern Neuchâtel Jura, (Figs. 4.3 and 4.5) allowed Xavier Tschanz (from Neuchâtel University) to observe subsurface outcrops along a continuous profile of an anticline from the Neuchâtel Haute Chaîne Jura. This profile, running perpendicular to the fold axis and located along the same trace as the seismic Section 3 (Panel 1 and Plate 2), is more or less between intersections 4 and 2. Results from observation of fresh outcrops in the drilled tunnel are discussed in more detail in the paper by Tschanz & Sommaruga (1993). The rocks displayed surprisingly few deformation features. Vein volumes generally represent less than c.a. 0.5% of the rock and very few tectonic stylolites are pre- sent. The more intense deformations occurred only within sharp kink and hinge zones. These are charac- terized by an increased number of seemingly chaotic calcite veins, representing in places up to about 5% of the total rock volume. Locally, reverse and normal faults with a decimetric throw were identified. Meter scale offset faults are associated to the major kink and hinge zones. On seismic Section 3 (on Panel 1, 2 km North of intersection 4), these zones are interpre- ted as related to a backthrust. Bedding parallel slip surfaces are present along the whole anticline. Stylolites, veins, striae and twins are the expression of strain at the outcrop and sample scale, respecti- vely and demonstrate strains at scales smaller than the wavelength of the folds. At sample scale, calcite twin strain analyses in bioclastic coarse grained Dogger limestones from the Neuchâtel Jura (Val de Ruz), revealed small intracrystalline deformations on the order of 1 to 4% shortening. All twins observed in this study are thin and straight (micro-)twins. These microstruc- tures are indicative of minor deformation at very low temperatures (<<150°C) (Groshong et al., 1984; Burkhard, 1993). This local study can be integrated with the regional study of Tschanz (1990), which presents the same results analyzed on calcite twins from the whole central Jura. Minor scale décollement levels, producing small scale folds, observed at the base of the lower Malm marls ("Argovian" facies) and the Cretaceous (Hauterivian) marls (see Figure 2.30) are discussed in two cases below. A detailed study at the outcrop scale has been made by Pfiffner (1990) ¡n lower Malm limestone and shale interlayered beds (for rheology, see Figure 2.30) at the frontal hinge of a large scale, SE-ver- gent anticline (St-Sulpice, western edge of the Val de Travers syncline, Neuchâtel Jura, see Figure 4.3 for location on the map). Figure 3.17 shows several zoom sections at different scales. The geological cross-section (Fig. 3.17b), located halfway between seismic Sections 13 and 15 (Fig. 3.17a), has been modified from that of Pfiffner. Seismic data has improved the geometry at depth and the thickness of the Dogger, Liassic and Triassic units. 96 3. Structures Figure 3.16: Portion of the dip seismic Section 71 located at the internal limit of the Jura Haute Chaîne. The interpretation shows a fold related to a thrust which roots in a minor décollement level at the base of the lower Malm unit ("Argovian" formation). Legend for the top of the layers: B = upper Malm; C = lower Malm; D = Dogger; F = Liassic. Partie du profil sismique 71 transversal localisé à la limite interne de la Haute Chaîne jurassienne. L'interprétation présente un pli en relation avec un chevauchement qui s'enracine dans un niveau de décollement mineur à la base du MaIm inférieur ("Argovien "). Légende pour le toit des couches: B = MaIm supérieur; C = MaIm inférieur; D = Dogger; F = Lias. Vl 3. Structures The seismic and geological sections show, at regio- nal scale, a hinterland-vergent anticline related to a thrust fault, rooting in the main décollement level (Triassic Unit 2). The Val de Travers syncline shows two thrust faults in the Triassic units. This shorte- ning is accommodated in the overlying layers by fish-tail structures. In the field, several disharmonie folds can be seen; these are of small wavelength and developed above a minor décollement level located in lower Malm shales ("Argovian") (Figs. 3.17c and 3.17d). According to Pfiffner (1990, p.585), "indi- vidual limestone layers maintain compatibility within large scale structures by folding, bedding plane slip, conjugate contractional and extensional faults, and duplexes. The thickness of individual limestone layers appears not to be altered by a signi- ficant amount. Some of the contractional faults indi- cate considerable layer-parallel shortening in the early history of the folds. The ductile behavior indi- cated by round fold hinges is mainly linked to small scale faulting." Metric and decimetric disharmonie folds are also common in the thin bedded limestones of the Lower Cretaceous (Hauterivian). A décollement level is present between the Cretaceous beds and the stiff Malm limestones, where strong disharmony in fol- ding style is observed. Within the Cretaceous, décollement can also occur within the Hauterivian marls. The geometry and the kinematics of these folds have been analyzed in detail by Droxler & Schaer (1979) at one outcrop of the Neuchâtel Jura. According to these authors, ruptures induced by shearing and traction during the fold evolution have transformed the layers into semi-independent groups of fragments, the external geometry of which has been modified by dissolution in pressured zones. Part of the dissolved material in stylolitic planes is recrystallized in extension cracks. These folds in the Cretaceous layers are attributed to Miocene thrusting e.g. in the Verrières syncline (Haute Chaîne Jura, France, Fig. 4.3) (Martin et al., 1991), in contrast to the gravity sliding hypothesis ("collapse structure") suggested first by Castany (1947). 3.2.3.5. Interpretation of the thrust-related folds In the Haute Chaîne Jura, high amplitude folds, though apparently simple, result from the superposi- tion of a number of processes active at metric to kilometric scale. Stylolites, veins, striae and twins demonstrate strain at scales much smaller than the wavelength of the folds (§3.2.3.4). Though the wavelength of the Haute Chaîne fold- geometry seems to be determined by the thickness of the Malm layers, the overall rheology of the sedi- mentary cover (see Figure 2.30) has an important influence on the shape of Jura folds. The thickness of the sedimentary cover decreases from SW towards NE, so that higher amplitude folds are loca- ted in the southwestern area (higher topographic relief in Canton Vaud, Canton Geneva and in France), whereas in the East topographic relief is much lower (Canton Aargau and Jura). The presence of a thick, very weak sole layer, in contrast with more competent overlying layers, also determines the fold type. The very weak zone consists of evaporites, salt and clays (see description and discussion in §2.6.2) of the Triassic Unit 2 layers. This interval shows clearly thickness changes in the Plateau Jura and the Molasse Basin. In the central part of the Haute Chaîne, this interval appears to be thick and of constant thickness in some synclines (Val de Travers and Val de Ruz, Sections 4, 6, 8 on Panel 3). In the northern part of the Neuchâtel Jura however, the La Brévine syncline (Section 2 on Panel 3) shows duplication of the Triassic Unit 2 along the whole valley, which may explain the high elevation (1000 m) of this syncline, in comparison with the Val de Travers and Val de Ruz synclines (600 m to 800 m). The poor quality of seismic data beneath anticlines does not allow for accurate interpretation. Although the combination of the interpretation of several dip and strike lines (seismic grid) suggests that the Triassic Unit 2 inter- val is thick, in some cases this thickness results from an obvious tectonic duplication. The sedimentary cover of the central Jura repre- sents a multilayer sequence with a thick and particu- larly weak layer at the base of the sequence, corres- ponding to the décollement zone. The overlying layers consist of alternating marls (incompetent layer) and limestones (competent strong layers, Fig. 2.30). A high viscosity contrast also exists between the very weak Triassic Unit 2 layer and the weak to strong overlying layers. The Haute Chaîne Jura folds were initiated first as buckle folds in response to the layer-parallel compression. Evaporites, clays and salt rock infilled the space generated at the base of the sequence by inflowing mechanisms. These first stage buckle fold, also called detachment fold, then developed into fault-propagation folds and fault- bend folds after breakthrough of thrusts, with pro- gressive deformation. The deformation within the stiff layers is accommodated mainly by bedding- 98 3. Structures plane slip, pressure solution and brittle faulting. The Chaumont (southern anticline on Sections 1 and 3 on Panel 1) and SomMartel anticlines (Intersection 2 on Section 5, Panel 1) are examples of detachment folds that evolved into fault-propagation style folds. On the geological cross-section of Figure 4.5, loca- ted more or less parallel to the seismic Section 3, the Chaumont anticline presents a breakthrough in the steeper forelimb (see High-Angle Breakthrough in Suppe & Medwedeff (1990). The anticlines of the Nouvelle Censière (Section 11 on Panel 2) and the Mt-Risoux (Section 111-87 on Panel 9) are examples of detachment folds developed later over a ramp - flat geometry (fault-bend fold style). Liassic marls favor minor décollement levels. Beneath the anticline, duplication of the Mesozoic cover is observed. In the study area, thrust-related folds are a mixture between the three types of fault-related folds described above. Dixon & Liu (1992) have observed a similar evo- lution from centrifugal structural models; these represent a stratigraphie succession composed of six units with alternating bulk competency (low compe- tence at the base). The models were subjected to horizontal, layer-parallel compression and show three mechanisms of shortening: layer-parallel shor- tening, buckling and thrust faulting. The relation- ship between folding and faulting evolves through time: firstly, detachment buckle folds form above a zone of décollement, secondly, a fault ramp propa- gates upward across the lowermost competent layers at the position of a foreland dipping limb (fault-pro- pagation fold) and thirdly, when the fault has propa- gated through the competent unit its trajectory bends into the overlying incompetent unit and with further transport the hangingwall is modified by fault-bend folding. Limbs of the low-amplitude folds form shortly afterward, cut by foreland verging thrust faults. The question, about the core infill of Jura anti- clines, has been debated by geologists since the beginning of the century (Buxtorf, 1907). Using the same dip data, based on surface geology, several type of cross-sections have been drawn. As discus- sed by BiTTERLi (1992), the core of an anticline may be filled by salt flow, duplexes, duplication of the entire cover over a thrust or thrust sheets presenting several generations of thrusting. In the absence of seismic data, several different methodologies have been tested in the Jura foreland fold and thrust belt, by different geologists, in order to answer this ques- tion: analogical modeling, comparison with other foreland belts, 2D and 3D modeling and new models of folding (BiTTERLi, 1988; Philippe, 1995). In the central Jura (this work), the wealth of seis- mic data has allowed clarification of the geometry beneath the anticlines. The Haute Chaîne folds are first related to evaporite stacks within the Triassic layers and then evolved over thrust faults, which implies duplication of the Jurassic cover. In the Plateau Jura, the cores of the broad folds (not rela- ted to thrust) are filled with evaporite stacks. For the southern French Jura, the recent PhD the- sis of Philippe (1995) interprets the evolution of folds as asymmetric detachment folds with salt flow that later evolve into fault-propagation folds. This is compared herein with analog modeling and with examples from the Canadian Rocky Mountain Foothills and Front Ranges described by Dobson & McClay (1992) and Langenberg (1992). Folds are first related to evaporite flow and then to thrust faults that duplicate the strata. In the eastern Jura, cross-sections were construc- ted for many years in the style of lift-off folds (box folds), supported by thickening of lower Jurassic to Triassic sediments in their core, as drawn already by Buxtorf (1916) at the beginning of the century (see Figure 1.5, Section 2). However, modeling in two (balanced cross-sections) and three dimensions (block mosaic) of the geometry and the kinematics of one of these eastern anticlines (Weissenstein), has lead to a completely different interpretation (Bitterli, 1990). The huge lift-off box folds are reinterpreted in terms of complex fault-bend folds presenting at least two generations of thrusting. However, these large thrusts are nowhere exposed at the surface. Laubscher (1986, 1992), using seismic evidence, has suggested that some of the eastern Jura box folds e.g. the Grenchenberg (Fig. 1.5) are supported by hinterland-dipping stacks, within the Middle Triassic Anhydritgruppe (= Triassic Unit 2) duplexes. Thrusts may be hidden in the subsurface. In the eastern Jura also, Jordan & Noack (1992) have discussed the geometry of thrusts related to thick ductile soles. The model they propose differs from the classical fault-bend fold model, which has only a discrete sole thrust fault. The backlimb is much longer than the present ramp and has a lower backlimb angle with respect to the ramp angle. Several differences concerning rotation and migra- 99 3. Structures O 1 2lm LEGEND Tops Cretaceous Malm Dogger Liassic -----------Triassic Unit 1 ........... Triassic Unii 2 »-^ Faults NW 0 1 2km a) NW Vallée de la Bravine 1000m — Om — -1000m -2000m— 1000m — 0m -1000m _-2000m b) Figure 3.17 (pages 100, 101): Deformation at different scales in a hinterland-vergent anticline from the Neuchâtel Jura (Val de Travers area). Figures 3.17c and d are according to Pfiffner ( 1990). a) Line drawing (in TWT depth scale) of seismic Section 13 and 15 illustrating the regional scale. b) Geological cross-section according to seismic interpretation and outcrop data. The rectangle shows the location of the small scale folds illustrated in Figure 3.17c. For location of the cross-section, see Figures 4.7b and 4.9. c) Detailed cross-section showing relationships between meter scale folds and faults. Rectangle locates Figure 3.17d. d) Detailed analysis (meter scale) of one of the small scale folds of Figure 3.17c. 100 3. Structures C) d) Déformation à différentes échelles au sein d'un anticlinal à vergence vers le SE localisé dans le Jura neuchâtelois (Val de Travers). Figures 3.17c el d sont modifiées de Pfiffner (1990). a) Réflecteurs (profondeur en temps) des profils sismiques 13 et ¡5 (échelle régionale). b) Coupe géologique en accord avec I 'interpretation sismique et les données de la géologie de surface. Le rectangle montre l'empla- cement des plis dècamétriques illustrés dans la Figure 3.17c. Localisation de la coupe, voir Figures 4.7b et 4.9. c) Coupe géologique détaillée montrant les relations entre les plis dècamétriques et les failles. Le rectangle localise ta Figure 3.l7d. d) Analyse détaillée d'un pli métrique de la Figure 3.17c. 101 3. Structures tion of hinges are presented, in this model, a major part of the backlimb is deformed only by layer parallel slip, because it does not migrate through the flat-ramp hinge. These authors present some field evidence (a more or less intact backlimb), which seems to confirm the last point. This model also presents some features compatible with the central Jura, notably a thick ductile sole thrust and long shallow dipping backlimbs e.g. Chaumont anticline (Fig. 4.5). In conclusion, the Haute Chaîne Jura folds of the central Jura developed first as buckle folds (detach- ment folds) and then evolved into fault-propagation or/and fault-bend fold types. The presence of a thick and very weak sole thrust and the final geometry of the folds has led to this model. The first stage buckle folds were most probably similar to the Plateau Jura evaporite-related folds. Seismic pro- files are a good tool for understanding the geometry of the folds, but 3D kinematic modeling would allow to further constrain the geometry and also include the lateral continuation of the anticlines. 3.3. TEAR FAULTS 33.1. Definitions In the following paragraphs, the general context of strike-slip faults is discussed first, followed by more specific discussion and definition of tear faults. a) Strike-slip faults (sensu lato) Strike-slip faults correspond to the end member of the spectrum in a kinematic classification of faults (Reíd et ai, 1913). According to the defini- tion of Bates & Jackson (1987), they represent "faults on which most of the movement is parallel to the faults' strike". They are generally vertical and accommodate horizontal shear within the crust or/and the lithosphère. Displacement along these faults may be either right-lateral or left-lateral. Sylvester (1988), reviewing strike-slip faults (Fig. 3.18), suggested that such faults can be classified either as transform faults, which cut the lithosphère as plate boundaries, or as transcurrent faults which are confined to the crust. The latter category, which includes indent-linked strike-slip faults, tear faults, transfer faults and intracontinental transform faults, is of particular interest in the study of thin-skinned fold belts, such as the Jura. 102 Indent-linked strike-slip faults juxtapose pieces of continental lithosphère, especially in zones of plate convergence and tectonic escape. They are not true transform faults, because they do not cut the lithos- phère. Tear faults accommodate the differential dis- placement within a given allochthon or between the allochthon and adjacent structural units. They are generally oriented transverse to the strike of the deformed rocks and are sometimes called transverse faults or transcurrent faults. The term transfer fault is used for strike-slip faults that connect overstep- ping segments of parallel or en echelon strike-slip faults. Commonly located at the ends of pull-aparts, they transfer the displacement across a stepover from one parallel fault segment to the other. Intraplate or intracontinental transform faults are regional strike- slip faults, which are similar to indent-linked strike- slip faults in that they are restricted to the crust, but they need not to be genetically related to indentor tectonics. They typically separate regional domains of extension, shortening or shear. Transfer faults and intracontinental transform faults are of larger scale than tear faults and also may accommodate larger amounts of slip (Twiss & Moores, 1992). Tear fault seems to be the appro- priate term to characterize the strike-slip faults observed in the Jura. b) Tear faults To our knowledge, there is no generally admitted, precise definition of tear fault. In this paragraph, definitions proposed by different authors are presen- ted to avoid confusion with this term. Dahlstrom (1970) probably published one of the first definitions and classifications of tear faults: "a tear fault is a species of strike-slip fault which ter- minates both upwards and downwards against movement planes, that may be detachments or thrust faults or low angle normal faults". He distinguishes two basic types of tear faults: 1) transverse (primary or secondary) or oblique tear fault within a defor- med thrust sheet; 2) tear fault as an integral part of a thrust sheet boundary. The first type is discussed below; for the second, one may refer to Dahlstrom's explanations. In primary tear faults, the amount of shortening on either side is consistent, but the mechanisms may be different. Such compensated differences in rock shortening mechanisms demonstrate that the tear fault is an integral part of the structural fabric, which developed in the very early stages of deformation. 3. Structures Secondary transverse tear faults provide a mecha- nism for transferring displacement between pairs of existent thrust faults. Such tear faults transect the intervening thrust sheet, thus permitting adjacent parts of the same thrust sheet to have markedly dis- parate displacements. Formation of these tear faults post-dates thrusting and they are therefore secon- dary phenomena. According to the Sylvester's (1988) classifica- tion of strike-slip faults (Fig. 3.18), tear faults belong to the category of transcurrent faults found in inrraplate settings (thin-skinned). For the exact definition of tear faults, Sylvester refers to Biddle & Christie-Blick (1985): "a strike-slip fault or oblique-slip fault within or bounding an allochthon produced by either regional extension or regional shortening. Tear faults accommodate differential displacement within a given allochthon and adjacent structural units". In this classification, strike-slip fault as mentioned above, is not a specific term, but a generic term. In structural geology books e.g. Twiss & Moores (1992), the term tear fault is used in a general sense, describing as a small-scale, local strike-slip fault, that is commonly subsidiary to other structures such as folds, thrust faults, or normal faults. They are steeply dipping and oriented subparallel to the regional direction of displacement. They occur in the hangingwall blocks of low angle faults and accommodate different amounts of displacement, either on different parts of the fault or between the allochthon and adjacent autochthonous rocks. The discontinuity in displacement is then taken up by tear faults. Tear faults within a deformed sheet per- CLASSIF1CATION OF STRIKE-SLIP FAULTS INTERPLATE (deep-seated) INTRAPLATE (thin-skinned) TRANSFORM Faults (delimit plates, cul lithosphère, fully accommodate motion between plaies) TRANSCURRENT faults (confined to the crust) Ridge transform faults* • Displace segments of oceanic crust having similar spreading vectors • Present examples: Owen, Romanche, and Charlie Gibbs fracture zones Boundary transform faults* • Join unlike plates which move parallel to the boundary between the plates • Present examples: San Andreas fault (California), Chaman fault (Pakistan), Alpine fault (New Zealand) Trench-linked strike-slip faults* • Accommodate horizontal component of oblique subduction; cut and may localize arc intrusions and volcanic rocks; located about 100 km inboard of trench • Present examples: Semanko fault (Burma), Atacama fault (Chile). Median Tectonic Line (Japan) Indent-linked strike-slip faults* • Separate continent-continent blocks which move with respect to one another because of plate convergence • Present examples: North Anatolian fault (Turkey); Karakorum, AIlyn Tagh, and Kunlun fault (Tibet) Tear faults • Accommodate differential displacement within a given allochthon, or between the allochthon and adjacent structural units (Biddle and Christie-Blick, 1985) • Present examples: northwest- and northeast-striking faults in Asiak fold-thrust bell (Canada) Transfer faults • Transfer horizontal slip from one segment of a major strike-slip fault to its overstepping or en echelon neighbor • Present examples: Lower Hope Valley and Upper Hurunui Valley faults between the Hope and Kakapo faults (New Zealand), Southern and Northern Diagonal faults (eastern Sinai) lnt racorni nenia! transform faults • Separale allochthons of different tectonic styles • Present example: Gariock fault (California) ?See Woodcock (1986, p. 20) for additional examples, both ancien and modem, and for their geometric and kinematic characteristics. From Sylvester 1988 Figure 3.18: Classification of strike-slip faults from Sylvester (1988). See also Woodcock (1986). Classification des décrochements (sensu ¡aio) d'après Sylvester (¡988). Voir aussi Woodcock (¡986). 103 3. Structures mit abrupt changes in the pattern of deformation through differential movement between component parts of the sheet. Displacement may be either right- lateral or left-lateral. Twiss & Moores define a strike-slip fault as a vertical fault that accommo- dates horizontal shear within the crust. Strike-slip faults exist on all scales, in both oceanic and conti- nental crust. In his glossary of thrust tectonic terms, McClay (1992) defines tear faults as strike-slip faults paral- lel to the thrust transport direction and separating two parts of the thrust sheet, each of which has a different displacement. He distinguishes a tear fault from a lateral ramp; the latter is a ramp in the thrust surface parallel to the direction of transport of the thrust sheet. Ramp angles are generally between 10° and 30°. He notes that if the lateral structure is ver- tical then it becomes a thrust transport parallel tear or strike-slip fault and should not therefore be ter- med a lateral ramp. All these explanations or definitions highlight many geometric and kinematic peculiarities of tear faults admitted by most authors. In summary: - a tear fault belongs to an allochthonous sheet and has a transcurrent movement - a tear fault terminates rightward and leftward into a thrust fault and downward into a décollement zone a tear fault is steeply dipping: Lateral ramps like tear faults are subparallel to the transport direction. Tear faults have a subvertical fault plane, whereas lateral ramp fault planes dip 10° to 30° (Fig. 3.19). the same amount of shortening may be accommo- dated differently on each side of the fault the formation of a tear fault may be earlier and contemporaneous to thrusting (primary tear fault) or subsequent (secondary tear fault): In primary tear faults, shortening may be accommoda- ted differently on each side of the fault i.e. one thrust- related fold on one side may correspond to two thrust- related folds on the other side. In this case it is difficult to determine a sense of movement. Therefore on a geo- logical map, the sense of movement of primary tear faults is only apparent and the true displacement will be deduced from restored maps or from fault/striae out- crops in the field. However, in the case of secondary tear faults, the sense of movement is real and fold axes are offset as passive markers. the tear fault is oriented subparallel to the regio- nal transport direction of displacement: A regional transport direction is difficult to determine in fold and thrust belts, where no direct access to the basal thrust planes exists. Intuitively, transport direc- tion is perpendicular to fold axes. But in many cases, local transport direction is oblique to the regional direc- tion. Therefore it would be more appropriate to define the trend of the tear faults in comparison with the fold belt orientation. Lateral ramp Figure 3.19: Three-dimensional view of a tear fault and different ramp types (lateral, frontal and oblique ramps). Vue tridimensionelle d'un décrochement (sensu stricto) et de différentes rampes (rampes latérale, frontale et oblique). 104 3. Structures Ramps are often associated with tear faults. There are basically three types of ramps: frontal, oblique and lateral (Fig. 3.19). These ramps are defined with respect to the regional transport direction, per- pendicular, oblique and parallel respectively (Apotria et ai, 1992). Fault planes of these ramps generally show dips between 10° to 30°. 3.3.2. Geomorphological evidence The tear faults in the Jura are well known from geological maps but can almost as well be recogni- zed on topographic maps, where they have a clear morphological expression (Fig. 3.20). A close gene- tic relationship between folding and tear faults was postulated by Heim (1915), who noted the radial arrangement of the tear faults when viewed on a map of the entire Jura arc. Major, apparently sinis- tral, tear faults are oriented NW-SE in the southern Jura, NNW-SSE to N-S in the central Jura and NNE-SSW in the eastern Jura (Fig. 3.20a). Somewhat shorter, apparently conjugate dextral tear faults are often associated. Folds, thrusts, sinistral and dextral tear faults define a set of structures compatible with a horizontal shortening in a WNW- ESE, NW-SE, and NNW-SSE direction respectively (Laubscher, 1972, indenter model). On the geomorphological map (Fig. 3.20b), tear fault traces appear to be straight lines even across rugged topography. Tear faults are marked by pro- minent continuous topographic features, such as narrow linear depressions. The topographically high part of the fault changes from one side to the other along the fault trace, because of the juxtaposition of different structures along the fault (synclines and anticlines) e.g. Pontarlier fault, or the juxtaposition of lithologies with different resistance to erosion. It is important to highlight this.morphologic evidence, because as will be discussed below, seismic charac- terization of tear faults may be poor in some cases (e.g. La Ferrière fault, La Tourne fault). In addition, geological maps present also evi- dence for tear faults; fold axes tend to terminate against tear faults and they do not have obvious direct correlation from one side of the fault to the other (see later discussion). 3.3.3. Geophysical evidence from seismic profiles Generally, geophysical evidence for an important tear fault includes a transparent zone without reflec- tions, a different succession of stratigraphie reflec- tors on either side of the fault and an offset of the corresponding seismic reflectors from one side to the other. The transparent zone may be wide or nar- row (<1 km). On the studied lines, large transparent zones without reflectors are recognized in Panel 7 and Panel 8. In this respect, seismic lines and cross-sections, are not ideal for the analysis of strike slip tectonics. Strike-slip faults are best analyzed on the horizontal plane of geological or geomorphological maps, which are natural sections at a high angle and contain the dominant displacement vector. Strike- slip faults may produce a series of characteristic fea- tures on seismic sections such as the well known positive and negative flower structures. Positive iden- tification of such structures is not easy, however, and would ideally require a three-dimensional survey or at least a series of lines across the same fault zone. 3.3.4. Examples illustrated by seismic profiles Some seismic lines cross major mappable Jura tear fault zones, e.g., the Morez (France), Mouthe (France), Pontarlier (France-Switzerland), Mt Chamblon-Treycovagnes (Canton Vaud), La Tourne and La Ferrière-Vue des Alpes (Canton Neuchâtel) zones (Fig. 3.19; Panel 3, 5, 6, 7 and 8). On seismic lines, it can be seen that these faults affect the whole Cenozoic and Mesozoic layers of the Jura and Molasse Basin cover. In the Canton Vaud, seismic data, crossing the southern part of the major Pontarlier tear fault, are of high quality on both sides of the fault (Panel 7). Section 46 (Panel 7) displays a succession of well layered reflectors, whose stratigraphie interpretation is well constrained laterally. From one side to the other, stratigraphie thickness changes are observable within the Dogger - Liassic beds and within the Triassic Unit 2 layer. The latter is thicker on the western side of the fault, which may explain the higher elevation for the same beds on the western side of the fault. Section 50 (Panel 7) presents also a succession of layered reflectors on both sides of the tear fault. They are unfortunately not well constrai- ned by other seismic lines (Figs. 1.4 and 4.1). The attempted interpretation shows a thickening of the Malm and the Triassic Unit 2 layers on the western side of the fault. Further north, in the Lake Joux area (Panel 8), the quality of seismic data along the Pontarlier tear fault 105 Figure 3.20: a) Location of tear faults on Jura anticline map drawn by Heim ( 1915). b) Geomorphological evidence of tear faults in the central Jura. a) Situation géographique des décrochements sur une carte des axes des anticlinaux de Heim (1915) de la chaîne du Jura. b) Evidence géomorphologique de décrochements dans le Jura central. 3. Structures is poor. Unfortunately the seismic lines end close to the fault. In this case, no stratigraphie correlation is possible on the western side of the fault. The Sarraz tear fault forms the conjugate dextral system to the sinistral Pontarlier fault and is displayed on Section 26 (intersection 36, Panel 4) and on Section 36 (intersection 26, Panel 6). Along the Pontarlier tear fault, seismic data do not give evidence for base- ment offsets. The tear fault appears to terminate in the décollement level of Triassic Unit 2. The Yverdon area (Figs. 1.2, 1.4 and 4.3) is ano- ther region with conjugate tear faults (Jordi, 1993). Recently, a careful structural investigation (CHYN, 1995) for hydrogeological purposes has led to a detailed interpretation of seismic lines in this region (Section 27 on Panel 5; Section 38 on Panel 6). The regional setting and results will be discussed in Chapter 4. These faults appear to be tear faults and thrust faults. The associated structure is a fold rela- ted to a north ward-vergent thrust, cutting the frontal fold limb (Fig. 4.18). Thickness changes within the Dogger beds are observable, showing a thickening towards the South. This local change (not clearly visible on the regional isopach maps of the western Molasse Basin, Fig. 2.26) may be due to lateral facies changes. In the Neuchâtel area, seismic data along tear fault zones of La Tourne (Section 7 on Panel 1 and Section 4 on Panel 3) and La Ferrière-Vue des Alpes (Section 3, Panel 1) are of poor quality, probably due to the presence of an anticline on one side of the faults. These faults appear on seismic lines as trans- parent zones. No offset of the basement top on either side of the fault could be detected from contour maps. Accordingly, these faults are either tear faults restricted to the cover or lateral ramps. No evidence for an extension of these faults into the basement could be found. 3.3.5. Description from outcrops Four major, N-S oriented tear faults are found within the study area (Fig. 3.20): Pontarlier, La Tourne, La Ferrière and Treycovagnes. All these faults have apparent sinistral offsets of a few hun- dred meters and their traces can be followed over several kilometers. Conjugate dextral faults e.g. La Sarraz, Mt-Aubert oriented 120° are associated with them. In addition to these large, map scale tear faults, an important number of minor faults are observed in the competent lithologies of the central Jura e.g. (Llyod, 1964). These minor faults have a less important throw (on the meter scale) and crosscut anticlines, but they are often too small to appear on geological maps. The poor outcrop quality, in many parts of the Jura, usually prohibits the direct obser- vation of the effect of such minor structures on geo- logical limits. In the Val de Ruz (Neuchâtel Canton), small scale (cm to dm offsets) striated faults are ubiquitous in limestones and most outcrops show a sufficient number (>20) of fault/slickenside pairs for statisti- cal determination of "paleo-stress" or strain axes directions. Tschanz & Sommaruga (1993) conduc- ted a kinematic analysis of fault/slickenside pairs using Angelier & Mecheler's (1977) right dihedra method. Similar observations have been made in other parts of the Jura and have led different authors (Droxler & Schaer, 1979; Laubscher, 1979; Pfiffner, 1990; Tschanz, 1990) to independently draw a comparable schematic diagram of the rela- tion between faults and folds (Fig. 3.21). Strike-slip and thrust faulting occurs during the early stages of folding and is interpreted as due to large parallel shortening preceding folding. During fold amplifi- cation, these earlier faults are passively rotated with the fold limbs and partly reactivated to accommo- date internal deformation related to folding. Fault/slickenslide analyses from the vicinity of large strike-slip movement zones indicate systemati- cally subhorizontal movement directions, despite variable bedding orientations. Horizontal striations on vertical fold limbs are frequently observed (e.g. La Tourne or La Ferrière-Vue des Alpes) and clearly indicate that some fault motion post-dates folding. Local maximum compression axes, determined from fault/slickenside pairs and twin strain analyses, are systematically oriented subperpendicular to the local fold axis trend, but major discrepancies exist between this general NW-SE compression direction and the map scale fold axis trends. Fault/slickenside analyses preferentially measure late (with respect to folding) strain increments, due to the sampling of fault planes, because late, subver- tical tear faults are much easier to detect in the field than supposedly earlier, layer-parallel fault planes. The predominance of subhorizontal shortening and extension directions as determined from fault/slic- kenside pairs is partly due to this sampling effect. Nevertheless, the results show that NNW to NW directed shortening was active throughout the fol- 107 3. Structures Figure 3.21: Relation between faults and folds during amplification of a metric fold in limestone rocks. A) Layer parallel shortening before folding; B) Fold amplification, 30% shortening; C) 60% or more shortening. From Droxler & Schaer (1979). Relations géométriques entre faille el pli durant la mise en place d'un pli métrique dans des roches calcaires. A) Avant le plissement, lorsque le rac- courcissement est parallèle aux couches; B) Durant l'évolution du pli, lorsque 30% de rac- courcissement est ohser\'é; C) à 60% ou plus de raccourcissement. Tiré de Droxler & Schaer (1979). From Droxler & Schaer 1979 ding history. This shortening seems to be still active today, as evidenced by in situ stress determinations (Becker, 1987; Becker, 1989; Schaer et al, 1990; Becker & Werner, 1995) and focal mechanisms from earthquakes (Pavoni, 1984; Deichmann, 1992). According to Deichmann, the focal mecha- nisms are of strike-slip or normal fault type, indica- ting a regional shortening with a NNW-SSE orienta- tion, roughly perpendicular to the strike of the Alpine and Jura belt and a corresponding WSW- ENE extension parallel to the main axis of the Molasse Basin. Hypocenters below northern Switzerland are distributed throughout the entire depth range of the crust. This distribution of focal depths contrasts with what is observed below the Alps and in most other intracontinental settings. 3.3.6. Interpretation of the central Jura and Molasse Basin tear faults Major tear faults (from West to East: Vuache, Morez, Pontarlier and La Ferrière, Fig. 3.20) cut the Jura belt at angles of 60°-70° to the fold axes. Many minor faults are associated to the major ones. All these faults have an apparent sinistral movement and change from a NW-SE to N-S trend along the Jura arc. Conjugate dextral sets of tear faults are less developed, nevertheless, the Sarraz and the Treycovagnes faults are good examples. The major tear faults are located in the Haute Chaîne Jura and are connected to thrust planes at the transition Haute Chaîne - Plateau Jura. On seismic lines these faults represent important transparent zones, indicating that these faults are characterized by broad deforma- tion zones. Folds tend to terminate against tear faults and do not match from on side of the fault to the other (e.g. Pontarlier fault, Fig. 3.22). Aubert (1959), Laubscher (1965), Philippe (1995) and Schönborn (1995) have attempted correlation from one side of Pontarlier fault to the other. Each author has propo- sed a different correlation, requiring lengthy argu- ments to justify a far from obvious choice. Most probably, there is no match to be found because shortening was accommodated by different fold trains on either side of the fault. These observations lead to the interpretation of the Pontarlier fault as a primary tear fault, i.e. differential movements along this fault occurred during folding (Dahlstrom, 1970). Fault/slickenside outcrop observations on tear faults cutting anticlines show that striae dip is rarely bedding parallel on the vertical limb, but is mostly horizontal. The latter observation favors post-folding movement along the fault. Although offsets of geological limits appear to be mostly 108 3. Structures HRMfM Fault, tear fault 40 Window J Quaternary deposits I I Molasse | Cretaceous [•.'•I'll Kimmeridgian—Portlandian E¡3 Sequanian" ¡ '•' J Argovian" Dogger From Aubert 1959 Figure 3.22: Simplified geological map along the Pontarlier tear fault (see for location Figures 1.2 and 3.20). From AUBERT ( 1959). Carte géologique .simplifiée le long du décrochement de Pontarlier (pour localisation voir Figures 1.2 et 3.20). Tiré de Aubert (1959). 109 3. Structures MS ro ui uoisiAipqns MS ui iMi »s i o ui uoisiMpqns oes ui iMi i !•= B f- 2d «i — où « O Ç 3 — cr •; Iff-fl C S D c£ ° « Ui ¦a -S C „ — "* D .2 « ** O o c c/i ¦ — i/3 a= a U 3 3 -O JS « .5 h OC T3 P II .S 3 §¦8- C «á — otJ'C u T3 "O C 3 >» g C to 2 73 ¿ g .3» « eus s g C/, C ^3 XJ — 3 U D< C ¦s. 12 M O — o S? ,O c. <£ r. * B - .2 rf ¦o tu SI c3B « L. - i ? a t> .2" *N 1 ° 1 »4 .s ^ <»5 O IU •a rc <: *> "^ ¡« Ç -¾ ^ 11 J3 •5J" tu Q o ¡S ^ ft. If <5i 3 1J Ü.5» 3 ï «j o 5 S *âf ,11 o S» -S" ;r a K s ? K P I ï « s ¿ g.^..S1 •t ü ss :r "** Q ^ ti ? K. a¡ > 3 13 Z Si lu *¿* »J VU PN 110 3. Structures sinistral on the map scale, there is no objective way to determine absolute displacements along this tear fault, because folding occurred at least partly syn- faulting. It is therefore concluded, that the Jura tear faults show mainly syn-folding and post-folding movement. In this study, no evidence for deeply rooted strike- slip faults has been observed on the seismic sec- tions. Faults root in the main ductile zone (Triassic Unit 2) and apparently do not offset the underlying basement. Heim (1921, p.614) already suggested that tear faults rooted in the cover only. Consequently, these faults have been correctly named as tear faults, because they belong to an allochthonous sheet. 3.4. "REEF-LIKE" FEATURES As explained in preceding paragraphs, the Triassic Unit 2 interval varies in thickness and often presents oblique reflectors on the seismic lines. Some of these features have been interpreted as tec- tonic structures (duplexes, see Figure 3.8 and Fig. 3.12) just above the main décollement horizon. They show a thickening within the Triassic Unit 2, with folds in the overlying beds. However, other examples that also show oblique reflectors in the Triassic Unit 2 layer, are not associated with thicke- ning in the Triassic beds or buckling of the over- lying strata (Fig. 3.23); accordingly, they seem to be formed during Triassic times. The nature of these features is the subject of this section. These features resemble biogenic reef structures or algal mounds like those of Mississippian- Pennsylvanian age, found in carbonate-anhydrite facies from the Canadian Arctic Archipelago (Davies, 1977) (Fig. 3.23). The Zechstein salt facies also show this type of feature. Jenyon & Taylor (1987) and Taylor (1993) discuss the possible confusion of such features with buildups and pro- pose the terms "reef-like features" or "pseudo- reefs", respectively. According to Jenyon & Taylor, some of these positive features resting on basal Zechstein, consist of material of much higher velo- city than salt and may be bryozoan-algal reefs from their shape and location. However, it is equally pos- sible that they may be primary-or relict pods of anhydrite resulting from dissolution and removal of the surrounding salt. For Taylor, the mound-like fea- tures are related to swelling in Zechstein evaporite intervals. Virtually all are situated below former pillows in the overlying salt and many are related to faults. Careful seismic interpretation in the Zechstein basin reveals that the reflector defining the top of these structures is broken into segments, each of which is offset. In many cases, there is no velocity anomaly beneath these pods. Therefore the included material should have more or less the same velocity as the adjacent rocks. Pure salt (~ 4500 m/s) does not have the same velocity as carbonates (~5500 m/s to more than 6000 m/s), whereas anhy- drites may be in the same range of velocity as car- bonates. Algal mounds within the German Triassic facies are not known from field observations (A. Baud at the Musée de Géologie in Lausanne, oral communi- cation). However, only very few Triassic beds crop out in the Jura Mountains and none in the Molasse Basin. Even in areas where Triassic strata crop out, such as around the rim of the Vosges and Black Forest, evaporite (NaCl)-bearing series are excee- dingly rare at outcrop and always strongly perturbed by surface weathering. In the absence of high reso- lution seismic lines and with the sparse drill hole information available, it is impossible to discard any of these hypotheses. This issue is not without bea- ring on the hydrocarbon potential of the area: carbo- nate buildups within evaporite series, if present, may be interesting traps. Ill 4. REGIONAL GEOLOGY 4.1. INTRODUCTION In this Chapter the lateral continuity of structures in the central Jura and the Molasse Basin is discus- sed, supported by regional examples. The availabi- lity of a dense seismic grid (Fig. 4.1) has given the opportunity to follow, at large scale, the structures along their strike. Interpretation of dip and espe- cially strike seismic lines highlights some large scale continuities and/or discontinuities in the struc- tures. Rather than presenting lengthy details of folds, thrust faults and ramps,in order to discuss the lateral continuity of the central Jura structures, we will emphasize some characteristics of the Jura and Molasse Basin anticlines and synclines. This Chapter is subdivided into regions (Neuchâtel Jura, Risoux Jura, Champagnole-Mouthe region, western Swiss Molasse Basin; for location see Figures 1.3 and 1.4), because the structures turn out different from one area to another. 4.2. NEUCHÂTEL JURA 4.2.1. Previous studies Too many authors have worked on the structures of the Neuchâtel Jura to give due credit to all of them and cited here are but some of the major works that have contributed to a better understanding of the geology in Neuchâtel Canton. During the 19th century, pioneers of Jura geology are von Buch, Thurmann, De Montmollin, Desor and Gressly. Von Buch (1867) is best known for his cross-section of 1803 from the Alps to the Jura (see comments in §1.4) and also in the Neuchâtel Jura for his drawing showing the profile and the plunge of an anticline (Chaumont). Thurmann (1836a, 1856) made a morphological map of the whole Jura and De Montmollin (1839) published the first geo- logical map of the whole Neuchâtel Jura. Desor & Gressly (1859) presented a detailed 1:25'000 scale map. Other interesting contributions of authors wor- king for the second Neuchâtel Academy have been recently reviewed by Schaer (1994). Contributions on the Jura Mountains from the beginning of this century are collected in the review OfHEiM (1921), De Margerie (1922, 1936) and Bailey (1935). Key references for the Neuchâtel Jura are Schardt (1906), Schardt & Dubois (1903), Rickenbach (1925), Frei (1925, 1942), Thiébaud (1936), Kiraly (1969) and Meia (1969). More recently, published and unpublished works (Diploma, PhD thesis and maps deposited at the Neuchâtel University, see Tab. 1.1) have contributed to increase the knowledge of the surface and subsur- face geology of the Neuchâtel Jura. The cross-section of Schardt & Dubois (1903) (Fig. 4.2) is of particular interest for the Neuchâtel Jura, because it crosses the Creux du Van anticline and the Areuse syncline (Fig. 4.3). Beyond the fact that the Creux du Van anticline is formed over a foreland-vergent thrust, with a vertical northern limb, the top of the anticline shows an impressive steep cliff resulting from a glacier cirque develop- ment. For details about glacial deposits and the Quaternary history, the reader is referred to the appropriate literature (Ritter, 1888; Du Pasquier, 1893; Aubert, 1965; Matthey, 1971). 4.2.2, Eastern part The geology of the Neuchâtel Jura is shown on the tectonic sketch of Figure 4.3. The geological units of this map have been compiled from several 1:200'000 scale tectonic sketches of 1:25'000 map sheets (see Tab. 1.1). Many tear faults appear in the eastern part, in contrast to the West. This is in part due to the dif- ferent style of mapping from author to author. Many thrust faults have been added on this general map, following our interpretation of seismic lines, e.g. in front of the Chaumont anticline. At the surface, most thrust faults are capped by a veneer of Quaternary sediments and therefore it is sometimes difficult or impossible to locate them exactly in the field. In the eastern part of the Neuchâtel Jura, the tec- tonic sketch highlights thrust-related anticlines with 113 4. Regional geology 6°00' 7°00' o o, o I I Avants Monts \ Tabular Jura [~~\ Tertiary Basins I Faisceau Jura i rWJ Subalpine Molasse .___. > External Jura , . , ¡J Plateau Jura J ^_J Préalpes 1 1 Haute Chaîne / Internal Jura LZZJvariscan basement massifs Figure 4.1: Seismic grid located on a tectonic sketch of the central Jura and the western Molasse Basin. Legend: AM = Avants- Monts; Fb ¦ Faisceau bisontin; Fl ¦ Faisceau lédonien; Fsal = Faisceau salinois; FSy = Faisceau de Syam; PC = Plateau de Champagnole; PL = Plateau de Levier; Pl = Plateau lédonien; PO = Plateau d'Ornans. Grille sismique reportée sur la carte tectonique du Jura central et du Bassin molassique occidental. Légende: AM = Avants-Monts; Fb = Faisceau bisontin; Fl = Faisceau lédonien; Fsal = Faisceau salinois; FSy = Faisceau de Syam; PC = Plateau de Champagnole; PL = Plateau de Levier; Pl = Plateau lédonien; PO = Plateau d'Ornans. a lateral continuity of about 10 km. Anticlines appear to end against tear faults (La Tourne and La Ferrière faults) and have axes oriented either ENE- WSW or NNE-SSW. The combination of both orientations results in rhomb shaped structures such as the Val de Ruz basin and the Le Locle syncline. The relationships between both trends are enhanced on the three-dimensional structural contour map representing the base of the competent upper Malm layer (top "Argovian") in the Neuchâtel Canton (Fig. 4.4a). The rhomb shaped structure of the Val de Ruz basin, with sharp bends of more than 35°, has been analyzed at different scales by Tschanz & Sommaruga (1993) in order to better constrain the various possible models for its development. Detailed structural studies within the differently oriented portions of the anticlines show that ENE- WSE trending anticlines are formed as cylindrical folds, whereas the NNE-SSE trending anticlines are non-cylindrical, with important discrepancies of up to 30° between measurable strike (bedding and local 114 4. Regional geology Fi:;. ¡)ï. — Coni« du Crem Figure 4.4 meters ABOVE 1250 1000- 1250 750- 1000 500- 750 250- 500 0- 250 -250- 0 -500- -250 -750- -500 -1000- -750 -1250- -1000 -1500- -1250 BELOW -1500 4. Regional geology fold axes directions) and the map scale fold axes directions. The direction of map scale anticlines was determined from the geological maps of Neuchâtel and Val de Ruz at 1:25'000 scale (Bourquin el ai, 1968; Frei et al., 1974) and a compiled structural contour map. Local anticline directions have been determined from dip and azimuth data compiled from several maps and then plotted on stereograms in order to obtain a best fit local fold (Mancktelow, 1989, Stereoplot computer software). Tschanz & Sommaruga interpret cylindrical folds as folding above a frontal ramp and non-cylindrical folds as folding above an oblique ramp, with an overall transport direction to the NNW (335°). This inter- pretation is corroborated by the direction of paleo- stress axes determined from fault/slickenside pairs as well as from twin strain analyses. Local maxi- mum compression directions are invariably NNW- SSE to NW-SE oriented, regardless of the position within the rhomb shaped structure. Strike seismic profiles within the Val de Ruz basin are of excellent quality and show strong and flat reflectors with a good lateral continuity (Section 8, and eastern part of Section 4 on Panel 3). Seismic intervals have been defined on strike Section 8 as discussed in §2.4.1 (Fig 2.15). No major structural features are recognized in those two lines. The slight bends of the basement top reflector are interpreted as velocity pull-up beneath the anticlines and pull- down beneath the Val de Ruz syncline. Due to a strong increase in thickness of Tertiary and Quaternary sediments from NNW to SSE within the syncline (Mornod, 1970; Schnegg & Sommaruga, 1995) reflectors are pulled down in the southern part of the Val de Ruz basin. On the dip seismic lines crossing the Val de Ruz basin (Sections 1, 3, 5 on Panel 1), the changes in width of the syncline are clear: on the eastern line the syncline is already broad, in the middle it is the widest (Section 3) and then it narrows towards the West to finally end against the La Tourne tear fault. In this area, seismic data are of poor quality, probably due to the topo- graphic effects and/or the complexity of the struc- tures. Therefore surface geological data are required in order to obtain a clear understanding. Three major N-S tear faults have been mapped around the Val de Ruz syncline (Fig. 4.3): the La Tourne fault in the western part, the La Ferrière fault in the northern and central part and the Monruz fault in the south-eastern part. These faults, of kilometric length, have an apparent sinistral off- set of few hundred meters on the map and coincide (especially the La Ferrière fault) with the position where the mapped fold axes change direction. This suggests that these fracture zones have possibly pre- determined the position of oblique and/or lateral ramps and can thus be interpreted as faults reactiva- ted during the late Miocene folding of the Jura. Alternatively, these faults may have developed only during folding. Fault/slickenside analyses from the vicinity of tear faults indicate that these were active still after the major folding phase, since sub-hori- zontal striations are frequently found on faults cross-cutting vertical bedding planes (Tschanz & Sommaruga, 1993). Directions of paleo-stress axes for the past 10 Ma are indistinguishable from pre- sent day maximum horizontal stress directions. Seismic lines do not offer any evidence for inheri- ted or newly developed Oligocene tear faults, as sug- gested by Aubert (1972) and Bergerat (1987). On the seismic lines, described in Chapter 3, tear faults appear as transparent zones, thus explaining why the La Ferrière and La Tourne faults are not clearly reco- gnizable on Sections 3 and 7, respectively. Val de Ruz cross-section A geological cross-section (Fig. 4.5) based on surface dip data and completed at depth with infor- mation from seismic data, has been constructed parallel to seismic Section 8 (Panel 1) in the Val de Ruz area. This cross-section has been modified from that presented by Sommaruga & Burkhard (1997). The availability of a dense seismic grid has better constrained the depth to the basement, which now lies at greater depth in this modified version (Fig. 4.5). From SE to NW, the geological section crosses two major anticlines. These thrust-related anticlines, previously discussed in Chapter 3, formed firstly as detachment folds (or buckle folds) and then develo- ped into fault-propagation or fold-bend folds, thus forming a complex structure e.g. La Vue des Alpes anticline (Sommaruga & Burkhard, 1997, see map of Figure 4.3). The interpretation of the seismic line (Section 8) suggests the presence of a rather smooth thrust, separating a relatively uniform dip domain with gently SSE dipping layers in the footwall, from a more strongly folded and/or faulted hangingwall. A very good correlation between SSE-vergent kink folds at the surface and SSE vergent (blind?) thrust faults at depth is observed. The southernmost of these backthrusts seems to be located above the area where the NNW thrust branches off from the basal 118 4. Regional geology décollement zone. Other backthrusts are unrelated to bends in the main thrust fault. Further North, two more foreland vergent thrust faults have been postulated, based on surface geo- logy in combination with interpretations of seismic lines. The La Chaux de Fonds syncline is located at a high elevation possibly due to a thrust fault within the Triassic beds. Further North (1 or 2 km), an anticline is related to this thrust fault. The geological section (Fig. 4.5) crosses the La Ferrière fault which adds to the complexity of the broken up anticline near La Vue des Alpes. The extension of this fault at depth is unknown. In constructing the cross-section, this fault has been ignored, because no evidence for its presence has been found on seismic lines. If this tear fault repre- sents a post folding feature only, its overall apparent sinistral offset of ca. 500 m should be compensated by a stretching of the section by a similar amount. On seismic Section 8, however, reflectors on either side of the supposed fault trace at depth could be correlated without major offset, suggesting that the La Ferrière fault could be a superficial tear fault. 4.2.3. Western part The western part of the Neuchâtel Jura is located between two tear faults: the La Tourne fault to the East and the Pontarlier fault to the West. In its nor- thern part, this region is characterized by anticlines and synclines oriented NE-SW with a lateral conti- nuity of about 15 km to 20 km e.g. the La Brévine and Morteau synclines. The central part appears much more complex, as shown on the tectonic sketch (Fig. 4.3). A detailed map analysis has been conducted in the Travers region, in order to better visualize the relations between the structures. Since no 1:25'000 scale map of this region has been publi- shed yet, a compilation (Fig. 4.7a) has been made of several unpublished maps (Schardt & Dubois, 1903; Rickenbach, 1925; Thiébaud, 1936; Frei, 1942; De Pury, 1963; Meia, 1969; Frei et al, 1974; Meia, 1986; Müller, 1958). The Travers area is characterized by several large scale structures illustrated on the tectonic sketch of Figure 4.7b. To the North, the St-Sulpice- Trémalmont-Les Combes Derniers anticline forms a continuous, NE-SW oriented, feature with lower Malm and Dogger layers outcropping in the core. This structure grades northward into the narrow, flat bottomed La Brévine syncline. In the southern part of the area, the anticline is thrust towards the South over adjacent synclinal structures along a south-ver- gent fault surface. To the South, the Travers area is bordered by the NE-SW trending Nouvelle Cens i ère-Creux du Van-Montagne de Boudry anti- cline. This structure is large and flat topped in the SW (Nouvelle Censière), whereas in the NE erosion allows insight in the tighter anticlinal core (Creux du Van-Montagne de Boudry). Along the southern border of the Areuse valley, the core is related to a north-vergent thrust and represented by Dogger strata. In the central portion, this anticline is cut by a conjugate set of tear faults, consisting of a N-S oriented set with sinistral offset and a NW-SE orien- ted set with a dextral offset. This is a typical orienta- tion for tear faults in the central Jura. Between the two anticlines, a complex set of structures has deve- loped. The most prominent are, to the NE, the La Sagne syncline oriented NE-SW and, to the SW, the ENE-WSW trending Val de Travers syncline. Both synclines are broad, flat bottomed and covered by Tertiary and/or Quaternary sediments. They are also overridden both from the NW and the SE by south- vergent and north-vergent thrusts respectively. Along its northern limit, the Val de Travers syncline is bordered by two small synclines oriented NE-SW and oblique to the Val de Travers, but pinches out to the East. Further to the E, the very tight Areuse syn- cline, like the Val de Travers syncline, is overridden to the North and the South by south-vergent and north-vergent thrusts respectively. The southern thrust (along the northern limb of the Montagne de Boudry-Creux du Van anticline) extends eastward into the N-S oriented, left-lateral La Tourne tear fault. The very narrow width of the Areuse syncline is unusual for the Neuchâtel Jura, where synclines are generally quite large, as shown by the Val de Ruz-La Sagne-Le Locle Valleys (Fig. 4.3). Most remarkably, the Solmont anticline runs NE- SW in its eastern part, changing to a WNW-ESE orientation in its western part, where it separates the La Sagne and Val de Travers synclines. Its continua- tion is probably the Crei Pellaton anticline, whose central portion is cut by a fault. Thus between two continuous NE-SW oriented major anticlines, the southern of which is related to an foreland-vergent thrust fault and the northern to a hinterland-vergent fault, we observe a complex transfer zone, where displacement is relayed bet- ween two large en echelon synclines (La Sagne, Val de Travers). Its further noteworthy, that the La Tourne tear fault grades into a north-vergent thrust at the base of the Montagne de Boudry anticline. 119 4. Regional geology LU (J) 120 4. Regional geology m E...... í = O Ol £ m io fö uj I e w e j j eng 3 P I 3 J VI CD yer hes OJ re . E J= O C3 •§ LL 2 J= o 3 Cs- C £ 3 tu ^ 1? C 3 •v tj 3 Sj O > CQ a O C T3 ¦2 C •S so !" re ¡v. C e O w S "B ¿ a» 11 clii j=i ^ ì, nti ¦t" re C 3 Sa re .SP 1-3 C C s~ U U v: C 0 O 3 res ^. ft. a "- îP C ¦5 > O re 0 he sect Ui O U-, 1.1 H C 3 S 0 >y il VO tn ¦d- w G\ 3 *ì ?¡ U •H) £ sJ Vl ¡~. 3 Freí i ft. O U- ^j O 3 0 X sec ¦ U O1 .½ (A a: N^, k° O ^- ^ O »M 5 -J "re .t: gic D -StS olo S-s di re is ^ T3 0 'C 3 -¾) ft. .¾ J= V3 ^ § 5. jS "re y = a C D MS J^ 3 -¾ J= 3 ¦*' ^ -3 y O U S J= Fìg O 121 4. Regional geology 122 4. Regional geology -205- xx Cross section location Intermediate layer boundary j Tertiary (Molasse) I^ Cretaceous Purbeckian --> Cenomanian ] Upper Malm Sequanian --> Portlandian I Lower Malm Argovian ] Dogger Callovian Figure 4.7b: Tectonic sketch of the structural map of Travers presented in Figure 4.7a with location of cross-sections. Esquisse tectonique de la carte structurale de Travers, présentée dans la Figure 4.7a. Localisation de plusieurs coupes présentées dans ce travail. Figure 4.7a (page 122): Structural map of the Travers region. This map is a compilation and interpretation of several published or unpublished geological maps. See text for references. Small dots (SE Les Ponts-La Sagne valley and Lake Neuchâtel) correspond to end points of three unpublished cross-sections by Frei (1946). Carte structurale de ¡a région de Travers. Cette carte est une compilation et une interprétation de diverses cartes géologiques publiées ou non publiées. Voir le texte pour les références. Les petits points (au SE de la vallée Les Ponts-La Sagne et dans le lac de Neuchâtel) correspondent à la fin de la trace de trois coupes géologiques non publiées de Frei (1946). 123 4. Regional geology Thus, fold and thrust appear to be contemporaneous with the tear fault, at least in this specific case. Inspection of the tectonic sketch of the Neuchâtel Jura (Fig. 4.3) shows that transfer zones such as that of the Travers area are common in e.g. the Le Locle- La Brévine area or the NW of lake Biel region. However, the Travers area presents the best out- crops. Unfortunately the seismic line crossing the Creux du Van and Solmont anticlines (Section 9, Panel 2) is of poor quality, probably due to the steep topogra- phy and the karstification of the Jurassic limestones. The geology of the Travers area has been illustrated since the beginning of the century in cross-sections by Schardt & Dubois (1903) (Fig. 4.2) and Frei (1946) (Fig. 4.6). The first section oriented N-S, crosses the western part of the Solmont anticline, the Areuse syncline and the Creux du Van anticline. On this section, the latter appears related to a thrust fault, whereas the Solmont anticline does not. The unpublished cross-section by Frei (Fig. 4.6) shows a modern structural concept, in that anticlines are related to thrust faults which root in the Middle Triassic evaporite décollement zone. The top of the basement is flat, showing a minor bend beneath the southern limb of the Montagne de Boudry anticline. Its depth is much shallower than the depth to the top basement presented in the maps of Chapter 5. The results of the seismic interpretation from the wes- tern Val de Travers area have shown that the Dogger, Liassic and especially Triassic layers are much thicker than predicted from surface geology. The structure of the western region of the Travers area (Fig. 4.7b) is illustrated by two seismic lines (Sections Il and 13 on Panel 2, for location see also Figure 4.3) and by two geological cross-sections (Figs. 3.17b and 4.8). The cross-section of Figure 4.8, running from Lake Neuchâtel to Morteau (for location see Figures 4.3 and 4.9), is based on sur- face geological data and completed at depth with information from seismic lines. From SE to NW, this section crosses different types of structure: the Mt Aubert dextral tear fault system, the periclinal end of the Mt Aubert anticline, the Nouvelle Censière anticline, the Val de Travers syncline, the Mont de Couvet anticline, the Trémalmont anticline, the Brévine syncline, the Les Gras anticline and the Morteau syncline. All the large scale anticlines are thrust-related structures. The broad Nouvelle Censière thrust-related anticline has already been discussed in Chapter 3. From the interpretation of the seismic lines, duplication of the Jura cover beneath the anticline can be demonstrated. Dips of 35° towards the South are observed in the northern portion of the Val de Travers syncline, whereas the central portion dips gently to the S, extending far below the broad Nouvelle Censière. The Mont de Couvet area presents, at surface, two short wavelength anticlines related to a hinterland- vergent thrust fault (the southernmost of which cor- responds to the Corridor aux Loups anticline). Seismic lines (Sections 11 and 13) in this region present at depth a rather broad anticline, with thic- kening within the Triassic layers. Both these anti- clines and the steeper northern portion of the Val de Travers syncline are related to an important change in thickness of the Triassic series, which appears to originate at the point of development of a complex imbricate thrust fault system linked to the Mont de Couvet thrust fault. Further North, the Trémalmont anticline is related to a backthrust of the broad north-vergent Les Gras anticline that steps up over a ramp and flat thrust surface. This anticline also shows duplication of the Mesozoic cover series. The Triassic series is less important below the meridional portions of the Nouvelle Censière and Les Gras anticlines. The thickness of these layers increases toward the North of these structures, espe- cially beneath the Val de Travers and the Mont de Couvet, suggesting "flow" of the evaporites under the weight of the overriding broad anticlines. Thickness changes are also noticeable in Dogger - Liassic strata in the hangingwall of the Nouvelle Censière thrust-anticline, as well as south of the Mont Aubert tear fault; these are thicker compared to those in the footwall, indicating that strata thic- ken toward the South, in general agreement with the trend of Middle Jurassic facies. It must be emphasized that the northwestern part of the Neuchâtel Jura is characterized by synclines, that have a higher topographic elevation (e.g. La Brévine, Le Locle-La Chaux-de-Fonds, La Sagne synclines) compared to the southern synclines e.g. Val de Ruz and Val de Travers. This is well shown on the cross-sections of Figure 3.17 and Figure 4.5. Strike seismic Section 2, running parallel to La Brévine syncline, shows a succession of very strong flat reflectors, usefully constraining the seismic stra- tigraphy. At depth, duplication of the Triassic strati- graphy is observed, explaining the high elevation of the Jurassic, Cretaceous and Tertiary strata. The total thickness of Triassic is around 1000 m. In terms of balancing, the duplication of the Triassic beds represents an important shortening which must 124 4. Regional geology E o o O E C E o o O Uj 1 5 I I i i I I O) ¦ . H I Jr •(0 I M E I I J 05 .¾ ¦ Jb 7 /l r V/ f _i Z ¦ m ' 'VV. sr/ar vine .N QJ ? CD CO CD ---- . \ \ S S S \ \ N N S ' ¦ v-"-'v-' — 500 1km I Cretaceous I/ { I Malm I Dogger 'y| Basement Aalenian & Liassic Triassic Unit 1 & 2 c) Figure 4.10: Geological cross-sections of the transition from Faisceau Jura (Faisceau salinois) to Plateau Jura (Plateau d'Ornans). For location see Figure 4.9. a) Surface geological cross-section made by Dreyfuss (1960). b) Surface geological cross-section made by Aubert et al. in TrOmpy (1980). c) Deep cross-section based on surface data from Dreyfuss and Aubert et al. and subsurface interpretation of seismic lines. The sec- tion shows a thrust-related anticline in the Faisceau Jura and an evaporite-anticline or a pillow at the front of the high amplitude anti- cline. Coupes géologiques montrant la transition entre le Faisceau salinois et le Plateau d'Ornans. Pour la localisation, voir Figure 4.9. a) Coupe géologique de surface dessinée par Dreyfuss (1960). b) Coupe géologique de surface dessinée par Aubert et al. dans Trümpy (1980). c) Coupe géologique dessinée à partir des données de la surface de Dreyfuss et Aubert et al. et des interprétations des profils sis- nuques. La coupe montre dans le Faisceau jurassien un anticlinal en relation avec un chevauchement. Dans le Plateau jurassien, on observe un anticlinal d'évaporites ou un coussin d'évaporites au front d'un anticlinal de grande amplitude. 130 4. Regional geology 4.5. THE WESTERN MOLASSE BASIN 4.5.1. Previous studies Research on the stratigraphy and the sedimento- logy in the western Molasse Basin has a long his- tory. Lithostratigraphic logs (e.g. Fig. 2.9) have been established since the beginning of the century and detailed stratigraphie correlations along the Molasse Basin are not so straightforward, due to the lateral changes of facies and especially to the scarcity of outcrops (references concerning the stratigraphy of the western Molasse Basin have been cited in Chapter 2). Few structural studies of the Molasse Basin have been published, due to the scarcity and poor quality of outcrops and the extensive cover by ground moraine. Furthermore, the gentle dip of the bedding requires careful collection of much data to map structures. During World War II, many Molasse areas were precisely mapped for oil exploration, resulting in detailed structural maps (Althaus & Rickenbach, 1947, 1952; Schuppli, 1950), which show the main anticline and syncline structures and the major tear faults. Recently, the availability of modern industry seis- mic lines has allowed elucidation of the stratigraphy and the structures beneath the Tertiary sediments. Structural interpretations and a contour map of the base of the Molasse sediments have been presented by Jordi (1990, 1993) for the Yverdon-Lake Morat region, by Gorin et al (1993) for the Lausanne and Geneva Cantons region and by Signer & Gorin (1995) for the Geneva area. All these interpretations show that the Plateau Molasse consists of a weakly deformed Tertiary clastic wedge thinning towards the Jura belt and onlapping the top Mesozoic strata towards the N-NW. 4.5.2. Interpretations and contour maps Surface geological maps from the Plateau- and Subalpine Molasse (Althaus & Rickenbach, 1947; Schuppli, 1950; Berster, 1952; Vernet, 1972; Weidmann, 1988, 1992; Jordi, 1994) give an over- view of the major structures that involve the Cenozoic and Mesozoic strata i.e. tear faults, broad folds and thrust sheets. These features have also been recognized in the subsurface and are interpre- ted on Panels 4, 5, 6. Seismic lines are mostly loca- ted in the Plateau Molasse, except for two lines, Sections 43 and 45, which extend into the Subalpine Molasse, because of their greater length. In the Molasse Basin, Cenozoic strata are onlap- ping the Mesozoic beds. On most strike lines, they appear strictly parallel. In some dip lines, located in the southern Molasse Basin e.g. Section 43 on Panel 5, Tertiary sediments ontap clearly onto the under- lying strata. Onlaps have been observed mostly on south dipping limbs of structures. The interpretation of these onlaps is discussed in Chapter 3 and Chapter 5. At a large scale, the Mesozoic strata of the Plateau Molasse dip to the SE, as demonstrated by subcrop contour maps of the top of the Cretaceous, upper Malm, "Argovian", Dogger, Triassic Unit 1 and Triassic Unit 2 layers (Figs. 4.11 to 4.17) within the western Molasse Basin. These contours maps are based on depth conversion of the seismic lines and have been calibrated by well log data. Seismic velocities used for each seismic unit are presented in Appendices 3. Contours have been calculated by computer, using a bilinear interpolation method. Tear faults, as known from geological maps and sur- face data, as well as seismic interpretation, have been imposed as discontinuities in the contour cal- culations. As a result small zones due to insufficient data density (blank zones in the maps) appear during contouring in the vicinity of these faults. Furthermore, minor closed contours may in some cases also represent contouring artifacts. Despite these contouring artifacts, the complete original computer contoured maps are presented here, because they clearly and objectively show large scale regional trends. All these maps highlight a hinterland-dipping Mesozoic monocline with a NE-SW structural trend, except along the south-western and northern edges which are unfortunately not well constrained by data and are crossed by several faults. Cretaceous, upper Maim and Triassic Unit 1 depth maps clearly show a depression located between the two major tear faults (on the West Pontarlier fault, on the East La Sarraz fault). This structural low has already been noted by Gorin et al. (1993) on their schematic Base Molasse map. This depression is located exactly above a thin thickness of Triassic Unit 2 beds (com- pare Figure 4.11 and Figure 3.6) and therefore seems to be related to it. Other thinning or alternati- vely thickening shown on the isopach map of Triassic Unit 2 beds (Fig. 2.29, Fig. 3.6) does not seem to have any direct impact for the overlying unit maps. Of course the spacing (200 m) of the depth intervals in Figures 4.11 to 4.17 is too large to resolve structures involving 100, 150 or even 200 m thickness changes. 131 4. Regional geology Isopach maps of Triassic Unit 2 are presented in Figures 2.29 and 3.6. The first figure (Fig. 2.29) has been contoured automatically by computer and the second (Fig. 3.6) has been contoured by hand, but based on the same data set. Both maps highlight elongated or elliptical thickening or thinning of Triassic Unit 2 along a NE-SW trend, parallel to the general trend of the Jura fold belt. The maximal thickness coincides with the most internal Jura anticlines. Some major sinistral tear faults oriented NNW-SSE (Pontarlier fault) to N-S (Yverdon-Treycovagnes area) and the conjugate dextral system WNW-ESE (La Sarraz fault) cross- cut and offset these pillows. In the hand contoured map, computer artifacts have been neglected to produce better lateral continuity of structures and the position of geological surface structures has also been taken into account. Furthermore, the Top of the Cretaceous unit meters ABOVE 400 200- 400 0- 200 •200- 0 -400- -200 -600- -400 -800- -600 -1000- -800 -1200- -1000 -1400- -1200 -1600- -1400 -1800- -1600 -2000- -1800 -2200- -2000 -2400- -2200 BELOW -2400 185000- 180000- 175000- 170000- 165000- 160000- 155000- If)OCOOH 145000- 140000 560000 Figure 4.11: Contour map of the top Cretaceous unit in the western Molasse Basin. Depths are in meters. Black dots are shotpoints used for depth control of contour map. Note that blank areas and tight closed circles are computer-related artefacts in areas of sparse control. Numbers on the side and bottom are in meters, that relate to the Swiss national geographical coordinate system. Lakes Geneva and Neuchâtel are outlined for general reference. Carte structurale des contours du toit des couches du Crétacé dans le Bassin molassique occidental (profondeurs en mètres). Les points noirs représentent la position géographique des points de tir, utilisés pour le contrôle des profondeurs de la carte de contours. Les zones vides et les petits cercles fermés sont des artefacts, dus au programme de contourage, dans des régions où la densité des données est faible. Les coordonnées géographiques (en mètres) se réfèrent à la grille suisse. Les lacs Léman et de Neuchâtel permettent une localisation générale de la carte. Figure 4.12 (page 133, top): Contour map of the top upper Malm unit in the western Molasse Basin. Depths arc in meters. For explanation see legend of Figure 4.11. Carte structurale des contours du toit des couches du MaIm supérieur dans le Bassin molassique occidental (profondeurs en mètres/. Pour les explications, voir la légende de la Figure 4.11. Figure 4.13 (page 133, bottom): Contour map of the top "Argovian" unit (lower Malm) in the western Molasse Basin. Depths are in meters. For explanation see legend of Figure 4.11. Carte structurale des contours du toit des couches de ¡'"Argovien " (MaIm inférieur) dans le Bassin molassique occidental (profon- deurs en mètres). Pour les explications, voir la légende de la Figure 4.11. 132 4. Regional geology Top of the upper Malm unit ¦ 11111111111111111111111111111111111111 meters ABOVE •100 200- 400 0- 200 LI -200- 0 -400- -200 -600- -400 ¦i -800- -600 ¦ -1000- -800 WM -1200- -1000 ¦¦ -1400- -1200 ^M -1600- -1400 WM -1800- -1600 H -2000- -1800 -2200- -2000 -2400- -2200 H BELOW -2400 185000- 180000- 175000- 170000- 165000- 160000- 155000- ISOOQO- 140000- I I I I I I I I I I I I I I I I I I I I I I I I I I I I 1 I I I I I I I 510000 520000 530000 540000 550000 500001 ' Figure 4.12 Top of the "Argovian" (lower Malm) unit meters I I above -500- -700- -900- -1100- -1300- -1500- -1700- -1900- -2100- -2300- -2500- -2700- -2900- -3100- -3300- BELOW -300 -300 -500 -700 -900 -1100 -1300 -1500 •1700 -1900 -2100 -2300 -2500 -2700 -2900 -3100 -3300 i i i i i i i i I i i ¦ i i i i i i I i i i i i i i i i I i i i......1 i i i i i i i i i l 185000- 175000- 170000- 165000- 160000- 146000- 140000 i I i i i i i i i i i i i i i i i 510000 520000 I I I I I I I I I I I I I I I I I I I I I I I Ì ! I I I I I I I I I I I I 530000 540000 550000 560000 Figure 4.13 133 4. Regional geology contour intervals are closer (each 50 m). The broad folds from the Plateau Molasse, observed on these maps, are due to tectonic thickening of evapo- rites, salt and clays within the Triassic Unit 2 (see discussion in Chapter 3). The Subalpine Molasse is characterized by a stack of thrust sheets of Tertiary sediments deta- ched along an incompetent décollement zone at the base of the Oligocene layers ("Grisigen shales" formation or "Schistes à Meletta" formation; Trümpy, 1980). Unfortunately, seismic resolution is really poor within the Molasse and therefore does not allow clear observation of these struc- tures. Nevertheless, foreland-vergent thrust faults have been recognized on seismic lines crossing the Subalpine Molasse (Sections 43 and 45 on Panel 5; Plates 6 and 7). Beneath the Subalpine Molasse, the Mesozoic layers present small thrust faults. Furthermore, seismic Section 43 (on Panel 5) ? meters I ABOVE -300 -500- -300 -700 - -500 -900- -700 -1100- -900 -1300- -1100 -1500- -1300 -1700- -1500 -1900 - -1700 -2100- -1900 -2300- -2100 -2500 - -2300 -2700- -2500 -2900- -2700 -3100- -2900 -3300- -3100 BELOW -3300 185000- IKi)OOO • 175000- 170000 165000- IMOOO- ISSOOO-I 150000- 146000- M oooo Top of the Dogger unit I I I I I I I I I I I I I 1 I I I I I I I, L-I I I I I I I I I. I I I I I I I I I I I, 510000 I I i l i i l i l l i 530000 540000 i i i I I 1I i i i i i i i i I i i i ! i' 550000 560000 Figure 4.14: Contour map of the top Dogger unit in the western Molasse Basin. Depths are in meters. For explanation see legend of Figure 4.11. Carte structurale des contours du toit des couches du Dogger dans le Bassin molassique occidental (profondeurs en mètres). Pour les explications, voir la légende de la Figure 4.11. Figure 4.15 (page 135, top): Contour map of the top of the Liassic unit from the western Molasse Basin. Depths are in meters. For explanation see legend of Figure 4.11. Carte structurale des contours du toit des couches du Lias dans le Bassin molassique occidental (profondeurs en mètres). Pour les explications, voir la légende de la Figure 4.11. Figure 4.16 (page 135, bottom): Contour map of the top Triassic Unit 1 in the western Molasse Basin. Depths are in meters. For explanation see legend of Figure 4.11. Carte structurale des contours du toit des couches de l'Unité 1 du Trias dans le Bassin molassique occidental (profondeurs en mètres). Pour les explications, voir la légende de ¡a Figure 4.11. 134 4. Regional geology Top of the Liassic unit meters ¦ ABOVE -800- -1000- -1200- -1400- -1800- -2000- -2400- -3000- -3200- -3400- -3600- ¦ BELOW -600 -600 -800 -1000 -1200 -1400 -1600 •1800 -2000 -2200 -2400 -2600 -2800 -3000 -3200 •3400 -3600 185000- 180000- 175000- 170000- 165000-1 160000a 150000- 140000- 510000 560000 Figure 4.15 meters J ABOVE -1100 -1300- -1100 S " -1900- -1700 -2100- -1900 -2300- -2100 -2500--2300 -2700- -2500 -2900- -2700 •3100-2900 -3300- -3100 I -3500--3300 ¦ BELOW -3500 Top of the Triassic Unit 1 111111 » 11111111 » 111 » i......i.......iii 185000- 175000- 170000- 165000- 140000- 510000 i i i l I i i i i i i i i i I i i i i i 520000 530000 ' I i l i i l i i i i I 'l l l i i i i i i i l l l l I 540000 550000 560000 Figure 4.16 135 4. Regional geology Top of the Triassic Unit 2 i meters ABOVE •1400 -1600- -1400 1800- -1600 2000- -1800 2200- -2000 2400- -2200 2600- -2400 2800- -2600 3000- •2800 3200- -3000 3400- -3200 3600- -3400 3800- -3600 ¦ LOW -3800 I ' ' ' ' I----' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' M ' ' ' ' I 505000 510000 515000 520000 525000 530000 535000 540000 545000 550000 555000 560000 565000 Figure 4.17: Contour map of the top Triassic Unit 2 in the western Molasse Basin. Depths are in meters. For explanation see legend of Figure 4.11. Carte structurale des contours du toit des couches de l'Unité 2 du Trias dans le Bassin molassique occidental (profondeurs en mètres). Pour les explications, voir la légende de la Figure 4.1L shows Cenozoic and Mesozoic foreland dipping reflectors in its southernmost part. The latter may be interpreted to be linked, through a simple, bend to the south dipping reflectors further to the North in the Molasse Basin. The interpretation of this bend is difficult and is clearly related to a structure underlying the Triassic strata. One interpretation involves a slice of basement, related to a thrust fault rooting in the basement. This thrust fault may step up from the basement into the basal Triassic evapo- rites and then evolves as the main decoupling zone for the Jura fold and thrust belt (Mosar et al., 1996). Another hypothesis could be that of Gorin et al. (1993), who proposed an inverted Permo- Carboniferous graben and a third suggestion could be a thick evaporile pillow. All these interpretations have profound implications for the formation of the Jura foreland fold and thrust belt, which will be dis- cussed in Chapter 5. It should also be noted, that these structures occur at the end of a non migrated line that runs obliquely to the direction of local structures, cautioning against overinterpretations. 4.5.3. The Yverdon - Treycovagnes area 4.5.3.1. Previous studies The Yverdon-les-Bains area has attracted much attention, because of hot water springs. During the 1970's, Shell conducted a seismic survey (Swisspetrol) and drilled a well (Treycovagnes, see Appendix 2) for oil exploration (Figs. 1.4 and 4.1). Besides the wells drilled by oil industry during the 1940's (Cuarny, see Appendix 2), the 1960's (Essertines) and 1970's (Treycovagnes), many shal- low holes were made recently for geothermal pur- poses or for mineral water production (Burger & Gohran, 1986). The area has been the subject of several geologi- cal studies: the earliest were based on surface geo- 136 4. Regional geology logy (Renevier, 1854; Jordi, 1955) and later studies were based on surface geology observations and subsurface interpretations of seismic lines and wells (Jordi, 1990, 1993,1994,1995). The recent investigation of the Yverdon area for hydrogeological purposes (CHYN, 1995; Muralt et al, 1997) has led to a new detailed interpretation of seismic lines in this region which is presented below. 4.5.3.2. Geological setting The Yverdon-Treycovagnes area extends along the foot of the internal Jura, at the southern end of Lake Neuchâtel (Fig. 4.3). The region is dissected by two conjugate sets of tear faults that trend N-S (sinistral) and W-E to WNW-ESE (dextral). A com- plex dextral fault zone, the Pipechat-Chamblon- Chevressy fault zone (PCC, see Figure 4.3) (Jordi, 1993) with at least two distinct faults (North and South fault), is joined by several southward trending sinistral faults: one along the western border of Mont de Chamblon and a second from the Plaine de Baulmes towards the South. All these faults also have a reverse compressional component and are transpressive faults. The areas located SE of the junctions of the sinistral with the dextral faults consequently form NW-pointing "indenters". They are either uplifted above NW-vergent thrust faults (e.g. Chamblon) or depressed below SE-vergent backthrusts. Near Yverdon, major water sources are all located along these faults. 4.5.3.3. Interpretation The interpreted seismic lines (see Figure 4.1 for the seismic grid) were depth converted according to velocities presented in Appendices 3. Compilation and correlation of all data led to a well supported structural model of the Yverdon area (Fig. 4.19), except for the faulted zone along the dextral trans- pressive PCC-fault. Seismic reflectors within this zone are blurred and the sparse surface data have had to be projected laterally over large distances and downwards across décollement horizons. On all seismic lines crossing the PCC-fault, the reflectors dip gently south outside the faulted zone. Within the faulted zone, reflectors dip gently south on the westernmost line (Section 38 on Panel 6) and are subhorizontal on the next line to the East (Section 24, see Figure 1.4 for location). On both lines the reflectors within the fault zone are well displayed and thus the south-dipping faults on either side are constrained. Section 25, the next line to the east, cuts the fault zone at a highly oblique angle. The faulted zone is blurred, but a gentle northward dip of some Cretaceous-Malm reflectors can be inferred nonetheless. Since both faults have a com- pressional component and the hangingwall as well as footwall of the whole fault zone are clearly dis- played, the inter-fault zone can be established with some confidence, once its general dip direction is known. The resulting structure (Fig. 4.18) is a fault- related fold with two faults breaking through the frontal limb. A third thrust fault is inferred at depth to account for the gentle northern dip of the strata immediately to the north of the faulted zone. Seismic and drill hole data show thickness changes of various strata across the PCC fault zone. The Dogger sequence decreases from 420 m in the South to 290 m in the North (Fig. 4.18). This may be due to a Jurassic normal fault, reactivated during the Jura folding in late Miocene-Pliocene time. Reactivated faults often show tortuous geometries in detail, so local complexities must be expected. Such an inherited fault could also have influenced the orientation of the younger fault zone presently observed. Further, tectonic thickening of the ductile beds during the Miocene deformation cannot be excluded. Tn any case two different regions which were not juxtaposed during Jurassic sedimentation time, are today juxtaposed. The interpretation and the depth conversion of the seismic lines discussed above have led to a structural model of the top of the Dogger unit. Figure 4.19 represents a structural contour map of the Dogger in the Yverdon-Treycovagnes area. This figure highlights the structure of the Chamblon area, which appears to form a NW-pointing inden- ter uplifted above a NW vergent-thrust fault. The vergence of the structures along the PCC fault changes further towards the East, where the nor- thern side is uplifted above a steep southeast ver- gent thrust fault (Fig. 4.19). In conclusion, the Chamblon hill (Yverdon- Treycovagnes area) shows a north-vergent fault- related fold with two to three faults breaking through the frontal limb. The thrust faults sole out in the Triassic décollement zone, but there are also suggestions that the Alpine faults reactivate earlier, possibly Liassic normal faults. 137 4. Regional geology NW PCC fault zone SE -2000— -2500 1km ® o Dextral strike-slip motion ^J -I Tertiary I Cretaceous m Malm I I "Argovian" EZl Dogger Aalenian Liassic Triassic Unit 1 & 2 Basement Figure 4.18: Geological cross-section oriented NW-SE based on surface geological data and completed at depth with information from seismic Section 25, Section 27 (Panel 5) and Section 38 (Panel 6). This geological section crosses the Chamblon hill (for loca- tion see Figs. 4.3 and 4.9). Modified from Muralt et al. (1997). Coupe géologique orientée NW-SE construite à partir des données de la surface et complétée en profondeur par les informations des profils sismiques 25, 27 (Panneau 5) et 38 (Panneau 6). Cette coupe traverse la colline du Mont de Chamblon (pour localisation voir Figs. 4.3 et. 4.9). Modifiée de Muralt et al. (¡997). 138 4. Regional geology Figure 4.19: Contour map of the top Dogger in the Yverdon-Treycovagnes area, western Molasse Basin. Depths are in meters, below sea level. Numbers on the side and bottom are in kilometers that relate to the Swiss national geographical coordinate system. Modified from MURAure/ al. (1997). Carte structurale des contours du toit des couches Dogger dans la région d'Yverdon-Treycovagnes, Bassin molassique occidental (profondeurs en mètres, en-dessous du niveau de la mer). Les coordonnées géographiques (en kilomètres) se réfèrent à la grille suisse. Modifiée de Muralt et ai. (!997). 139 5. SYNTHESIS AND DISCUSSION The synthesis and discussion emphasize the follo- wing important points: - the boundary conditions such as the geometry of the basement top, the distribution of the very weak décollement zone - the geometry of structures in the Jura and the Molasse Basin - correlation between inherited structures in the basement and in the cover - the Molasse Basin, example of a foredeep basin - the link from the Jura to the Alps. Structure of the basement top The depth conversion of all seismic lines has allo- wed a mapping of the depth to top basement in the central Jura and the Molasse Basin (Fig. 5.1). In the Jura area, a simple velocity model, attributing a constant velocity to each major interval (Tertiary, Cretaceous, Jurassic and Triassic), has been used. In the Molasse Basin, however, more complex depth- dependent conversion functions from NAGRA (Naef & Diebold, 1990) were used in order to account for increased velocities due to the conside- rable thickening and facies changes of Tertiary sedi- ments (see Appendices 3 for seismic velocities)^ The contour map highlights a smooth basement that dips uniformly at Io to 3° to the SSE. Some broad irregularities in Figure 5.1, e.g. Treycovagnes, are better illustrated in the three-dimensional view of the top basement map (Fig. 5.2). An apparently flat area (with a high at Treycovagnes) at the SW edge of Lake Neuchâtel is visible. The Treycovagnes area has been analyzed carefully (see Chapter 4) and no major basement high appears on the interpreted seismic lines. Depths from well data (e.g. Treycovagnes, Laveron, Essavilly, Valempoulières) fit to +/-200 m the depths obtained from depth converted seismic time lines. Discrepancies may be due to the use of inappropriate seismic velocities, especially in the weathering layer (Schnegg & Sommaruga, 1995). However, seismic velocities used in this work are compatible with the overall well information, but do not satisfy precisely indivi- dual drill hole data. It seems that the velocity model used here lies on the fast side (i.e. seismically obtai- ned depths are slightly deeper than those obtained from drill holes). The trend of the basement is E-W in the Neuchâtel Jura and NE-SW or ENE-WSW in the other regions (Risoux, external Jura and Molasse Basin, Fig. 5.1). The E-W trend of the Neuchâtel Jura differs by an angle of 30° from the orientation of structures in the Jura fold and thrust belt. The contour map presented here is coherent with the crude map from Buchi & ETHZ (1981). Even if the values of the contour map are not absolute, Figures 5.1 and 5.2 clearly demonstrate the morphology and orientation of the basement structures, which are important in understanding the formation of the Jura foreland fold and thrust belt. It has to be emphasized, that the major tear faults (e.g. Pontarlier, La Sarraz) presented in Chapter 3 do not show any significant offset of the basement top. Along these faults, data are reasonably well constrained in the Molasse Basin and no major bend is visible (Fig. 5.1). The tear faults are therefore limited to the sedimentary cover of the Jura. In addi- tion, evaporite-related folds and thrust-related folds are floored by subhorizontal layers at their base. No evidence has been found (see description in Chapter 3) for construction of thrust faults downward into the pre-Triassic basement. Nowhere in the study area has this basal décollement layer been disrupted by later thrusts. Any irregularities which exist in the top of the basement are small compared to the thickness of Triassic Unit 2 (see Figure 3.6), which ranges from 100 m to more than 1000 m. On the south-eastern edge of the map (Fig. 5.1), near the front of the Subalpine Molasse, the basement dips toward the North showing an important uplift. The latter, high- lighted by only one seismic line (Section 43), has been already discussed in Chapter 4 (§4.5). This map shows a basement high resulting either from an inversion of a Permo-Carboniferous graben or else a basement slice. No reflector appears beneath the supposed basement top and therefore it is difficult 141 5. Synthesis 480 500 520 540 560 Figure 5.1 (pages 142,143): Map of the depth to top basement in the central Jura and western Molasse Basin. Depths are in meters with reference to the sea level. Depths next to drill holes correspond to top basement depth from drill hole data. Minimal depths (>...) are from drill holes which ended within the Triassic layers. No depth indication is given for drill holes which did not reach the Triassic. Coordinates (in km) are according to the Swiss national geographical coordinate system. a) Contours made by hand. Gray thin lines represent the seismic grid. Modified from Sommaruga (1995). b) Contours made automatically by computer. Compare with Figure 5.1a. Black dots correspond to location of depth control at shot points along seismic lines. Carie du toit du socle du Jura central et du Bassin molassique occidental. Les profondeurs sont exprimées en mètres par rapport au niveau de la mer. Les chiffres à côté des forages indiquent la profondeur du toit du socle dans le forage. Les profondeurs minimales (>...) sont indiquées pour les forages se terminant dans les couches du Trias. Aucune indication n'a été donnée pour les sondages n 'atteignant pas les séries du Trias. Les coordonnées (en km) correspondent à ¡a grille de référence géographique de la Suisse. a) Contourage interpolé à la main. Les fines lignes grises fines représentent la grille sismique. Modifié de Sommaruca (1995). h) Contourage automatique assisté par l'ordinateur. Comparer avec la Figure 5.1a. Les points noirs représentent la position géo- graphique des points de tir. utilisée pour le contrôle des profondeurs de ¡a carte de contours. 142 5. Synthesis i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i l i i ^i i i i i i i i ,1 i ^ Figure 5.1b: For legend seepage 142. Pour ¡a légende, voir page ¡42. med by well data (see Figs. 2.11 to 2.13), by sample analyses (Jordan, 1994) and also by interpretation of seismic lines. This very weak zone consists of salt, gypsum, anhydrite and clay, of which salt is by far the weakest lithology. In the Jura, thicknesses of pure salt range between 20 m and 300 m (Laveron drill hole). The Triassic Unit 2 zone is bound by two décollement horizons (roof and basal décollements). The style of deformation within this zone is very different from the overlying weak Triassic Unit 1 layer and the strong Jurassic layers, as well as from the underlying crystalline (or sedimentary) base- ment. In the Molasse Basin, the isopach map of Triassic Unit 2 (Fig. 3.6) clearly shows changes in thickness along a NE-SW trend. These changes in thickness contrast significantly with the smooth and planar top basement (Fig. 5.1), suggesting "ductile" defor- to interpret the nature of this high. It has also to be recalled, that Section 43 is a non migrated line run- ning at a low angle to the direction of local struc- tures and caution is required in interpretation. There is a problem related to seismic interpretation of fact or artifact. It should be noted that the presence, beneath the Jura fold and thrust belt, of a continuous basement dipping Io to 3° toward the South and underlying a weak décollement level was previously proposed by Buxtorp (1907) almost ninety years ago, from sub- crop observations along the Grenchenberg railway tunnel (see Figure 1.5, cross-section 2). Extension of the Triassic evaporites In the central Jura and the Molasse Basin, the pre- sence of the very weak (Fig. 2.30) Triassic Unit 2, at the base of the sedimentary cover has been confir- 143 5. Synthesis 144 5. Synthesis mation within Triassic Unit 2. In the eastern Jura, both, Triassic Unit 1 and Unit 2 layers seem to host décollement horizons. An isopach map of both Triassic Units in the Jura and its vicinity has been compiled in order to examine possible correlation between the distribution of Triassic strata and the extent of the deformed Jura cover (Fig. 2.5). A rather good correlation between both may be infer- red from Figure 2.5. The thickest regions corres- pond to the central Haute Chaîne Jura and to the southern part of the external Jura. Towards the East (e.g. German Molasse Basin), even where Triassic sequences are preserved, they do not contain evapo- rites (Bachmann et al., 1987). Towards the South- west, several drill holes have also demonstrated the absence of evaporitic layers (Philippe, 1994). It the- refore appears likely that the existence, as well as the arcuate shape of the Jura fold and thrust belt, is intimately linked to the presence of Triassic evapo- rites. Within the whole Molasse Basin, the change in structural style along strike, from a transported basin (western Molasse Basin) to an autochtonous basin (eastern Molasse Basin), is therefore due to the thinning and then absence of evaporite horizons. The presence of a thick and very weak basal layer beneath the Jura and the western Molasse Basin is thus important for the understanding of the forma- tion of this foreland fold and thrust belt. Characteristics of fold and thrust belts developed over a very weak basal décollement A common feature of foreland fold and thrust belts is the presence of a basal décollement surface or zone, which dips towards the hinterland and below which relatively little deformation occurs (Chapple, 1978; Davis & Engelder, 1985). The basal layer bounding the thin-skinned belt is gene- rally composed of particularly weak rocks, e.g. salt, evaporite or shale. The undeformed sediments below this layer often do not differ much in rheo- logy from those above the basal décollement and which have participated in the deformation. According to Chapple (1978), it is not the mechani- cal contrast between basement and cover but rather the presence of a weak layer that seems to determine the thin-skinned nature of such folded belts. Many fold and thrust belts around the world (e.g. Melville Island Fig. 3.13, Appalachian Plateau Fig. 3.14, Alberta and British Columbia Rocky Mountains,...) have developed above a weak basal layer (salt and/or evaporites and/or shales). A com- parison of these belts has allowed some authors e.g. Bally et al. (1966), Davis & Engelder (1985, 1987) to characterize these compressional terranes as broad belts with a low-angle cross-sectional taper, laterally continuous symmetric folds, broad synclines, anticlinal salt flow and forward as well as backward verging folds and thrusts. The critical taper is the cross-sectional wedge profile maintained when an entire thrust belt is on the verge of horizontal compressive failure. The magnitude of the angle between surface topography and the décollement surface of the critical taper of belt or accretionary prism is governed by the rela- tive magnitudes of the frictional resistance along the base and the compressive strength of the wedge material (Dahlen, 1990). Therefore, the contrast in competence between the basal décollement zone and the overlying cover series is responsible for the minimum (and maximum) permissible critical taper angle. The critical angle is therefore the sum of the angles of the décollement dip, which is towards the hinterland, and the topographic slope towards the foreland (Chapple, 1978; Davis & Engelder, 1985; Dahlen, 1990). The lowermost permissible critical taper angle determines the locus of the thrust front in any transport parallel cross section. Propagation of this front toward the foreland is achieved by thic- kening at the back of the thrust wedge which results in an increase of the topographic slope. Foreland fold and thrust belts riding above salt décollements typically have extremely low critical taper angles of less than Io (Dahlen et al., 1984; Davis & Engelder, 1985, 1987) and in these cases topographic slopes may be virtually absent. In such a low angle taper, internal deformation of the wedge may take place by symmetric foreland- or hinter- land-vergent thrusts. This is in contrast to higher angle tapers (commonly in excess of 8°), where a predominance of shallow foreland vergent thrusts is both predicted and observed (Davis & Engelder, 1985; Dahlen, 1990). Recently Philippe (1995) cal- culated a critical taper angle of 3,4° for the Jura fold and thrust belt, based on an angle of 1,6° for the topography and 1,8° for the basement. This is in good agreement with results herein, since an angle of Io to 2° for the basal décollement is observed on the top of basement map (Fig. 5.1). However, the topographic slope, when considered from the toe of the Jura to the crest of the Alps, does show a conspi- cuous hinterland-dipping portion located at the tran- sition from the Jura to the Molasse Basin. This hin- terland-dipping slope can be explained by the important glacial erosion that occurred during 145 5. Synthesis 1) Plateau Jura: evaporite-related fold mm/ Laveron8.5km WNW ce LEGEND -^N^>WVs^V>^s. Tops Cretaceous « Malm _-----mmm Dogger ¦¦¦¦¦—i— Liassic Triassic Uniti ### . Triassic Unit 2 Triassic Faults 2) Haute Chaîne Jura: thrust-related fold NVV SE 3) Molasse Basin: evaporite-related fold Essertines 6 km NE WNW ESE Figure 5.3: Line drawings of examples of typical fold structures in the Jura and the Molasse Basin (example numbers refer to Figure 3.1). Horizontal scale is in meters, whereas vertical scale is in seconds (two way travel time). Triassic Unit 2 is highlighted in gray. Examples 1 and 3 show low amplitude evaporite-related folds, whereas example 2 presents a high amplitude thrust-related fold. Modified from Sommaruga ( 1995). Exemples de plis dans le Jura central et le Bassin molassique (numéros d'exemples réfèrent à la Figure 3.1). L'échelle horizontale est en mètres, par contre l'échelle verticale est en secondes (temps double). L'Unité 2 du Trias est soulignée en grisé. Les exemples 1 et 3 montrent des plis de faible amplitude en relation avec des évaporites, par contre l'exemple 2 présente un pli en relation avec un chevauchement. Modifié de Sommaruga (1995). 146 5. Synthesis Pleistocene time in the Molasse Basin. Deformation behind this line is considerably less than in the Jura itself, apparently in contradiction with a simple foreland fold thrust belt wedge model which would predict increasing deformation intensity from the foreland to the hinterland. Perhaps the most outstan- ding feature of the Jura arc is its position at the out- ward rather than the inward side of the foreland basin, which is directly related to the Triassic evapo- rite distribution. Thrust-related folds in the Haute Chaîne Jura are either foreland- or hinterland-vergent (see Panels and descriptions in Chapter 3). Most evaporite-rela- ted folds in the Molasse Basin are symmetrical, but some folds show either a forward or backward ver- gent asymmetry. In the Plateau Jura, most of the broad folds end against tear or thrust faults and the- refore do present an aborted geometry; though some examples show entire symmetric folds (Panel 10). Anticlines and synclines have a lateral continuity over 30 km in the Risoux Jura, whereas in the Neuchâtel Jura the lateral continuity is limited to 10 km. In the Haute Chaîne Jura, synclines are mainly broad e.g. Val de Ruz, Vallée de la Sagne, but some exceptions exist (e.g. Areuse syncline in the Neuchâtel Jura). In conclusion, the Jura presents most of the cha- racteristics of fold and thrust belts developed above a weak basal layer. Evolution of the centrai Jura and Molasse Basin folds The surface geology and especially the seismic subsurface data of the central Jura and the Molasse Basin display two different types of folds: thrust-rela- ted folds and evaporite-related folds (Fig. 5.3). The first type is located in the Haute Chaîne Jura, whe- reas the second type has been observed not only in the Plateau Jura at the very front to the NE of the Haute Chaîne Jura folds, but also beneath the Molasse Basin. According to Bally et al. (1966) and critical taper models (Dahlen el al,. 1984) fold and thrust belts evolve by progressive deformation from the hinterland to the foreland. If we admit this consi- deration as a rule, the southernmost structures of the Jura fold and thrust belt should have formed first, to then have evolved into the present day structures. The northern folds should therefore represent a later stage of deformation, with less evolved geometries. This corresponds to what we have observed and inferred from the surface and especially subsurface data. The Plateau Jura evaporite-related folds (Panel 10) may also represent an early stage of the Jura folds. The folds would have been initiated as low amplitude, apparently symmetric, buckle folds in response to layer-parallel compression. The very weak rocks of the Triassic Unit 2 infilled the space generated at the base of the sedimentary cover by flow mechanism. The core of the folds present a thickening within the very weak basal zone, whe- reas the strong layers buckle without any change of thickness (concentric folding). With progressive deformation, a fault ramp nucleates in the hinge area at the base of the strong layers in order to accommodate further the strain. Fault ramps will then propagate upward within the stiff layers or bend their trajectory within overlying weak or incompetent layers. With further transport the cover may be doubled, as has been observed in the present day Haute Chaîne Jura folds (e.g. Mt-Risoux or Nouvelle Censière anticlines). The spacing between adjacent early stage buckle folds should be large, since in the Haute Chaîne Jura the formation of sub- sequent folds does not alter the geometry of the adjacent fold. In the northern part of the area, seismic data are poor and structures are only revealed with difficulty. Even if from geological maps no major anticlines can be inferred, it is possible that embryonic folds as described by Harrison (1995), i.e. early stage buckle folds may exist. Moreover, the Plateau Jura buckle folds are located at the front of the high amplitude Jura folds. The Laveron well drilled into one of the Plateau Jura low amplitude folds penetra- ted 1400 m of Triassic strata (300 m of rock salt). This seems rather thick for an early stage Jura buckle fold compared to the Molasse Basin folds. From the location of the Plateau Jura folds, it appears that part of the thickening may be due to salt flow resulting from the load beneath the thrust- related anticlines of the Haute Chaîne (e.g. Risoux). Folds beneath the Molasse Basin are located in a more internal position of the Alpine foreland belt than the Jura. The folds observed on the seismic lines represent typically early stage buckle folds and their cores seem to be filled with reasonably well organized evaporite duplexes, whereas those of the Plateau Jura suggest salt flow features. The question remains as to why the Molasse Basin folds did not further evolve into thrust-related folds as did the Haute Chaîne folds? The most obvious reason seems to be the load of Tertiary sediments. The Tertiary Molasse Basin has a wedge shape, with 147 5. Synthesis strong thickening of the sediments from North to South. Furthermore, the amplitude of folds within the Molasse Basin increases toward the Northeast, i.e. toward the Jura belt (e.g. Section 43 and Section 45). There may be two explanations for this: on the one hand, the thickness of Tertiary sediments decreases towards the North and on the other hand the thickness of Triassic sediments decreases slightly towards the South in the Molasse Basin (see Figure 2.22). With progressive erosion and deforma- tion of the Tertiary wedge, it can be supposed that the Molasse Basin anticlines may evolve into Jura folds supported by antiformal duplexes, such as is seen in examples from the eastern Jura (Noack, 1989; Laubscher, 1992). Inherited structures and their consequences The décollement at the base of a fold and thrust belt follows frequently the contact between the crys- talline basement and sedimentary cover (Buxtorf, 1907; Bally et al, Î966; Rodgers, 1990). Pre-exis- ting structures along this contact may form an important boundary condition and play a role in the location of thrust faults during the deformation of the sedimentary cover. Local irregularities in the basal décollement may act as stress concentrators to determine instabilities in the sedimentary cover and therefore act as a nucleation point for a thrust fault (Laubscher, 1977). In the eastern Jura fold and thrust belt, many authors have seen a relation bet- ween thrust nucleation and inherited structures e.g. Paleozoic graben system or Oligocene N-S oriented normal faults related to the Rhine-Bresse graben system (Laubscher, 1985; 1986; Naef & Diebold, 1990; Noack, 1995). In the western Molasse Basin, especially the Geneva area, recent work based on seismic data (Gorin et al., 1993; Signer & Gorin, 1995) highlights the presence of NE-SW and NW- SE trending Permo-Carboniferous lineaments which would have been reactivated several times until the present day. Further East in the Molasse Basin (Bern area), the Hermrigen anticline (for location see Figs. 1.3 and 1.4) is interpreted by some authors as a result of inversion tectonics, reactivating a Permo-Carboniferous graben (Pfiffner, 1994; Pfiffner et al., 1997a). This latter interpretation, based on one dip seismic line only, is however not well constrained and an alternate interpretation of an evaporite pillow in the Triassic beds, has been suggested by Erard (oral communication). In this study area, thickening in the Triassic Unit 2 and duplication of the Mesozoic cover represent the only clear tectonic features visible beneath the anticlines. Reflections attributed to Permo- Carboniferous sediments have been recognized beneath the Molasse Basin. In the Neuchâtel Jura, many reflectors, visible beneath the top of the base- ment, may represent either Permo-Carboniferous strata or multiples. Seismic data interpreted in this work do not present any positive evidence for inver- sion of Permo-Carboniferous grabens or for thrust fault nucleation related to inhomogeneities in the basement. Clear evidence for normal faults on seismic data has been observed within the Bavarian Molasse Basin (Bachmann et al., 1982; Bachmann et al., 1987), whereas in the Swiss Molasse Basin no dis- tinct evidence is visible. Early interpretations by Vollmayr & Wendt (1987) in the central Swiss Molasse Basin showed many normal faults of presu- mably Oligocene-Early Miocene age offsetting the whole Mesozoic cover. These authors later reconsi- dered their interpretation and replaced the normal faults by an embryonic thrust system, as illustrated and discussed by Laubscher (1992). However, nor- mal faults, confined below the Middle Muschelkalk Triassic layers, have been identified by Hauber (1993) from drill hole data in the Rhine valley of northern Switzerland. These faults have offsets of 50 m or less (Fig. 5.4) that which would corresponds to 0.01s or 0.02s TWT on the seismic lines (more or less one reflector). Unfortunately, the resolution of the seismic lines is too low to observe such faults. N-S oriented tear faults in the North Alpine fore- land are interpreted by many authors as inherited features related to the Oligocene opening of the Rhine-Bresse Graben system (Laubscher, 1973a; Elmohandes, 1981; Illies. 1981; Bergerat, 1987; Laubscher, 1992). Aubert (1972) was indeed able to identify N-S oriented karst crevasses with Oligocene Molasse infill in the Pontarlier region. Accordingly, Aubert (1972) postulated an Oligocene age for these N-S trending structures, without being specific about their tectonic signifi- cance. During Miocene folding and thrusting of the Jura, preexisting faults and joints represented major anisotropics within the Mesozoic cover. They were reactivated during and after folding and played an important role in localizing bends and discontinui- ties in folds during shortening deformation. The N- S orientation of preexisting faults and joints thereby induced a reactivation with sinistral transcurrent deformation, compatible with the overall N to NW directed Alpine push-(Laubscher, 1972). However, 148 5. Synthesis Figure 5.4: Geological cross-sections exhibiting normal faults within the Triassic layers calibrated from drill holes (Rhine Valley of northern Switzerland). Modified from Haubl:r (1993). Coupes géologiques montrant des failles normales au sein des couches du Trias, calibrées à partir de forages (Vallée du Rhin, Nord de ¡a Suisse). Modifié de Hauber (¡993). seismic lines in the central Jura do not present, in general, arguments in favor or against inherited faults. In the Treycovagnes area, minor thickness changes within the Liassic and Dogger beds are visible from one side of the fault to the other one (Muralt et al., 1997). These thickness changes may be due to Liassic synsedimentary normal faults, which may have been reactivated during the Miocene deformation. The Molasse Basin: aforedeep basin The wedge shaped Molasse Basin is presently considered to be a foreland basin. Foredeep or fore- land basins are defined as sedimentary basins, loca- ted between the front of a mountain chain (orogen) and the adjacent craton and developed in the context of a A-type subduction (Bally & Snelson, 1980). Price (1973) and Beaumont (1981) suggested that foredeep basins result from lithospheric flexuring in response to overthrust loading in the mountain chain. Foreland basins are characterized by a down- warp flexure, the foredeep basin, and an upwarp flexure or forebulge located in the foreland. Models predict an outward-migrating peripheral bulge, which is often responsible for a characteristic basal foredeep unconformity. Because the thrust load is inherently mobile, the foreland basin itself becomes eventualy involved in deformation. Dickinson (1974) was the first to define the North Alpine Tertiary Molasse Basin as a peripheral foreland basin located against the outer arc of the orogen. The Alpine Molasse Basin system extends from the Jura to the Prealps and even to the Helvetic nappes. Therefore the Jura and especially Plateau Molasse subsurface data interpreted in this work, represent only the distal parts of the North Alpine foreland basin system. According to Bally (1989, idealized foredeep Figure 4), foredeep basins pre- sent many common features and units. These are recognized on seismic lines crossing the Plateau Molasse: (1) a basement: a crystalline basement has been confirmed by drill holes in the Jura and has been inferred from seismic interpretation beneath the Molasse Basin Mesozoic strata; (2) a rifting 149 5. Synthesis en 'U) TS * S e ">< = LUTÒ CO O I I T x X X XNX / í" .' NXX yyy N x' /y y y%ry , /\ x V^ x y AhC^x ^'••^/'¦•f* x' \$^*X\\\ O \ rx N X W -¦' ^- ^ y y y y . (H* \ X N XNXX y y y y ¿ m OI ^s¿ y y y y .¦ XXXN O S ;î= «o i .•• y y y j XXSN SfSS; "An Boye y yyy, ¦-i a ^ / • / s 3 îsi > ¡y y s s ? í í * - ¡y yyy o (ímP¿s^ S SÍ Vr^T ¡y y y y 1982 i /íís'¡-|/ ss*-¦p /aî'/l XXNV C { t*~/*\ S S S f ¦S, tí VV L^ •' ' ' o> \*j±*r} XXX-' ra C S JÎRfVxA'v * « (M/l\'s\'+ Ea W££w- SI OT/-, vv CÜÍ ¦ 150 sequence: no real rift sequence has been identified on the seismic lines, although Permo-Carboniferous grabens present characteristics of extension tecto- nics; (3) a passive margin or platform sequence: during the Triassic an epicontinental shallow water environment developed and then during the Jurassic and Cretaceous interlayered marl and limestone beds were deposited in a platform or lagoonal envi- ronment. These strata, 1 to 3 km thick, overly the basement; (4) a deep water phase representing the inception of the foredeep: this early sequence consists of deposits located in the hinterland parts of the Molasse Basin. In Switzerland, the oldest depo- sits of the foreland basin are Flysch series, which are involved in the Helvetic nappes. Moreover the Molasse underlies the Prealpine klippen. This unit is not visible on the Plateau Molasse seismic lines; (5) a prograding sequence: the sequence of Tertiary shallow water Molasse sediments has been observed on the seismic lines. These five units are separated by basically diffe- rent unconformity types (see also Bally, 1989): (a) The pre-rift unconformity at the base of the Permo- Carboniferous grabens. (b) The breakup unconfor- mity at the base of the Mesozoic strata. Unconformities (a) and (b) are inherited and are not related directly to the formation of the foredeep. (c) The basal foredeep unconformity which marks the beginning of the flexural response to the Alpine loa- ding of the European plate. Subhorizontal Tertiary sediments onlap the Cretaceous (Mesozoic) strata dipping a few degrees (the same dip as the base- ment) toward the South. This unconformity is the most obvious on all Molasse Basin seismic lines. Unfortunately reflections are poor within this sequence and do not give the possibility to reco- gnize minor unconformities due to sea level changes. The possibility of interpreting the Tertiary sediment onlaps in relation to salt flow deformation has been discussed in Chapter 3. However it appears most reasonable to attribute them to the basal fore- deep unconformity. Evidence that the Molasse Basin represents a flexural basin has already been provided, based on stratigraphy, sedimentology, subsidence profiles and gravity anomaly studies (Lemcke, 1974; Karner & Watts, 1983; Naef et al, 1985; Homewood et al, 1986). Laubscher (1992) suggested that the fore- bulge developed during two different phases: the Helvetic phase (Late Oligocene-Early Miocene) and the Jura phase (Middle to Late Miocene). Recently Crampton & Allen (1995), using modeling work 5. Synthesis and surface outcrop data, proposed the existence of the forebulge flexure throughout the early evolution of the Alpine foreland basin, although its topogra- phic expression has changed over time. It is beyond the scope of this work to join this discussion. Further arguments of the evolution of the Molasse Basin and its structural relations with the Jura and the Alps are developed by Burkhard & Sommaruga (in press). The present work, neverthe- less, shows what was previously inferred, that the basement is homoclinal and can be followed with reasonable continuity from the foreland to the Alps beneath the foreland basins. The foreland basement (the Jura basement) is the shared link with the Alpine belt. Jura -Alps links The two fundamental questions, i.e. the cover- basement relationships within the Jura and the Molasse Basin and the compensation of the Jura cover shortening, were discussed in a short review in Chapter 1. The review shows that answers to both questions remain open and various authors still argue the possibility of shortening the basement beneath the Jura and the Molasse Basin, based on debatable seismic evidences (Ziegler, 1982; Guellec et al., 1990; Gorin et al, 1993; Pfiffner, 1994; Signer & Gorin, 1995; Pfiffner et al, 1997a). However, the interpretation of more than 1500 km of seismic reflection lines across all the Jura tecto- nic units (external Jura: Plateau and Faisceau; inter- nal Jura: Haute Chaîne) and also the Molasse Basin tectonic units (Plateau- and Subalpine-Molasse) has not revealed any obvious examples where cover structures can be related to observable, reliable deformation in the underlying basement. No defor- mation features are observed in the basement, ins- tead the overlying Triassic Unit 2 layers are defor- med, showing mainly evaporite swells. Any irregula- rities which might exist in the top of the basement (Fig. 5.1) are small compared to the thickness of Triassic Unit 2. This leads to the key conclusion that the Jura and Molasse Basin cover has been defor- med over a main décollement zone located in Triassic Unit 2. This conclusion corresponds to the "Fernschub theory" formulated at the beginning of the century by Buxtorf (1916). Nevertheless the link between the Jura fold and thrust belt and the Alps chain remains an item for discussion. Many authors have proposed various hypotheses (Fig. 5.5) based on field work, fission track data, balancing 151 5. Synthesis concepts and recently refraction or reflection seis- mic data (see also review by Burkhard, 1990; Laubscher, 1992). One hypothesis connects the Jura basal thrust to the basal Helvetic thrust and implies that uplift of the external crystalline massifs would post-date the Jura-thrusting (Laubscher, 1973b). The internal deformation within the exter- nal crystalline massifs would be explained by verti- cal pure shear (Marquer, 1990) or by differential isostatic uplift (Neugebauer et ai., 1980). Another hypothesis, first expressed by Boyer & Elliot (1982), states that the Jura basal thrust continues beneath the Molasse Basin and then roots in the frontal part of the external crystalline massifs. This hypothesis is the most widely accepted today, even if some authors see contrary seismic evidence. This work provides a map of the basement top showing a basement dipping gently towards the South under- neath the Jura and the Molasse Basin but does not provide new critical information beneath the exter- nal crystalline massifs. Therefore it is beyond the scope of this work to embark in a detailed discus- sion of the large scale relationship of the Jura - Alps system. The relationship of the Jura system to the deeper parts of the crust and the mantle has been elucidated by the results of the NFP20 research pro- jects (Pfiffner et a!., 1997b) and has been discus- sed recently by Laubscher (1992). 152 6. CONCLUSIONS Apart from some stratigraphie observations within the Jurassic and Cretaceous layers for which the reader can refer to Chapter 2, the interpretation and correlation of more than 1500 km seismic reflection lines has revealed some major structural elements, which have a considerable consequences for the understanding of the formation of the Jura foreland fold and thrust belt. This work has confirmed that the cover of the central Jura and the western Molasse Basin has been deformed over a smooth basement surface dipping Io to 3° to the S-SE, within the limited constraints of seismic resolution. This pre-Triassic basement is not significantly affected by the deformation of structures such as the folds, thrust faults and tear faults observed in the cover. The contrast in defor- mation between the basement and the cover is due to an unexpectedly thick and very weak décollement zone. This zone, composed essentially of evaporite and salt rocks belonging to seismic Triassic Unit 2 defined in this work (corresponding ± to Middle Triassic age), represents the lowermost seismic stra- tigraphie unit involved in the deformation of the cover. The regional thickness of this unit ranges from 200 m at the NW periphery of the Jura belt to more than 1000 m in the central part of the Jura, due to structural thickening. Thus Triassic evaporites control the development of the Jura and the Molasse Basin folds during the Late Miocene as well as the arcuate shape of the fold belt. The analyses of the geometry of structures within the central Jura and the Molasse Basin has shown two types of folds: 1) Evaporite-related folds, located in the Molasse Basin and the external Plateau Jura. The develop- ment of these low amplitude buckle folds is related to evaporite stacks within the Triassic Unit 2 layer. The axis of these folds is oriented NW-SE, parallel to the general trend of the Jura belt structures. 2) Thrust-related folds, located within the Haute Chaîne Jura. These high amplitude folds are related to NW- or SW-vergent thrusts of at least kilometric dipslip displacement, that results in a duplication of the entire Mesozoic sequence. These thrusts step up from the main décollement zone (Triassic Unit 2) through the entire Mesozoic and Cenozoic cover series. These two types of folds are respectively interpre- ted as embryonic and evolved stages of the Late Miocene cover deformation. The Haute Chaîne Jura folds began as buckle folds, to then evolve into the present thrust-related folds. In the Molasse Basin, however, low amplitude folds represent an early stage which did not evolve because of loading by the overlying thick Tertiary sedimentary wedge. Both types of folds show that the Triassic Unit 2 is clearly involved in the development of fold and thrust structures in the cover of the Jura and control- led their formation. Furthermore, the cover structures are also influen- ced by the rheology of the whole stratigraphie column. The increase of the thickness ratio between strong (competent) and weak (incompetent) layers, associated with an increase of the Mesozoic strati- graphic column from the north-eastern towards the south-western Jura, results in the development of low relief box-folds or detachment folds in the eas- tern Jura and high relief thrust-related folds in the western Jura. Some seismic lines cross major tear faults which, within the limit of seismic resolution, do not show any significant offset of the basement top from one side to the other. These faults are restricted to the cover and do not present evidence of an offset base- ment. In conclusion, interpretation of the seismic lines confirms the "Fernschub" hypothesis concerning the formation of the Jura: the Mesozoic cover of the Jura foreland fold and thrust belt and the Molasse Basin has been pushed to the NW over a main décollement zone. The seismic lines have not shown evidence for inverted Permo-Carboniferous grabens or thrust faults continuing downwards into the pre- Triassic basement. Therefore, the shortening of the 153 6. Conclusions basement is compensated, not beneath the Jura, but probably under the external crystalline massifs of the Alps. This work has provided the opportunity to com- plete the surface geological observations with sub- More than 1500 km of industry seismic reflection lines in the Neuchâtel and Vaud Jura of Switzerland, the French Jura and the Swiss western Molasse Basin have been interpreted. Through the seismic grid, constrained by drill hole data, each intersection of seismic profiles was controlled in order to obtain an internally consistent interpretation. These new data have shed new light on the stratigraphy of the buried Mesozoic layers and the defor- mation style of the subsurface structures from the Subalpine Molasse to the external Jura. Interpretation of the seismic lines demonstrates that the Mesozoic and Cenozoic cover of the Jura fold and thrust belt and the adjacent Molasse Basin has been deformed over a weak basal décollement and displaced for many kilometers toward the NW. Folding and thrusting took place above a very weak décollement zone within the thick seismic Triassic Unit 2 (Middle Triassic age) composed essentially of evaporites, salt and clays. The thickness of this interval ranges from 200 m at the NW periphery of the Jura belt to more than 1000 m in the central part of the Jura. The low amplitude broad folds of the Molasse Basin and the external Plateau Jura are controlled by evaporite pillows or anticlines within Triassic Unit 2. As seen on an isopach map, these structures are aligned along a NE-SW trend parallel to the general trend of the Jura fold and thrust belt. The high surface data and thus has allowed to present a regio- nal "three-dimensional view" of basement to cover relationships. Thanks to the subsurface data, new light has been shed on the formation of the Jura, which remains a classical evaporite foreland fold and thrust belt. amplitude folds of the Haute Chaîne Jura, however, are related to NW- or SE-vergent thrusts with at least kilo- metric dipslip displacement that result in doubling of the entire Jurassic sequence. These thrusts step up from the main décollement zone through the entire Mesozoic and Cenozoic cover series. The Plateau Jura evaporite-related folds located in the foreland are interpreted as early stage buckle folds. With progressive deformation a fault ramp nucleates and this fold type develops into thrust-related folds as observed in the Haute Chaîne. Thus, within the Jura fold and thrust belt, deformation increases toward the hinterland. In the Molasse Basin, however, low amplitude folds represent an early stage which could not develop further due to load of the overlying thick Tertiary clastic wedge. The depth to the basement map of the studied area derived from depth conversion of the seismic lines, shows a smooth flat basement dipping Io to 3° to the S- SE. No significant change in depth and trend of the base- ment top can be seen below major tear faults which affect the layers of the sedimentary cover. No structural relation can be detected, when comparing isopach map of Triassic Unit 2 series and the basement top contour map. A key conclusion is therefore, that basement is not involved in the formation of folds, thrusts and tear faults in the cen- tral Jura and the Molasse Basin. ABSTRACT Geology of the central Jura and the Molasse Basin: new insight into an evaporite-based foreland fold and thrust belt. 154 RESUME Geologie du Jura central et du Bassin molassique: nouveaux aspects d'une chaîne d'avant-pays plissée et décollée sur des couches d'évaporites. Plus de 1500 km de profils sismiques mis à disposition par l'industrie pétrolière, provenant du Jura suisse neu- châtelois et vaudois, du Jura français et du Bassin molas- sique occidental ont été interprétées. Chaque intersection du profil sismique a été contrôlée à travers le réseau de profil sismique et calibrée sur les données de forage, afin d'obtenir une interprétation cohérente. Ces nouvelles données ont apporté des connaissances plus approfondies sur la stratigraphie des couches mésozoïques et sur Ie style de déformation des structures de la subsurface depuis la région de la Molasse subalpine au Jura externe. L'interprétation des profils sismiques démontre, que la couverture mésozoïque et cénozoïque de la chaîne plissée et chevauchée du Jura et du Bassin molassique a été déformée au-dessus d'un décollement basai et a été déplacée de plusieurs kilomètres vers le NW. Les plis et les chevauchements se sont développés au- dessus d'une zone de décollement à faible viscosité, loca- lisée dans les couches du Trias moyen (Unité 2 du Trias) et composée essentiellement d'évaporites, de sel et d'ar- giles. L'épaisseur de cet intervalle varie entre 200 m à la périphérie NW de la chaîne du Jura et 1000 m dans la partie centrale du Jura. Les plis de faible amplitude du Bassin molassique et des Plateaux jurassiens sont contrô- lés par des coussins d'évaporites ou des anticlinaux d'évaporites au sein de l'Unité 2 des couches du Trias. Comme cela a été démontré sur une carte d'isopaques, ces structures sont alignées NE-SW parallèlement à la direction des structures majeures du Jura. Les plis de grande amplitude de la Haute Chaîne jurassienne sont, In questo lavoro sono state interpretate oltre 1500 km di linee sismiche acquisite dall'industria petrolifera nel Giura Svizzero (Cantone Vaud e Neuchâtel), nel Giura francese e nel Bacino occidentale della Molassa. La cor- relazione degli orizzonti da una linea sismica all'altra, attraverso tutta la maglia dei profili, ha reso possibile un'analisi strutturale a scala regionale, calibrata dalle informazioni stratigrafiche provenienti dai pozzi. Questi nuovi dati hanno permesso di approfondire le conoscenze scientifiche sulla stratigrafia dei sedimenti mesozoici attualmente sepolti e sullo stile di deformazione delle strutture profonde, dalla regione della Molassa subalpina al Giura estemo. Dall'interpretazione delle linee sismiche emerge che la copertura mesozoica e cenozoica della Catena a pieghe del Giura e del Bacino della Molassa si sono deformate al cependant, en relation avec des chevauchements à ver- gence vers le NW ou le SE montrant un déplacement d'ordre kilométrique qui se traduit par le redoublement de toute la séquence jurassique. Ces chevauchements montent depuis la zone de décollement principale à tra- vers toute la série mésozoïque et cénozoïque. Les plis des Plateaux jurassiens, localisés dans Tavant-pays, sont interprétés comme des plis de flambage correspondant à un stade précoce de la déformation. En augmentant la déformation, une rampe se génère et ce type de pli se développe associé à un chevauchement comme ceux observés dans la Haute Chaîne. La déformation aug- mente, donc, vers l'arrière-pays au sein de la chaîne plis- sée et chevauchée du Jura. Dans le Bassin molassique, les plis de faible amplitude représentent, cependant, un stade précoce, qui n'a pas pu évoluer à cause de la surcharge de sédiments tertiaires. Une carte du toit du socle de la région étudiée a pu être réalisée grâce à la conversion en profondeur des profils sismiques. Cette carte montre un socle plat et lisse plon- geant de Io à 3° vers le S-SE. Aucun changement impor- tant de profondeur et de direction, qui affecterait les couches de la couverture sedimentale, n'a pu être décelé sous les décrochements principaux. Aucune relation structurale n'a pu être déduite entre la carte d'isopaches de l'Unité 2 du Trias et celle des contours du toit du socle. Le socle n'est donc pas impliqué dans la formation des plis, des chevauchements et des décrochements du Jura central et du Bassin molassique. di sopra di un livello di scollamento basale e sono state dislocate per diversi chilometri verso NW. Le pieghe e gli accavallamenti si sono sviluppati al tetto di una zona di scollamento molto duttile, localizzata entro l'Unità 2 del Triassico medio e costituita soprattutto da evaporiti (prevalentemente sale) ed argille. Lo spessore di questa unità varia da 200 m, alla perifieria NW del Giura, a 1000 m, nella parte centrale del Giura. Le pieghe che si sono formate nel Bacino della Molassa e negli altipiani del Giura ("Plateaux jurassiens") hanno bassa ampiezza a causa della presenza di cuscini ed anticlinali di evaporiti nell'Unita 2 del Triassico. Come si può notare dalle carte delle isopache, queste strutture hanno la stessa direzione delle strutture principali del Giura, allineate NE-SW. Nella parte alta della Catena del Giura ("Haute Chaîne") le pieghe hanno un'ampiezza maggiore e sono RIASSUNTO Geologìa del Giura centrale e del Bacino della Molassa: nuovi sviluppi sulla conoscenza delPavampaese di una catena a falde e pieghe al tetto sulle evaporiti triassiche. 155 associate ad accavallamenti vergenti sia verso NW che SE, che causano uno spostamento superiore al chilometro ed il raddoppio di tutta la sequenza giurassica. Questi accavallamenti si originano lungo la zona di scollamanto principale e risalgono verso la superfice, interessando tutta la serie mesozoica e cenozoica. Le pieghe degli alti- piani del Giura ("Plateaux jurassiens"), che si trovano nella parte frontale della Catena, sono interpretate come pieghe di buckling formatesi durante uno stadio iniziale della deformazione. All'aumentare della deformazione si forma generalmente una rampa e le pieghe di buckling evolvono in pieghe associate ad un accavallamento. Simili pieghe sono state osservate nella Haute Chaîne e questo viene interpretato come indice dell'aumento della deformazione verso le zone interne della Catena. Le pie- ghe a piccola ampiezza osservate nel Bacino della Melassa rappresentano uno stadio precoce della deforma- Mehr als 1500 km seismische Reflexionsprofïle der Erdölindustrie aus dem neuenburger und dem waadtlän- der Jura, sowie aus dem französischen Jura und dem westlichen Schweizer Molasse Becken, wurden interpre- tiert. Um eine kohärente Gesamtinterpretation zu errei- chen, wurden die Schnittstellen der verschiedenen Profile im gesamten Netz kontrolliert und mit Bohrlochdaten korrelliert. Die daraus gewonnenen Daten haben es ermö- glicht neue Erkenntnisse über die Stratigraphie der nicht aufgeschlossenen mesozoischen Horizonte, sowie des Deformationstils zwischen der subalpinen Molasse und dem äußeren Jura zu gewinnen. Die Auswertung der seis- mischen Profile zeigt, daß die mesozoischen und käno- zoischen Deckschichten des Jura Faltengebirges und des angrenzenden Molasse Beckens über einer plastischen Basiszone abgeschert wurden und viele Kilometer nach NW überschoben wurden. Falten und Überschiebungen entstanden in dieser schwachen Abscherzone innerhalb der mächtigen "Trias Unit 2" (mittleres Trias), welche im wesentlichen aus Evaporiten, Salz und Tonen besteht. Die Mächtigkeit dieses Intervalls schwankt zwischen 200 m, am NW- Ende der Jurakette, und 1000 m im zentralen Teil des Jura Gebirges. Die Evaporit "pillows" oder Antiklinalen in der "Trias Unit 2" verursachen breite Falten mit schwachen Amplituden im Molasse Becken, sowie im äußeren Plateau Jura. Auf den Mächtigkeitskarten erkennt man daß die Strukturen NE-SW, parallel zum all- gemeinen Trend des Jura Deckengebirges streichen. Die Falten mit großen Amplituden im "Haute Chaîne" Jura, sind mit NW oder SW vergenten Überschiebungen ver- zione che non si è potuta evolvere ulteriormente, a causa del carico dei sedimenti terziari della Molassa. Nella regione studiata, la carta del tetto del basamento, compilata dalla conversione in profondità delle linee sis- miche, evidenzia un basamento con una superfice pla- nare, leggermente inclinata (da 1 a 3°) verso S-SE. Il basamento che si trova al di sotto delle zone in cui la copertura sedimentaria è notevolmente disturbata da faglie trascorrenti non mostra nessuna variazione struttu- rale significativa. Non si nota inoltre nessuna relazione strutturale tra la carta delle isopache dell'Unità 2 del Triassico e la carta del tetto del basamento. Importanti conclusioni di questo lavoro sono che il basamento non è coinvolto nel piegamento e nell'accavallamento che hanno prodotto la Catena del Giura ed inoltre che il basa- mento non risulta essere interessato dalle faglie trascor- renti del Giura centrale e del Bacino della Molassa. bunden. Die Überschiebungsbeträge im Kilometerbereich führen zu einer Verdopplung der gesamten jurassischen Serien, Die Überschiebungsflächen steigen von der Basisüberschiebung auf und durchschlagen die gesamten mesozoischen und känozoischen Deckensedimente. Die sich im Vorland befindenden, mit Evaporit ver- bundenen Falten werden als "buckle" Falten in einem frühen Entwicklungsstadium interpretiert. Mit anhalten- der Deformation und Verkürzung entsteht ein Rampenbruch und der sich entwickelnde Faltenstil ents- pricht einer Überschiebungs-verbundenen Falte, wie man es in der "Haute Chaîne" beobachten kann. Im Jura Faltengebirge ist die Deformation also größer in Richtung Hinterland. Im Molasse Becken dagegen, ents- prechen die Falten mit schwacher Amplitude einem frü- hen Stadium der Deformation, die sich wegen der darü- ber liegenden Mächtigkeit der tertiären Sedimente nicht weiter entwickeln konnte. Im untersuchten Gebiet zeigt die Tiefenkarte zur Oberfläche des Sockels, basierend auf Tiefenkonversionen von seismischen Linien, eine flache l°-3° nach S-SE hin geneigte Oberfläche. Unter den Blattverschiebungsflä- chen die die Deckschichten durchschneiden, gibt es keine Veränderungen im Streichen und in der Tiefe der Sockeloberfläche. Eine strukturelle Verbindung wird vom Vergleich der Mächtigkeitskarte der Trias Unit 2 und der Sockeloberfläche, ausgeschlossen. Die Schlußfolgerung daraus schließt eine Beteiligung des Sockels in der Entwicklung der Falten, Überschie- bungen und Blattverschiebungen im zentralen Jura und im Molasse Becken aus. ZUSAMMENFASSUNG Geologie des Zentraljuras und des Molassebeckens: Neue Erkenntnisse zu einem Evaporitverbundenen Vorland Uberschiebungs - und Faltengebirge. 156 APPENDICES APPENDIX 1 Thesis Numbering Survey Numbering Line trend British Petroleum, Sector A Neuchatel Jura 1 2 3 6 5 4 7 8 9 10 11 12 13 14 15 17 19 SW88-16 SW88-12 SW88-14 SW88-04W SW88-15 SW88-04E SW88-01 SW88-17 SW88-09 SW88-11 SW88-07 SW88-18 SW88-06 SW88-10 SW88-20 SW88-03 SW88-19 NW-SE, dip NE-SW, strike NW-SE, dip NE-SW, strike NW-SE, dip NE-SW, strike NW-SE, dip NE-SW, strike NW-SE, dip NE-SW, strike NW-SE, dip NE-SW, strike NW-SE, dip NE-SW, strike NW-SE, dip NW-SE, dip NW-SE, dip Shell Switzerland and SADH Sector B Molasse Basin 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 73 VD01 73VD04 73VD02-73VD34 74VD33 74VD35 74VD30 78SADH21 73VD05-74VD36-74SADH08 76SADH18 78SADH22 79SADH27 74VD31 72SADH07 74VD29 74VD38-74SADH09 74VD40 74SADH14 73VD06 75VD55 73VD07 76SADH15 74SADH11 73VD23 79SADH24 73VD21-73VD18 76SADH16 74SADH06-74VD52-N6 76SADH17 76VD57 76VD67 NE-SW, strike NW-SE, dip NE-SW, strike NW-SE, dip NE-SW NE-SW NE-SW NW-SE, dip NE-SW, strike NW-SE NE-SW NW-SE, dip NE-SW, strike NW-SE, dip NNE-SSW NW-SE, dip N-S NW-SE, dip N-S WNW-ESE NNW-SSE NW-SE NE-SW NW-SE NNE-SSW WNW-ESE NE-SW, strike NNW-SSE N-S NW-SE Appendix 1 (pages 157, 158): Inventory of seismic lines. Sector A, B, C3 D refers to Figure 1.4. Inventaire des profils sismiques. Se référer à la Figure .1.4 pour la définition des secteurs A, B, C, D. Appendices 50 76VD58 strike, NE-SW 51 76VD69 N-S 52 78VD81 NE-SW 53 73VD26 NW-SE, dip 54 78VD76 NE-SW 55 73VD24 NW-SE, dip 57 73VD19 NW-SE, dip 59 73VD22 NW-SE, dip 61 73VD25 NW-SE, dip 63 73VD14 NW-SE, dip 65 73VD20 NW-SE, dip 67 73VD15 NW-SE, dip 69 73VD16 NW-SE, dip 71 73VD17 NW-SE, dip 22-34- 73VD02-73VD34-74VD38-74SADH09- NNE-SSW 46-44 74SADH06-74VD52-N6-73VD18 NNE-SSW 35-37-39 74VD40-73VD06-73VD07 NW-SE 42-32-20-22 73VD23-72SADH07-73VD01-73VD02 NE-SW, strike 50-48-34-39 76VD58-76VD57-74SADH09-74VD38-{-73VD07} S-N after W-E Shell 80 74VD49 NE-SW, strike Sector C 81 74VD48 NW-SE, dip Mt-Risoux 82 74VD44 NE-SW, strike 83 74VD42 NW-SE, dip 84 73VD09-74VD53-73 VD10 NE-SW, strike 85 74VD45 NW-SE, dip 86 73VD12 NE-SW, strike 87 73VD08 NW-SE, dip 88 73VD13 NE-SW, strike 89 74VD46 NW-SE, dip 91 72M2 NW-SE, dip 93 74VD43 NW-SE, dip 95 74VD41 NW-SE, dip 111 CM7 NW-SE, dip 93-95-61 73VD43-73VD41-73VD25 NW-SE, dip 89-91-55 74VD46-72M2-73VD24 NW-SE, dip 111-85-87-91-55 CM7-74VD45-73VD08-72M2-73VD24 NW-SE, dip Shellrex 100 CM20-CM20N strike Sector D 101 CM13 dip Champagnole- 102 CM19Sud-CM19Nord strike Mouthe 103 CM1 dip 104 CM11-CM11Nord strike 105 CM42 dip 106 CM3Sud-CM3cenîre-CM3Nord (à double) strike 107 CM4 dip 108 CM5-CM5Sud-CMV7 strike 109 CM6 dip 110 CM10 strike 111 CM7 dip 112 CM2 strike 113 CM16Nord dip 115 CM16Sud dip 117 CM17 dip 119 CM18 dip 121 CM9 dip 123 CMV6 dip 125 CM14 dip Appendix I (pages 157,158): Legend on page 157. Légende à la page 157. 158 Appendices APPENDIX 2 exploration company exploration company 1) Courtion BP, I960 T.D. = 3083m 2) Tschugg KUS, 1976 T.D. = 704m elevation X y elevation X y 599m 572'415m 189'420m 463m 572'61Om 207'91Om top of... iopof... Formation depth elevation thickness Formation depth elevation thickness m m m m m m Tertiary 0 599 1322 Tertiary 0 463 509 Cretaceous -1322 -723 112 Cretaceous -509 -46 129 Malm -1434 -835 388 Jurassic -638 -175 66 Argo vi an -1822 -1223 216 T.D. -704 -241 Dogger -2038 -1439 400 Aalenian -2438 -1839 152 Schnegg 1992 Liassic -2590 -2690 -1991 -2091 100 180 Kcupcr MK dolom. -2870 -2271 63 4) Essertines SADH, 1963 T.D. = 2936m MK evap. -2933 -2334 150 T.D. -3083 -2484 elevation 660m X 539775m y I73'490m Fischerà Luterbachcr 1963 top of... Formation depth elevation thickness 3) Hermrigen Elf Aquitaine. 1982 T.D. = 2198m m m m Tertiary 0 660 337 elevation X y Cretaceous -337 323 194 480m 587'79Om 214'90Om Malm -531 129 501 Argovian -1032 -372 314 top of... Dogger -1346 -686 406 Formation depth elevation thickness Aalenian -1752 ' -1092 153 m m m Liassic -1905 -1245 397 Keupcr -2302 -1642 634 Tertiary 0 480 395 T.D. -2936 -2276 Cretaceous -395 85 8 Malm -403 77 436 Buchi et al. 1965b Argovian -839 -1078 -359 -598 239 380 Dogger Aalenian -1458 -978 92 5) Cuarny Vingerhoets, 1940 T.D. = 2229m Liassic -1550 -1070 177 Kcupcr -1727 -1247 280 elevation X y MK dolom. -2007 -1527 79 562m ' 543'54Om !80'38Om MK evap. -2086 -1606 112 TD. -2198 -1718 top of... Formation depth elevation thickness Housse 1982 m m m Tertiary Cretaceous 0 -480 562 82 480 240 Malm -720 -158 1240 Dogger T.D. -1960 -2229 -1398 -1667 269 Ahhaus& Rickenbach 1947 Appendix 2 (pages 159-162): Compilation of well data as found in the literature or in unpublished reports. Lithologies and abbre- viations are explained at the end of the table. Drilling company and year are mentioned for each hole. Total depth (T.D.) corresponds to the depth reached and corrected for deviations from vertical. Elevations are indicated above or below (-) sea level. Location coor- dinate X and Y refers to the Swiss coordinate system. All data are in meters. Données de forages compilées de la littérature ou de rapports non publiés. Les lithologies et les abréviations sont expliquées à la fin de cet annexe. Les compagnies eî l'année de forage sont mentionnées pour chaque puits. La profondeur (T.D.) correspond à la pro- fondeur maximaie atteinte et corrigée par rapport aux déviations de la verticale. Les altitudes sont indiquées par rapport au niveau de la mer. Les coordonnées X et Y se réfèrent au système suisse de coordonnées géographiques. Toutes les données sont en mètres. 159 Appendices 6) Treycovagnes Shell, 1978 T.D. = 322 Im elevation X y 7) Chapelle SADH, 1958 T.D.= 1540m 473m 536'135m 180'273m elevation X y lop of... 764m 547'305m 168'359m Formation depth elevation thickness m . m m top of... Cretaceous 0 473 177 Formation depth elevation thickness Malm -177 296 514 m m m Argo vi an -691 -218 203 Tertiary 0 764 1506 Dogger -894 -421 404 Cretaceous -1506 -742 25 Aalcnian -1298 -825 66 T.D. -1531 -767 Liassic -1364 -891 308 Keuper -1672 -1199 858 Lemcke 1959 MK doiom. -2530 -2560 -2057 -2087 30 121 MK evap. Bunisandstcin -2681 -2208 62 9) Lave ron PREPA, 1959 T.D. = 2485 Permian -2743 -2270 478 T.D. -3221 -2748 elevation 1080m X 503'00Om y 180'60Om Report deposited at the Musée géologique du Canton de Vaud in Lausanne; Schegg et al. 1997 top of... Formation depth m elevation in thickness m 8) Savigny SADH, I960 T.D. = 2486m Malm 0 1080 266 Raur. /Argov. -266 814 243 elevation X y Dogger -509 571 220 839m 546'271m I55'312m Aalenian -729 351 228 Liassic -957 123 118 top of ... Rhaelian / Keup> -1075 5 882 Formation depth elevation thickness Lettenkohlc -1957 -877 23 m m m MK dolom -1980 -900 63 Tertiary 0 839 2331 MK evap. -2043 -963 377 Cretaceous -2331 -1492 155 Buntsandstein -2420 •1340 65 T.D. -2486 -1647 T.D. -2485 -1405 Lemcke 1963 BRGM.Mouthc 1964 10) Risoux PREPA. I960 T.D. = 1958m ll)Eternoz T.D. = 2500m elevation X y elevation X y 1350m 5O0'3l0m 16l'020m 521m 491'800m 207'200m top of... top of... Formation depth elevation thickness Formation depth elevation thickness m m m m m m Upper Malm 0 1350 123 Bat h VB aj. 0 521 278 Argo vi an -123 1227 205 Aalenian -278 243 79 Dogger -328 1022 125 Liassic •357 164 196 Argo vi an -453 897 129 Keuper -553 -32 456 Dogger -582 768 160 Lettenkohle -1009 -488 18 Aalenian -742 608 15 MK dolom. -1027 -506 57 Lias -757 593 218 MK evap. -1084 -563 131 Lias -975 259 Buntsandstein -1215 -694 65 Malm -1234 116 265 Permian -1280 -759 1220 Argo vi an -1499 -149 417 T.D. -2500 -1979 Dogger -1916 -566 42 T.D. -1958 -608 BRGM, Quingey 1975 Winnock 1961 Appendix 2 (pages 159-162): Legend on page 159. Légende à la page 159. Ì60 Appendices 12) Essavilly SNPA, 1964 T.D. = 2067m elevaiion X >' 13) Toillon PREPA, T.D.= 1573m 795m 496'40Om 183'OOOm elevation X y top oi"... 844m 492'10Om I74'000m Formation depth elevation ¡hickness m m m (op of... Creiaceous 0 795 36 Formation depth elevation thickness Portlandian -36 759 359 m m m Raur./Callov. -395 400 279 Malm 0 844 180 Baj./Bath. -674 121 226 Argovian -180 664 240 Aaícnian -900 -105 102 Dogger -420 424 270 Lw AaI. / Lias. -1002 -207 333 Aalenian -690 154 225 RhVKeuper -1335 -540 327 Liassie -915 -71 193 LCU./MK dolom. -1662 -867 45 Keuper -!108 -264 465 MK e va p. -1707 -912 111 TD. -1573 -729 Buntsandsiein -1818 -1023 92 Permian tI9I0 -1115 46 BRGM. Champag noie 1965a Carboniferous -1956 -2024 -1161 -1229 68 43 Basement TD. -2067 -1272 16) Thésy T.D. = 1108m Cristalline rock BRGM.Champag noie 1965a elevation 703m X 484'20Om y I96'800m ères 1 PREPA. 1961 T.D.= 1421m 14) Valempoul 17) Saugeot T.D. = 1307m elevaiion X y 653m 481'40Om 186'500m elevation m X 476'00Om y 162'00Om 10p Ol'... depth elevation thickness Formaiion m m m 18) Salins-Les-Bains T.D. = 267m Dogger . 0 653 280 Keuper Aalentan/Lias. -280 373 190 elevation Keil per -470 183 370 347m Lciienkhole -840 -870 -187 -217 30 115 MK evap. Kenper -985 -332 120 ' 19)Buez T.D.= 1200m MK evap. -1105 -452 95 Banisaiidstein -1200 -547 72 elevation X y Permian -1272 -619 118 685 m 522'40Om 235'800m Basement -1390 -737 31 , T.D. -1421 -768 top of... ' ' Formation depth elevation thickness Biuerli 1972 î m m m Dogger 0 685 125 IS) Valempoulières 2 PREPA, 1962 T.D.= 1252m t Aalenian/Lias. -125 560 225 Rhactian -350 335 50 elevation X y Dogger -400 285 30 643m 480'60Om 186'00Om Aalenian/Lias. •430 255 250 Keuper -680 5 175 lop of... Lettenkohlc -855 -170 25 Formation depth elevation thickness MK dolom. -880 -195 25 m m m MKevap. -905 -220 200 RhVKeuper -466 177 361 Buntsandstein •1105 -420 75 Lettenkohlc -827 -184 208 Basement. •1180 -495 20 Middle Triassic -1035 -392 182 T.D. -1200 -515 Lower Triassic -1217 -574 35 TD. -1252 -609 Biuerli 1972 Bilierli 1972 Appendix 2 (pages 159-162): Legend on page 159. Légende à Ia page J 59. 161 Appendices 20) Humilly 2 T.D. = 3040m SNPA 1969 elevation X y 629m 480'50Om 108'25Om lop of... Formation depth elevation thickness m m m Tertiary 0 629 438 Cretaceous -438 191 374 Malm -812 -183 832 Argovian -1644 -1015 211 Dogger -1855 -!226 233 Aalenian -2088 -1459 28 Liassic -2116 -1487 410 Keuper -2526 -1897 383 MK evap. -2909 -2280 131 T.D. -3040 -2411 Persoz 1982, Wildieial. 1991. Jenny et al. 1995 Lit ho logics: Raur. = Rauracian; Argov. = Argovian; Lias. = Liassic; Bath. = tìathonian; Baj. = Bajocian Lett. = Lettenkohle: MK = Muschelkalk; MK dolom. - Muschelkalk dolomite: MK evap. - Muschelkalk evaporices Companies: PREPA = Société de Prospections, Recherches et Etudes Pétrolières en Alsace SNPA = Société Nationale des Pétroles d'Aquitaine SADH - Société anonyme des Hydrocarbures, Lausanne KUS = Konsortium Untertagespeicher RAP = Rcsie autonome des Pétroles Appendix 2 (pages 159-162): Legend on page 159. Légende à la page 159. 162 Appendices APPENDIX 3.1 Depth ¡n meters Depth in meters CO ¦o C O O CD V) CD E "CD > to >> CO O 5 O O O O O O O CJ O O O O O O O O O in O LO O in O m T- T- CN CN CO CO Tf Depth in meters Depth in meters to ¦o C O CJ CLi V) CD E "CD > JC CO o- 5 ¦o C O Ü CD V) CD > CO >> o 5 O O O O O O O O O O O O O O O O O m O m O m O in CJ CO CO CO ^t 4100 0.1 A "v.8» 3200 V Qi ^ 0.2 -\ 6250? 4444 -" -" 3750- Appendix 3.1: Seismic velocities deduced from correlation of seismic two way time interval (in seconds) and stratigraphie thick- nesses (in meters) based on well log data. Vitesses sis/niques déduites de la corrélation entre les intervalles sismiques (en temps double, en secondes) et les épaisseurs strati- graphiques (en mètres) basées sur les données de forages. 163 Appendices APPENDIX 3.2 A B B C C D E Neuchâtel Jura Molasse Basin interval Treycovagnes area interval Ri soux area CM. area BP Shell/SADH min-max veloc. Shell/SADH min-max veloc. Shell Shell rex 1988 1973-1976 1973-1976 1973-1974 1970-1974 Formation m/s m/s m/s m/s m/s m/s m/s Tertiary 2500 4119*2*0.133 2500-4300 3000 3000 . . Cretaceous 3500 4370+0.477 z 4370-5700 4370+0.477 z 4370-5000 - - Malm 4800 5015+0.393 z 5015-6217 5015+0.393 z 5015-5500 4600 4800 Argo vi an 4800 1864*zA0.12 3240-5042 1864*2*0.12 3931-4500 4600 . 4800 Dog. & Aal. 4800 4370+zA0.477 4418-6276 4800 4800 4600 4800 Liassic 4800 995*zA0.17 2178-4107 995*zA0.17 3300-3800 4600 4800 Keuper 5500 5435+0.184z 5453-6224 5500 5500 5500 5000 M K evap. 5500 5435+0.184z 5453-6252 5500 5500 5500 5000 A, B, C. D, E: refers to velocity sector in Appendix 3.3 *: multiplication function A: exponential function Appendix 3.2: Table of seismic velocities used in this work. In the Jura area a simple velocity model attributing a constant velocity to each major interval (Tertiary, Cretaceous, Jurassic and Triassic) was used. In the Molasse Basin, however, more complex depth- dependent conversion functions from NAGRA (Naef & Diebold, 1990) were used in order to account for increased velocities due to the considerable thickening and facies changes of Tertiary sediments. See also Appendix 3.3, Tableau des vitesses sismiques utilisées dans ce travail. Dans la région jurassienne, un modèle simple attribuant une vitesse constante à chaque intendile majeur (Tertiaire, Crétacé, Jurassique et Trias) a été appliqué. Dans ¡a région du Bassin molassique, on a utilisé un modèle plus complexe, nécessitant des fonctions qui tiennent compte de la profondeur des couches (Naef & Diebold, 1990, CEDRA). Voir aussi Annexe 3.3. 164 APPENDIX 3.3 Appendices i / b/ "7 p\A. \J w/ * y y3od5J ¿7 • .--' A i >^\ p><^4119 (2) I v X/ "t a k e -SrS B Geneva ^^\^ ____, J or Tertiary ~7—~^-J el inrii IH >5500 m/s ¦1 5000 m/s - 5500 m/s IH 4500 m/s -5000 m/s IH 4000 m/s - 4500 m/s IZZI 3500 m/s - 4000 m/s ZZ 3000 m/s-3500 m/s ZZl 2500 m/s -3000 m/s Appendix 3.3: Map showing the seismic velocities used for the depth conversion of the seismic lines. Carte montrant les vitesses sismiques utilisées pour la conversion en profondeur (en mètres) des profus sismiques (en secondes). 165 REFERENCES Allen, P.A., Homewood, P. & Williams, G.D. 1986. Foreland basins: an introduction. In: Allen, P.A. & Homewood, P.A. (eds.). Foreland basins, 8, 3-12. Spec. Pubi. int. Ass. sediment. Althaus, H.E. & Rickenbach, E. 1947. Erdölgeologische Untersuchungen in der Schweiz, I. Teil. Beitr. Geol Schweiz. Geotechi. Ser. 26, 88 pp. Althaus, H.E. & Rickenbach, E. 1952: Erdölgeologische Untersuchungen in der Schweiz, IV. Teil. 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Genèse des gisements d'asphalte de la Pierre Jaune de Neuchâtel et des calcaires urgoniens du Jura (Jura neu- chàtelois et nord-vaudois, Suisse). Thèse de Doctorat. Université de Neuchâtel 174 panels/panneaux: Remark: The reader will find here the captions of the Panels, which are fold outs of this Memoir. Remarque: Le lecteur trouvera ci-dessous la légende des Panneaux qui se trouvent hors-texte. Panel 1: Line drawings from migrated seismic lines of the Jura Mountains (Neuchâtel, Switzerland). Eastern dip lines. Sections 1,3, 5 and 7 in TWT time. Illustration des réflecteurs caractéristiques des profils sis- miques (migres) transversaux du Jura neuchâtelois oriental (Suisse). Profils 1, 3, 5 et 7 en temps double. Panel 2: Line drawings from migrated seismic lines of the Jura Mountains (Neuchâtel, Vaud, Switzerland). Western dip lines. Sections 9, 11, 13, 15, 17 and 19 in TWT time. Illustration des réflecteurs caractéristiques des profils sis- miques (migres) transversaux du Jura neuchâtelois occidental et vaudois (Suisse). Profils 9, 11, 13, 15 et 19 en temps double. Panel 3: Line drawings from migrated seismic lines of the Jura Mountains (Neuchâtel, Vaud, Switzerland). Strike lines. Sections 2, 4, 6, 8, 10, 12 and 14 in TWT time. Illustration des réflecteurs caractéristiques des profils sis- miques (migres) longitudinaux du Jura neuchâtelois et vaudois (Suisse). Profils 2, 4. 6, S. 10, 12 et 14 en temps double. Pane) 4: Line drawings of the Molasse Basin (Vaud, Switzerland). Strike lines. Sections 22, 26, 28 and 30 in TWT time. Illustration des réflecteurs caractéristiques des profils sis- miques longitudinaux du Bassin molassique (Vaud, Suisse). Profils 22, 26, 28 et 30 en temps double. Panel 5: Line drawings of the Molasse Basin (Vaud, Switzerland). Dip lines. Sections 27, 29, 37, 39, 43, 47s 45 and 49 in TWT time. Illustration des réflecteurs caractéristiques des profils sis- miques transversaux du Bassin molassique (Vaud, Suisse). Profils 27, 29, 37, 39, 43, 47, 45 et 49 en temps double. Plate IA: Uninterpreted, migrated seismic Section 1. Dip line, Neuchâtel Jura, Switzerland. Profil sismique 1 migré, non interprété. Profil transversal, Jura neuchâtelois, Suisse. Plate IB: Line drawing of Section 1. Dip line, Neuchâtel Jura, Switzerland. Illustration des réflecteurs caractéristiques du profil sismique I. Profil transversal, Jura Neuchâtelois, Suisse. Plate 2A: Uninterpreted, migrated seismic Section 3. Dip line, Neuchâtel Jura, Switzerland. See also Sommaruga & Burkhard (1997). Profil sismique 3 migré, non interprété. Profil transversale, Jura neuchâtelois, Suisse. Voir aussi Sommaruga & Burkhard (1997). Panel 6: Line drawings of the Molasse Basin (Vaud, Switzerland). N-S lines. Sections 34, 36, 38 and 48 in TWT time. Illustration des réflecteurs caractéristiques des profils sis- miques N-S du Bassin molassique (Vaud, Suisse). Profils 34, 36, 38 et 48 en temps double. Panel 7: Line drawings of the Molasse Basin. Pontarlier tear fault zone (Vaud, Switzerland). Strike lines. Sections 46, 50 and 54 in TWT time. Illustration des réflecteurs caractéristiques des profils sis- miques longitudinaux de la région de ¡a faille de Pontarlier (Vaud, Suisse). Profils 46, 50 et 54 en temps double. Panel 8: Line drawings of the Risoux, Jura Mountains (Vaud, Switzerland). Strike lines. Sections 80, 82, 84 and 86 in TWT time. Illustration des réflecteurs caractéristiques des profils sis- miques longitudinaux de la région du .Risoux (Jura vaudois, Suisse). Profils 80. 82. 84 et 86 en temps double. Panel 9: Line drawings of the Risoux, Jura Mountains (Vaud, Switzerland, France). Dip lines. Sections 53, 55, 61, 81, 83, 85, 87, 89, 91, 95 and 111 in TWT time. Illustration des réflecteurs caractéristiques des profils sis- miques transversaux de la région du Risoux (Jura vaudois, Suisse). Profils 53, 55, 61, 81, 83, 85, 87, 89, 91, 95 et 111 en temps double. Panel 10: Line drawings of the Champagnole-Mouthe area, Jura Mountains (France). Dip lines. Sections 107, 115, 117 in TWT time. Illustration des réflecteurs caractéristiques des profils sis- miques transversaux de la région de Champagnole-Mouthe (Jura français). Profils 107, 115. 1.17 en temps double. Plate 2B: Line drawing of Section 3. Dip line, Neuchâtel Jura, Switzerland. Illustration des réflecteurs caractéristiques du profil sismique 3. Profil transversal, Jura neuchâtelois, Suisse. Plate 3A: Uninterpreted, migrated seismic Section 8. Strike line, Neuchâtel Jura, Switzerland. See also Sommaruga & Burkhard (1997). Profil sismique 8 migré, non interprété. Profil longitudinal, Jura neuchâtelois, Suisse. Voir aussi Sommaruga & Burkhard (1997). Plate 3B: Line drawing of Section 8. Strike line, Neuchâtel Jura, Switzerland. Illustration des réflecteurs caractéristiques du profil sismique 8. Profil longitudinal, Jura neuchâtelois. Suisse. PLATES/PLANCHES : Remark: The reader will find here the captions of the Plates, which are fold outs of this Memoir. Remarque: Le lecteur trouvera ci-dessous la légende des Planches qui se trouvent hors-texte. 175 Plate 4A: Uninterpreted, migrated seismic section 11. Dip line, Neuchâtel and Vaud Jura, Switzerland. Profil sismique II migré, non interprété. Profil transversal. Jura neuchâtelois et vaudois, Suisse. Plate 4B: Line drawing of Section 11. Dip line, Neuchâtel and Vaud Jura, Switzerland. Illustration des réflecteurs caractéristiques du profil sismique 11. Profil transveral, Jura ncuclniiclois et vaudois, Suisse. Plate 5A: Uninterpreted, migrated seismic Section 14. Strike line, Neuchâtel and Vaud Jura, Switzerland. Profil sismique 14 migré, non interprété. Profil longitudinal. Jura neuchâtelois et vaudois. Suisse. Plate 5B: Line drawing of Section 14. Strike line, Neuchâtel and Vaud Jura, Switzerland. Illustration des réflecteurs caractéristiques du profil sismique 14. Profil longitudinal, Jura neuchâtelois et vaudois. Suisse. Plate 6A: Uninterpreted, seismic Section 43. Dip line. Molasse Basin, Vaud, Switzerland. See also Gorin et al. ( 1993). Profil sismique 43 non interprété. Profil transversal. Bassin molassique, Vaud. Suisse. Voir aussi Goris et al. (1993). Plate 6B: Line drawing of Section 43. Dip line. Molasse Basin, Vaud, Switzerland. Illustration des réflecteurs caractéristiques du profil sismique 43. Profil transversal, Bassin molassique, Vaud, Suisse. Plate 7A: Uninterpreted, seismic Section 45. Dip line. Molasse Basin. Vaud, Switzerland. Profil sismique 45 non interprété. Profil transversal. Bassin molassique, Vaud, Suisse. Plate 7B: Line drawing of Section 45. Dip line. Molasse Basin, Vaud, Switzerland. Illustration des réflecteurs caractéristiques du profil sismique 45. Profil transversal, Bassin molassique, Vaud, Suisse. Plate 8A: Uninterpreted, seismic Sections 111. 85 and 87. Dip lines. Jura, Vaud, Switzerland and France. Section 111 see also Bitterli(1972). Profils sismiques III, 85 et 87 non interprétés. Profils trans- versaux. Jura vaudois (Suisse) et France. Pour le profil III voir aussi Bitjf.ru (1972). Plate 8B: Line drawing of Section 111, 85 and 87. Dip line, Jura, Vaud, Switzerland and France. Illustration des réflecteurs caractéristiques des profils sis- miques III, 85 et 87. Profil transversal, Jura vaudois (Suisse) et français. Plate 9: Large scale balanced cross-section and restored section from the external Jura to the Alps (external crystalline massif) across the Molasse Basin. Data of the Moho are from Baumann (1994). Modified from Burkhard & Sommaruga (in press). Coupe équilibrée à grande échelle allant du Jura externe aux Alpes (massifs cristallins externes) en passant par le Bassin molassique. Les données du Moho sont de BAVMANN (1994). Coupe modifiée de Burkhard & Sommaruga (in press). 600' 700' I___I Tertiary Basins 1¾¾ Subalpine Molasse VA Préalpes [\\\ Variscan ^-^-1 basement massifs Jura fold belt I I Avants Monts \ Tabular 1-----' J Jura Faisceau Jura } External I Plateau Jura / Jura CU Haute Chaîne } l?{emai ----- J Jura f Thrust & Major unit contact Tear fault Seismic line 176 MÉMOIRES DE LA SOCIÉTÉ NEUCHÂTELOISE DES SCIENCES NATURELLES Tome I, 1835, 199 pp. (épuisé). Travaux de L. Agassiz, F-A. de Montmollin, A. de Montmollin, C. Nicolet, Ch. Allamand, J.F de Castella, J.-L. Borei, L. Coulon, J.-F. d'Osterwald, H. Ladame. Tome II, i 839, 283 pp. (épuisé). Travaux de C. Nicolet, Ch.-H. Godet, J.-J. Tschumi, L. Agassiz, C-L. Bonaparte, M. de Bosset, A. de Montmollin, A.-P. de Candolle. Tome III, 1845, 454 pp. (épuisé). Travaux de L. Lesquereux, L. Agassiz et C. Vogt, J. Marcou, J.-F. d'Osterwald, A. Guyot. TomeIV, Impartie, 1859,206pp. E. Desor & A. Gressly - Etudes géologiques sur le Jura neuchâtelois. Charles Kopp - Des variations du niveau du lac de Neuchâtel pendant les années 1835 à 1856. Tome IV, 2e partie, 1874, 225 pp. E. Desor & A. Gressly- Le bel âge du bronze lacustre en Suisse. P. de Loriol - Description de quelques Astérides du terrain néocomien des environs de Neuchâtel. P. de Loriol - Description de trois espèces d'Echinides appartenant à la famille des Cédéridées. M. de Tribolet - Recherches géologiques et paléontologiques dans le Jura neuchâtelois. Tome V, 1914,1090 pp. O. Fuhrmann & E. Mayor - Voyage d'exploration scientifique en Colombie. Tome VI, 1938,535 pp. G. Dubois - Monographie des Strigeida (Trematoda). Tome VII, 1943, 253 pp. (épuisé). D. Vouga - Préhistoire du Pays de Neuchâtel des origines aux Francs. Tome VIII, 1" fascicule, 1952, 121 pp. V. Aellen - Contribution à l'étude des Chiroptères du Cameroun. Tome VIII, 2e fascicule, 1953, 141 pp. G. Dubois - Systématique des Strigeida, complément de la monographie. Tome IX, 1958,202 pp. E. Mayor - Catalogue des Péronosporales, Taphrinales, Erysiphacées, Ustilaginales et Urédinales du canton de Neuchâtel. Tome X, 1er fascicule, 1968, p. 1-258. G. Dubois - Synopsis des Strigeidae et des Diplostomatidae (Trematoda). Tome X, 2e fascicule, 1970, p. 259-727. G. Dubois - Synopsis des Strigeidae et des Diplostomatidae (Trematoda). Tome XI, 1989,322 pp. J. Remane et ai. - Révision de l'étage Hauterivien (région-type et environs, Jura franco-suisse). Les Mémoires de la S.X.S.N. peuvent être obtenus à l'adresse suivante: Société neuchâteloise des Sciences naturelles, p. a. Bibliothèque publique et universitaire de la Ville, 2000 Neuchâtel (Suisse). PLATE 8B Line drawing location map Section 111-87 Jura fold belt I I Tertiary Basins | | Avants Monts j Tabular Jura WiYA Subalpine Molasse Jgg^ Faisceau Jura 1 r^—i ^ , , r-Tîm > External Jura rVI Prealpes IaMSfI Plateau Jura J Variscan basement massifs llll Plateau Jura 1 Haute Chaîne } Internai Jura r Thrust & Major unit contact Tear fault 30km LEGEND Tops Cretaceous rjm—*m MaIlTl Dogger Liassic Triassic Unit 1 Triassic Unit 2 Faults SECTION 111 NW Laveron 8.5km SECTION 85 SECTION 87 SE o.o — o CD 0.5 1.0 1.5 0.0 H — 0.5 H C/) CD O 1.0 1.5 MEM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII1 1997 PLATE 8A Seismic line 111-87 LOCATION MAP Jura fold belt I Tertiary Basins | I Avants Monts j Tabular Jura Subalpine Molasse Faisceau Jura K^l Préalpes External Jura vy] Variscan basement massifs lg|| Plateau Jura I | Haute Chaîne } Internai Jura ¿T* Thrust& * Major unit contact ( Tear fault 30km SECTION 111 2000m _, lOOOm Cross-section from BitterH 1972 SECTION 85 SECTION 87 Mt-Risoux riHHlÉIÉHHÉÉaiHhJ MÉM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII1 1997 PLATE 7B Line drawing Section 45 LOCATION MAP Tertiary Basins Subalpine Molasse KSH Préalpes Variscan basement massifs Jura fold belt I I Avants Monts j Tabular Jura fVVl Variscan U—Í—J Faisceau Jura Plateau Jura Haute Chaîne } Internal Jura > External Jura A M? Thrust & Major unit contact ( Tear fault 30km LEGEND Tops Cretaceous «t^-i iyialm ~- Dogger wj* Liassic • - Triassic Unit 1 .. Triassic Unit 2 Faults ESE \W Savigny SECTION 45 E o.o 0.5 — 1.0 H O) (D O 1.5 2.0 MEM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII, 1997 PLATE 7A Seismic Section 45 LOCATION MAP Jura fold belt Il Tertiary Basins | | Avants Monts j Tabular Jura £ííííj Subalpine Molasse Q| Faisceau Jura Plateau Jura t—i__C Préalpes Variscan basement massifs 3SZi _. . . f External Jura Haute Chaîne } Internai Jura A M? Thrust & Major unit contact ( Tear fault 30km SECTION 45 WNW H H O 0 2 km MÉM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII, 1997 PLATE 6B Line drawing location map Section 43 Jura fold belt I Tertiary Basins | | Avants Monts j Tabular Jura fcffi-j Subalpine Molasse K>i Préalpes Variscan 1 basement massifs Faisceau Jura . .. ... > External Jura ¿SI Plateau Jura J Haute Chaîne } Internal Jura Thrust & Major unit contact Tear fault 30km LEGEND Tops Cretaceous ~~— Malm Dogger ™ Liassic - Triassic Unit 1 . Triassic Unit 2 Faults MEM. SOC. NEUCHATEL. SCI. NAT., TOME XII, 1997 PLATE 6A Seismic Section 43 LOCATION MAP Jura fold belt I Tertiary Basins | | Avants Monts j Tabular Jura I Faisceau Jura Plateau Jura Haute Chaîne } Internal Jura 1¾¾ Subalpine Molasse h^-i Préalpes > External Jura |s/,/| Variscan basement massifs Thrust & Major unit contact Tear fault 30km SECTION 43 WNW 34 T H (D O 2 km MEM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII, 1997 PLATE 5A Seismic Section 14 LOCATION MAP A M? Thrust & Major unit contact Tear fault 30km l___jTertiary Basins féffi] Subalpine Molasse K^j Préalpes pr^ Variscan ^-^ basement massifs Jura fold belt I I Avants Monts } Tabular Jura [Hl Faisceau Jura 1 Jura J External Jura Wn Plateau Haute Chaîne } Internai Jura SECTION 14 SW Migrated^: *~__, .liW,« —!^¾**1***!^^ i^^^iW^iaiall^ailEiiNiA^i^ .....1V TJMMWtMrf^w" 'iM**jrtoà**m......„j-.'i'.IIWI__ ^"uMMiIWW ._M.fl"M I"in___'"'urn' ^1^^11-*^.!¾111½!-¾!- 1¾*-%M|'U LHl * "1WwIiJWMiIdWW^11JW*11 "1N1W* iS2g^^ ï^sSii^^ 0 2 km 1.5 CD O PLATE 5B Line drawing LEGEND Tops Cretaceous Malm Dogger Triassic Unit 1 Triassic Unit 2 Faults H '*" ----- " -------- ssss^~ \J. \J ' U) CD O MEM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII, 1997 PLATE 4A Seismic Section 11 LOCATION MAP Thrust & Major unit contact / Tear fault 30km [____J Tertiary Basins féffi] Subalpine Molasse K>j Préalpes |\VX] Variscan L¿-^J basement massifs Jura fold belt I I Avants Monts J Tabular Jura Hm Faisceau Jura. > External Jura Plateau Jura J Haute Chaîne } Internai Jura IiK PLATE 4B Line drawing LEGEND Tops Cretaceous rj~~*m Malm <— Dogger • - Triassic Unit 1 , . Triassic Unit 2 Faults MEM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII, 1997 SECTION 11 NW H O 2 T 4 T 6 T 10 T 14 T 0.0 0.5 1.0 1.5 0 2 km 0.0 0.5 (7) CD O 1.0 1.5 PLATE 3A Seismic Section 8 LOCATION MAP r A M? Thrust & Major unit contact ( Tear fault 30km I Tertiary Basins Kv>v| Subalpine Molasse K>| Préalpes FTv^] Variscan lji-iLi basement massifs Jura fold belt I 1 Avants Monts j Tabular Jura Hi Faisceau Jura 1¾¾! Plateau Jura I I Haute Chaîne } Internai Jura 1I > External Jura PLATE 3B Line drawing LEGEND •••••••• Tops Cretaceous Malm Dogger -- Triassic Unit 1 .. Triassic Unit 2 Faults MEM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII, 1997 SECTION 8 O CD SW 0.0-5 0.5- 1.0- 1.5 in CD o 3 T 1 T O CD (f) 1.5 PLATE 2A Seismic Section 3 LOCATION MAP r A Mf Thrust & Major unit contact Tear fault 30km I___I Tertiary Basins tëffi] Subalpine Molasse K^l Préalpes yy^\ Variscan Lj:LZj basement massifs Jura fold belt I 1 Avants Monts } Tabular Jura EH Faisceau Jura.. > External Jura Jura J ym\ Plateau Jl I [ Haute Chaîne } Internai Jura PLATE 2B Line drawing LEGEND Tops Cretaceous ~~~— Malm *— Dogger —-- -¦ Triassic Unit 1 ..........Triassic Unit 2 Faults MEM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII, 1997 SECTION 3~| NW c/> CD O 2 km 0.0 0.5- 1.0 1.5 0.0 0.5 H H CD o 1.0 1.5 PLATE 1A Seismic Section 1 LOCATION MAP Thrust & Major unit contact ( Tear fault 30km L___| Tertiary Basins Effiv] Subalpine Molasse Préalpes T^ Variscan ^-^ basement massifs Jura fold belt I I Avants Monts j Tabular Jura ÜÜÜ Faisceau Jura. pajan \ External Jura mi'..\ Plateau Jura J Haute Chaîne } Internai Jura PLATE 1B Line drawing LEGEND Tops Cretaceous Malm Dogger Triassic Unit 1 Triassic Unit 2 Faults MEM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII, 1997 SECTION 1~| NW o.o- 0.5- 1.0 1.5 CD O 0.0--------^ 1.0- 1.5 0.5------------- 0.5 H CD o WNW 0.0 ¦ 109 T 104 T EssaviIIy 106 T Laveron O (D U) 0.5 1.0 1.5 SECTION 107 ESE • o.o - 0.5 ^ tear fault zone 0 2 km 1.0 (D O 1.5 PANEL 10 DIP LINES CHAMPAGNOLE- MOUTHE, JURA MTS. FRANCE LOCATION MAP 480 500 200- 180- 160- Ste-Croix Essavilly \ // / . rV JoillonXsi (111)(108^ t\,» vallo* e ' T117) / \ T>r~vv ' Ô La Sarraz Sai/geof / "X V" _« V ÌÌ19) / \ Morez » % » Morges Of Í" Rolle JC&WQ ______________f"öeneva A/M/ o.o ¦ o 0.5- H 1.5 Faisceau Jura zone no data SECTION 115 SE ¦ o.o 0 2 km 0.5 H (D I- 1.0 Sl 1.5 LEGEND Tops ——— Malm ——— Dogger -----------Triassic Unit 1 ........... Triassic Unit 2 Faults well A/l/l/102 0.0 108 T Ü O 0.5 - 1.0 1.5 SECTION 117 SE ¦ o.o no data __ tear fault zone ? 0 2 km 0.5 h :> H (D 1.0 Ä 1.5 MEM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII, 1997 PANEL 9 DIP LINES RISOUX5 JURA MTS. (VAUD) SWITZERLAND FRANCE NW SECTION 81 SE NW SECTION 83 SE ---------— 0.0 0.0 0.5 H 0.5 — co CD O 1.0 1.0 1.5 1.5 0 1 2 km 1.5 NW 42 T SECTION 53 46 SE T 0.0 Ü CD ^0.5 h- h- 1.0 1.5 no data Pontarlier tear fault zone 0 1 2 km 0.0 0.5 1.0 1.5 U) (D O O CD U) \- NW Lave ron 8.5km SECTION 111 SECTION 85 FRANCE SECTK 84 T j 7 _____ —--------------------------" ----------------------— ^--------—-------------— ¦ 7 ~~------------- ^s^w ^zT ^=< -------- 7 0.0 — ^ssss^ __________________ ^=^ir<^ ~— ^=5*" 0.5 — "^==5^^: ^^.^- -----'^~=Z^^=^^=EE^l 1.0 — ~~-------------------------------^- ¦^^^" .---------------------------------------------- 7~ ............. 0 1 2 km SE o.o 0.5 1.0 1.5 U) CD O LEGEND Tops ----------- Cretaceous ——— Malm ——— Dogger ™ Liassic -----------Triassic Unit 1 ........... Triassic Unit 2 Faults Projected well LOCATION MAP 180—f 160 — 140 — 540 J_______1 Treycovagnes •»/ s ' #- ' Orbe » « Vallorbe Essertines Lake Joux V W # S) -'' Lausanne va Ai? KsJ** v Morges - \ _ \§y ' ^y Lake Geneva Rivers « Drillholes • Cities (9fi Seismic lines i0km NW Risoux 3km 82 T SECTION 89 84 SE T WN W SECTION 91 ESE o.o I 0.5 I- 1.0 1.5 SECTION 93 NW Risoux SECTION 95 SE WNW 42 T 46 T ^^ ? ^^ ~~__ ___—~^^.~. — no data ------------ — ^¾=^-. ~~^]^_ _—— - ? Pontarlier tear fault zone ™ 1 2 km SECTION 55 ESE o.o 0.5 1.0 1.5 H CD o A/A/IV SECTION 61 SSE o.o — o CD U) \- <: 1.0 1.5 \J . O mrsss. ¦¦ -------- 0.5 ^ C/) CD O ' . SOC. NEUCHÂTEL. SCI. NAT., TOME XII, 1997 SW SECTION 82 NE O CD CO H 95 93 89 TT T 0.0 — M^V////^^V////,^^V////^^V/'//I'MM^////^^v/yy/^^VA>v.<|^By/„/l^^v////^^V//v>/JaM» — 0.0 -----------------------------------------------------------------¦------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------¦----------------------------------------------------- '"''"' "*"""''^ O ------ f *—¦—+ H B ------ % 0.5 — —'"------""------"-------"-------—-----~~_____.—„______^„„__„, CO CD --------------------------------------------------------------------------------------------- Ci, 1.0 — — 1.0 ____ HHMHiIlH 1.5 — 0 1 2 km — 1.5 SlV 81 T SECTION 80 NE PANEL 8 STRIKE LINES RISOUX, JURA MTS (VAUD) CXAfITTC DI AMn — 0.0 :> — 0.5 ^¡ CO CD Ci, -1.0 1 R ^^^^SSSS'^^^^"'"'^^^ 0.0 — ^^ssss.^^^^^^r^sss.^^^mm^ssss.^mmmBÊ^rssj'S^mmimm^r-ssss.^^^^^^rss/rs^^^^^^rssssjm^^^^j'SSS^^^^^^r^ssS-^^^^^^'SSSS.^^^^^^ C } ---------- - ? CD CO "' " — ?- H- 0.5 — h- ----------------------------------------------------------- ////^^»////.^^»/x//«^^/''/''-^^»"_//*"^''''''/— 1.0 — 1 Fi 0 1 2 km ovvi I ¿tnLANU SlV 95 T 89 T 87 T 0.0 O CD ^0.5 I- 1.0 1.5 83 T no data SECTION 84 NE o 2 km Pontarlier tear fault zone 0.0 0.5 h CO CD O h- 1.0 1.5 180—T LOCATION MAP J_____I_____L 540 160 H 140 — SW 83 T SECTION 86 NE O CD Vi^ \- <: 0.0 0.5 1.0 1.5 no data Pontarlier — tear fault zone 0 2 km 0.0 0.5 h- 1.0 1.5 co CD O LEGEND Tops ------------ Cretaceous -~—— Malm —~—""- Dogger —-~ Liassic ------------Triassic Unit 1 ........... Triassic Unit 2 Faults MEM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII, 1997 SW 61 T 2.0 2.5 i 55 T 53 T 49 T SECTION NE no data no data Pontarlier tear fault zone 0 2 km 0.0 0.5 1.0 H C/> CD O — 1.5 2.0 2.5 PANEL 7 STRIKE LINES MOLASSE BASIN (VAUD) SWITZERLAND Pontarlier tear fault zone SW o.o- o CD S- \- i- 0.5 1.0 1.5 2.0 no data SECTION 50 48/VE T Pontarlier tear fault zone — 0 2 km 0.0 1 O ^ o 1.5 LEGEND Tops ------------ Cretaceous ———- Malm -^—~~ Dogger —— Liassic ------------Triassic Unit 1 ........... Triassic Unit 2 Faults 2.0 LOCATION MAP 540 560 180 — 160- 140 — ^g) Seismic lines Pontarlier tear fault zone Rivers ® Drillholes • Cities 10km WSW o.o O CD CO H 1.0 SECTION 54 ENE o.o no data Pontarlier tear fault zone MEM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII, 1997 O CD CO S o.o 0.5 1.0 1.5 2.0 2.5 PANEL 6 N-S LINES MOLASSE BASIN (VAUD) SWITZERLAND LOCATION MAP 180H 160- 140H 500 _L_ 520 540 560 560 Treycovagnes\ /TV - Orbe N^ y Vallorbe / S*' O CD CO h- S o.o 0.5 1.0 1.5 2.0 39 T Treycovagnes ft 27 T 22 T SECTION 38 N o.o 0.5 28 T 45 T 26 T 43 T SECTION 36 N o 2 km 0.0 0.5 1.0 H H co 1 .5 O 2.0 1.0 1.5 2.0 CO CD O LEGEND Tops ------------ Cretaceous ———- Malm ——— Dogger —— Liassic -----------Triassic Unit 1 ........... Triassic Unit 2 Faults Ä Well 2.5 SSW o.o — 2.5 SECTION 48 49 T 26 T 47 T 43 T 37 T SECTION 34 NNE 22 T ; •••. • ti 0 2 km 0.0 0.5 1.0 H co 1.5 8 2.0 2.5 MEM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII, 1997 ü CD (f) h- NW o.o 0.5 1.0 1.5 2.0 22 T Treycovagnes A 38 T SECTION 37 SE WNW Essertines A 26 T SECTION 27 SE o.o ^^^r^ — 0.5 1.0 1.5 2.0 SECTION 39 ESE r— 0.0 0.5 1.0 1.5 2.0 H en CD o SECTION 29 SE o.o ü -\ CD :> CO -\ h- üT <: CD i- O 0.5 H 1.0 H CD O 1.5 2.0 PANEL 5 DIP LINES MOLASSE BASIN (VAUD) SWITZERLAND 180 — 160 — 140 — LOCATION MAP 520 540 560 560 WNW o.o 0.5 34 ? 36 T 26 T o CD (f) H 1.0 1.5 2.0 2.5 30 T ESE INW SECTION 43 SE o 1 2 km 0.0 0.5 — 1.0 "H 00 CD O 1.5 — 2.0 2.5 SECTION 47 SSE WNW ESE \W Savigny SECTION 45 E o.o 0.5 — 1.0 H en CD O 1.5 2.0 SECTION 49 ESE o.o 0.5 1.0 h co CD 1.5 Ä 2.0 2.5 LEGEND Tops ----------- Cretaceous ——— Malm ——— Dogger ~—. Liassic -----------Triassic Unit 1 ........... Triassic Unit 2 Faults j| Well MEM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII, 1997 PANEL 4 STRIKE LINES MOLASSE BASIN (VAUD) SWITZERLAND sw 0.0 0.5 CD \- 1.0 <: 1.5 2.0 34 T 19 T 27 T 38 T 15 T 13 T SECTION 22 NE 11 T o.o 0.5 1.0 H co CD O — 1.5 0 2 km 2.0 SW 0.0 0.5 34 47 T T 36 ? 43 T 39 T 27 T SECTION 26 NE 0.0 0.5 o CD CO b 1.0 1.5 2.0 0 1 2 km 1.0 H CD O 1.5 2.0 SW 0.0 0.5 SECTION 28 NE o.o 0.5 CD CO h- 1.0 1.5 2.0 1.0 i 'co" CD O 1.5 2.0 SW 0.0 0.5 1.0 1.5 2.0 45 T 43 T 29 T LOCATION MAP 500 520 540 560 180 — 160 — 140' 560 J Courtion LEGEND Tops ----------- Cretaceous ——~- Malm ——— Dogger -™ Liassic ----------- Triassic Unit 1 ........... Triassic Unit 2 Faults SECTION 30 NE o.o 0.5 1.0 1.5 2.0 o CD CO H 0 2 km CO CD O MEM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII, 1997 PANEL 3 STRIKE LINES JURA MTS. (NEUCHATEL5 VAUD) SWITZERLAND sw 17 T 13 T 11 ? 9 7 T 5 T 3 T SECTION 2 NE o cd h- 0.0 0.5 1.0 1.5 0 1 2 km 0.0 0.5 1.0 1.5 co CD O 220- LOCATIONMAP 210 — 200 520 530 540 550 560 570 SW O CD H 1.5 17 T 15 T SW SECTION 4 NE H :> H o SECTION 6 13 T 11 T 9 T NE o.o 0.5 1.0 1.5 H co CD O SW SECTION 8 NE o CD CO 0.0 -i 0.5 H en CD O 1.0 1 5 LEGEND Tops ------------ Cretaceous ——— Malm ——— Dogger ------------Triassic Unit 1 ........... Triassic Unit 2 Faults SlV SECTION 10 NE O CD 0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5 H H To CD O SbV SECTION 12 NE 0.5 H CO CD O SH/ 19 Ü (D h- 0.0 0.5 — 1.0 1.5 17 15 ? 13 T 11 T 9 T SECTION 14 7 NE o.o 0.5 1.0 1.5 CO (D O MEM. SOC. NEUCHATEL. SCI. NAT., TOME XII, 1997 NW SECTION 9 SE O CD \- h- CO CD O PANEL 2 DIP LINES JURA MTS. (NEUCHATEL, VAUD) SWITZERLAND NW O CD GO h- SECTION 11 SE 0.5 H co CD O NW SECTION 13 SE co CD O LEGEND Tops ----------- Cretaceous ———- Malm ——— Dogger -----------Triassic Unit 1 ........... Triassic Unit 2 Faults NW SECTION 15 SE O CD CO 'Ti) CD O NW SECTION 17 SE O CD \- :> H 0.5 Î o LOCATION MAP 520 220- 210- 200- 190- 180- 530 _J__ 540 __I 550 560 _]_ 570 ¦yf----- La Chaux de Fonds VJ M ort e au / -210 10 km Seismic lines Rivers 570 -220 NW SECTION 19 SE ¦200 O CD CO I- -190 ¦180 H CO CD O MEM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII, 1997 PANEL 1 DIP LINES JURA MTS. (NEUCHATEL) SWITZERLAND LOCATION MAP 520 530 _1_ 540 I 550 _l 560 _L_ 570 220- S La Chaux de Fonds \J Morteau Y 210— 200- 190- 180— 520 10 km -210 Seismic lines Rivers 570 •220 -200 -190 ¦180 NW CD CO NW SECTION 1 SE -0.5 H SECTION 3 SE o.o 0.5 1.0 1.5 H U) CD O O) CD O LEGEND Tops ------------ Cretaceous —-——- Malm ——— Dogger -----------Triassic Unit 1 ........... Triassic Unit 2 Faults NW CD (A 0.0 0.5 1.0 1.5 SECTION 5 SE H CD O SECTION 7 SE H CD O MEM. SOC. NEUCHÂTEL. SCI. NAT., TOME XII, 1997 NW (335°) SE (155°) Avants Monts Faisceau Plateau Jura Faisceau Appennine frontal thrusts (Monferrato) Okm ¦10 km -20 km 10 km 20 km 30 km