\l\lO Am mm'* %a w^° Institut de Géologie Université de Neuchâtel Tectonometamorphic evolution of the Eastern Pennine Alps during Tertiary continental collision: Structural and petrological relationships between Suretta, Tambo, Chiavenna and Gruf units (Switzerland/Italy). Evolution tectonométamorphique des nappes Penniques orientales pendant la collision continentale Tertiaire: relations pétrologiques et structurales entre les unités de Suretta, Tambo, Chiavenna et Gruf (Alpes Suisses et Italiennes). Thèse présentée à la Faculté des Science de l'Université de Neuchâtel pour l'obtention du grade de docteur es sciences par Rachel K. Huber Janvier 1999 IMPRIMATUR POUR LATHESE Tectonometamorphic evolution of the Eastern Pennine Alps during Tertiary continental collision: structural and penological relationships between Suretta-, Tambo-, Chiavenna and Gruf units (Switzerland/Italy) de Mme Rachel Huber UNIVERSITE DE NEUCHATEL FACULTÉ DES SCIENCES La Faculté des sciences de l'Université de Neuchâtel sur le rapport des membres du jury, MM. F. Persoz (directeur de thèse), D. Marquer, M. Burkhard, M. Engi (Berne) et N. Mancktelow (Zürich) autorise l'impression de la présente thèse. Neuchâtel, le 6 janvier 1999 Le doyen: F. Stoeckli Résumé Les nappes de Tambo et Suretta, comme les unités de Gruf et Chiavenna font partie des nappes penniques orientales. Les unités sont situées dans la zone des "racines", dans la région de Val Bregaglia à l'Est de la Suisse. Les nappes de Tambo et Suretta représentent les nappes supérieures de la pile des nappes penniques. Elles appartiennent paléogéographiquement au domaine de Briançonnais. L'unité de Chiavenna est située sur la marge amincie nord du Briançonnais et l'unité de Gruf provient de la marge continentale Européenne. Les deux nappes sont essentiellement constituées d'un socle polycyclique et polymetamorphique composé de métasédiments, métagranitoides et amphibolites. Elles sont intrudées par des granito'ides Varisques et recouvertes par des sédiments Mesozoiques. L'unité sous-jacente, l'unité de Chiavenna, montre primordialement des roches basiques et ultrabasiques d'origine subcontinentale et océanique. L'unité la plus basse, l'unité de Gruf, est formée par des migmatites alpines et préalpines avec des reliques de métamorphisme granulitique. La déformation et la cinématique Tertiaire et les assemblages métamorphiques associés ont été défini dans les quatres unités, ce qui permet de proposer une évolution tectonométamorphique de la collision Tertiaire et de séparer cette evolution alpine de l'histoire préalpine. Les quatre phases Tertiaires se résume de la manière suivante: La première phase de déformation (Dl) est hétérogène et ductile. Les structures majeures sont des zones de cisaillement avec un mouvement chevauchant vers le NW. La schistosité et sa lineation plonge vers le NW. Les assemblages minéralogiques associés montrent des conditions HP (ex: nappe de Tambo: -13-10 kb, ~500°C). A la déformation Dl, se superpose une deuxième phase de déformation (D2), ductile et hétérogène. Sa schistosité est la schistosité principale de la région, peu inclinée vers le NNE avec une lineation subhorizontale E-W. A toute échelle, des zones de cisaillement indiquent un mouvement normal vers I1E. Cette phase décale les contacts lithologiques et tectoniques. Elle est responsable de l'amincissement des unités vers le SE. L'étude métamorphique de D2 dans la zone des racines démontre l'existence d'une décompression avec un maximum thermique au début de cette phase (ex: la nappe de Tambo: -11-6 kb, -6100C), cependant que dans la partie nord des nappes de Tambo et Suretta une décompression isotherme est enregistrée. Pendant le refroidissement, la déformation se poursuit, comme en témoignent des structures de style différent: aux structures ductiles de HT succèdent des structures fragiles de BT. Les conditions PT augmentent en traversant la pile de nappes depuis le haut vers le bas (la nappe de Suretta: -10-5 kb, ~550°C et l'unité de Gruf: -10-4 kb, -7300C). La troisième phase de déformation (D3) ne s'imprime au N, que dans des zones restreintes orientées E-W, tandis qu'au sud, toute la région est plissée d'une manière isoclinale. La schistosité et la lineation plongent fortement vers le S et elles sont associées à des plis orientés E-W. Pendant la même phase segénèrent des failles ductiles-cassantes chevauchantes vers le N et fortement inclinées vers le S. Des failles mineures conjuguées montrent une géométrie et une cinématique opposées. Cette phase est responsable de la réorientation et du redressement de S2 dans la zone des racines, ainsi que du soulèvement de la région de l'intrusion de Bergell. Au nord, les conditions PT indique la limite inférieure du faciès des schistes verts à la limite ductile-cassante, pendant qu'au sud, dans l'unité de Gruf, les conditions PT de D3 commencent à -4 kb et ~550°C. Pendant cette phase, l'intrusion de Bergell se met en place, ce qui provoque une auréole de métamorphisme de contact au toit et une fusion partielle in-situ à la base du pluton. La quatrième phase de déformation (D4) est caractérisée par des failles normales plongeant vers le NE, parallèles à la faille de Forcola et des mylonites et failles sénestres parallèle à la Ligne d'Engadine. La première phase esUnterprétée comme la phase principale d'empilement des nappes durant la subduction Eocène. La formation des nappes avec une direction de transport vers le NW est liée à la subduction du domaine Briançonnais et à la fermeture du basin Valaisan. La deuxième phase est due à une extension ductile syn-collision, orienté E-W, parallèle à l'axe de l'orogène Alpin. L'épaisseur anormale de la croûte, suite à l'empilement des nappes pendant la subduction, représente un déséquilibre isostatique. Ce déséquilibre conduit à une décompression isotherme pendant l'extension Eocène-Oligocène. Un lien entre la température maximale pendant D2, la formation du magma de l'intrusion de Bergell et l'initiation de l'uplift de D3 pourrait être établi avec un modèle du détachement progressif de la plaque lithosphèrique vers l'Ouest (slab break-off). La troisième phase a lieu pendant la collision continentale tardive et implique un soulèvement différentiel avec une composante maximale dans la région de l'unité de Gruf. L'érosion induite par le soulèvement mène à l'exhumation de cette région et à une augmentation du taux de refroidissement. L'intrusion de Bergell a lieu pendant cette phase Oligocène. Pendant la quatrième phase se forme des failles parallèles aux failles majeures de la Ligne d'Engadine, de Forcola et de la Ligne Insubrienne. La Ligne de Forcola pourrait présenter l'équivalent symétrique de la faille du Simplon à l'Ouest des Alpes Centrales. Cette phase est due à l'échappement latéral des blocs lithosphèriques. Ce fascicule correspond à une forme réduite d'une partie de la thèse. Le texte intégral de cette thèse a été déposé à la biblothèque principale et à la bibliothèque de l'Institut de Géologie de l'Université de Neuchâtel Liste des publications majeures Publications: 1. Huber, R.K. & Marquer, D. (1996): Tertiary deformation and kinematics of the southern part of the Tambo and the Suretta nappes (Val Bregaglia, Eastern Swiss Alps). Schweiz. Mineral. Petrograph. Mitt., 76, 383-397. 2. Huber, R.K. & Marquer, D. (1998): The tectonometamorphic history of the peridotitic Chiavenna unit from Mesozoic to Tertiary tectonics: a restoration controlled by melt polarity indicators (Eastern Swiss Alps). Tectonophysics, 296, 205-223. Résumés étendus: 1. Huber, R.K. & Baudin, T. (1995)a: The relationship between extension and uplift- erosion processes in the root zone of the Eastern Pennine Nappes (Val Bregaglia). Terra nova, 7, 123. 2. Huber, R.K., Baudin, T. & Marquer, D. (1995)b: Alpine tectonic and metamorphic evolution of the root zone of the Eastern Pennine nappes (Val Bregaglia, Central Swiss Alps). Journal of the Czech geological Society, 40, 3. 3. Huber, R.K. (1997): The tectonometamorphic evolution of the continental crust and the upper mantle during the Alpine Tertiary continental collision shown at the example of the Eastern Penninic nappes (Val Bregaglia, Switzerland). EUG9- Abstracts, Terra Nova, 9, 85. 4. Huber, R.K. & Marquer, D. (1998): Tectonometamorphic evolution of the Eastern Pennine Alps during Tertiary continental collision: Structural and petrographical relationships between the Suretta-, Tambo-, Chiavenna and Gruf units (Switzerland). Mem. Sci. Geol., 50. SCHWEIZ. MINERAL. PETROGR. MITT. 76,383-397,1996 TRAVAUX DE L'IfôSTITUT DE GÉOLOGIE DE UEUGHATEL (Stäissc) PUBLICATION Ko. g£ß Tertiary deformation and kinematics of the southern part of the Tambo and Suretta nappes (Val Bregaglia, Eastern Swiss Alps) by Rahel K. Huber' and Didier Marquer' Abstract The southern part of the Tambo and Suretta nappes (Eastern Swiss Alps) records several structural phases during Tertiary orogenesis. Based on micro- and mesostructural methods and kinematic indicator analysis, a tectonic mod- el with four deformation phases is proposed: (i) A top to the NNW shearing (Dl) correlated to the nappe stacking event. (ii)The strongest deformation of the area forming the main schistosity (D2), with vertical shortening and E-W extension showing a top to the E shear sense. This phase may be responsible for the bulk reduction of thickness in the SE part of the nappe pile, (iii) Localized deformation creating open N-verging folds and N-thrusting steep shear zones during differential uplift of the Penninic domain. This D3 deformation causes reorientation and steepening of nappe contacts and main S2 schistosity in the southern part of the Suretta and Tambo nappes, (iv) Normal faulting (D4) to the NE corresponding to a late tectonic event under brittle conditions. Keywords: deformation, tectonic evolution, extension, shear sense, Penninic domain, Central Alps, Switzerland. Introduction The aim of this work is to establish the tectonic evolution of the southern part of the Tambo and Suretta nappes (Eastern Swiss Alps) during Ter- tiary collision between European continent and Apulian microplate. The knowledge gained from this area may lead to a better understanding of the geometry of collision belts and of the kinematics and deformation during ongoing continental col- lision. We focus on the geometrical description of the different deformation phases and the struc- tural correlation between the Penninic and Aus- troalpine units in the Eastern Swiss Alps. Particu- larly interesting is the link of the deformations and kinematics in the studied area to the tectonic evolution established further north for the Tambo and Suretta nappes (Marquer et al., 1994,1996), to the Turba Mylonite Zone (Liniger, 1992; NiE- VERGELT et al., 1996), as well as to the ductile de- formation associated with the Tertiary Bergell in- trusion (Rosenberg et al., 1994,1995; Davidson et al., 1996). Structural cross cutting relationships of the Bergell intrusion with the country rocks al- low to deduce the relative timing of the deforma- tion phases. Special attention is given to the fol- lowing points: the geometry and kinematics of the syn-collision deformation phases; the reduction of thickness of the nappe pile towards the SE; the re- orientation of both tectonic contacts and main schistosity from N-S to E-W; and the steepening of the main tectonic contacts. Tb distinguish between Alpine and pre-Alpine structures, the Alpine structural phases were de- fined in the Permian intrusive complex (Truzzo granite, Tambo nappe) and in the autochthonous and allochthonous Mesozoic sedimentary cover of the Tambo and Suretta nappes (Baudin et al., 1995).The internal consistency of the deformation phases, the kinematics within the nappes and the intensity of deformation was studied at all scales. The geometry of the nappes is shown in cross-sec- tions which were constructed from structural maps. A deformation history has been established taking into account the superposition of all ob- servable structures and their deformation type (brittle/ductile). For all schistosities and lin- eations, trajectory maps are presented to illustrate 1 Institut de Géologie, Université de Neuchâtel, rue E. Argartd 11, CH-2007 Neuchâtel, Switzerland. E-mail: huber@geol.unine.ch. 384 R.K. HUBER AND D. MARQUER the superposition of deformation phases. The tra- jectories were drawn by extrapolation of the strike of each schistosity measurement. The kine- matics of each deformation phase were investi- gated, using mainly the shear sense from shear bands (C/S relationships, discrete shear zone [C] and penetrative schistosity [S]: Berthe et al., 1979; Extensional Crenulation Cleavage [ECC]: Plait and Vissers, 1980). In the last part of this paper, a tectonic model is proposed to integrate the structural data, at the scale of the Tambo and Suretta nappes, with con- tinental collision processes related to mountain building, such as syn-collision extension, vertical extrusion and uplift. Geological setting The field area in the Val Bregaglia extends E-W from Chiavenna in Italy to Lobbia in Switzerland, and includes the two slopes of the Val Bregaglia with a range in altitude from 300 to 3000 m. Its co- ordinates define a rectangle between [769-748] and [132-140] (Swiss federal topographic coordi- nategrid) (Fig. 1). The Tambo and Suretta nappes belong to the upper Penninic nappes in the eastern Swiss Alps (Figs 1 and 2) (Trümpy, 1980). Their basement rocks, as well as their autochthonous and al- lochthonous sedimentary covers (Stadera and Schams nappes), showing a typical stratigraphy of internal Briançonnais sediments, belong to the Briançonnais domain (Staub, 1924; Baudin et al., 1995). The Stadera nappe, recently defined by Baudin et al. (1995), is a sedimentary nappe, em- placed by thin skin tectonics on the top of the Tambo and Suretta Mesozoic cover prior to thrust tectonics affecting the Tambo and Suretta base- ment. At the top of the Suretta nappe (Fig. 2), the Stadera nappe is tectonically overlain by the Schams nappes (Schreurs, 1993), the Avers schistes lustrés, the Arblatsch flysch, the Mesozoic Piatta ophiolites, and covered by the orogenic lid of the Austro- and South-Alpine units (Trümpy, 1969; MiLNES and Schmutz, 1978; Liniger and Guntli, 1988; Guntli and Liniger, 1989). The Chiavenna ophiolites (Dal Vesco, 1953; Fig. 1 Location of the study area on a sketch map of the eastern Alps. Numbers refer to the Swiss coordinate grid. TECTONIC EVOLUTION OFTAMBO AND SURETTA NAPPES, VAL BREGAGLIA (BERGELL) 385 386 R.K. HUBER AND D. MARQUER S S Fig. 3 (a) Structural map with the trajectories of the first phase stretching lineation Ll. The lineation Ll on the stereoplot shows a gentle NNW plunge (black points), (b) Structural map with the trajectories of the first and the second phase schistosities (Sl and S2). On the stereoplots the poles of the first and second schistosity are represent- ed by black dots, (c) Structural map with the trajectories of the second phase stretching lineation L2. The lineation L2 on the stereoplot shows a moderate to sub-horizontal NE to SE plunge (black points), (d) Map of the trajecto- ries of stretching lineation L3 and schistosity S3. On the stereoplots the poles of the schistosity S3 and the lineation L3 are presented by black dots. The fourth phase is represented by NW-SE trending faults, represented by heavy lines (equal area stereograms, lower hemisphere). TECTONIC EVOLUTION OFTAMBO AND SURETTA NAPPES1VAL BREGAGLIA (BERGELL) 387 Schmutz, 1976), addressed as the Chiavenna unit the south, the Gruf unit is crosscut by the Tertiary in this paper on the basis of the lack of a typical Bergell intrusion (Wenk, 1970,1973; Moticska, ophiolitic sequence, are located between the Tarn- 1970; Gulson, 1973; Wagner, 1979; Tromms- bo nappe and the Gruf unit (Fig. 2). The Gruf unit dorff and Nievergelt, 1983; Rosenberg et al., has been correlated with the Adula nappe 1994,1995; Davidson et al., 1996). (Blanc, 1965; Pfiffner et al., 1990a, 1990b). In The Tambo and Suretta nappes are mainly 388 R.K. HUBER AND D. MARQUER composed of a polycyclic and polymetamorphic basement of paragneisses (Staub, 1921).Thin lay- ers of amphibolite and orthogneiss are intercalat- ed within the paragneiss. Permian acidic mono- cyclic intrusions crosscut the Tambo nappe in the south (Truzzo granite: Blanc, 1965; Weber, 1966; Marquer, 1991) and the Suretta nappe in the north (Roffna porphyries: Grünenfelder, 1956; Marquer et al., 1996). The basement of both nappes is unconformably overlain by a Permo- Mesozoic cover which, from older to younger sed- iments, is constituted of: conglomerates with quartz pebbles and albite-bearing quartzites which probably were formed from Permian vol- cano-detritic sediments. The Mesozoic cover con- sists of pure quartzites in the Suretta nappe and impure quartzites in the Tambo nappe respective- ly, dolomitic marbles, marbles and marly schists (see details in Baudin et al., 1995; Gieré, 1985). On the top of the Tambo and Suretta nappes, this AP 3 " N W-I Ve ¦')- © Avers schistes lustrés Starlera nappe ;::r:::j Orthogneisscs £| Porphyrie gneisses d Muscoviteschiste Slllllöj Truzzo granite (Variscan) I I Mesozoic cover (carbonates) I______I Permo-Triassic covers (quarzites) iiii Basement (micaschistes and gneisses) ^^^| Ultramafic and mafic rocks a Fig. 4 (a) The N-S profiles (see location on Fig. 2) show the steepening of the root zone due to the D3 folding (black lines = axial plane traces: AP3). Stereogram 1: poles of the fold axial plane of the third phase are indicated as large dots and the fold axes as small dots. Stereogram (2): poles of the brittle-ductile D3 shear zones (large dots) and the adjacent stretching lineation (small dots) (equal area stereograms, lower hemisphere). TECTONIC EVOLUTION OFTAMBO AND SURETTA NAPPES1VAL BREGAGLIA (BERGELL) 389 reduced autochthonous cover is overlain by a more complete allochthonous cover, the Starlera nappe (Fig. 2), consisting of Mesozoic banded marbles and dolomites, dark stink marbles, white marbles, thick polygenic breccias, and dark calc- schists (BAUDiN et al, 1995). The Alpine metamorphic grade increases from the top of the Suretta nappe to the bottom of the Tambo nappe and from the North to the South of the nappes from greenschist facies to amphibolite facies (see review in Baudin and Marquer, 1993). High temperature pre-Alpine mineral relics are preserved in basement domains poorly affected by Alpine deformation. From the root zone of the two nappes to the adjacent units in the south (Gruf and Chiavenna units) a metamor- phic gradient unusually high for regional meta- morphism exists (Bucher-Nurminen and Droop, 1983; Droop and Bucher-Nurminen, 1984). b Fig. 4 (b) The E-W cross-sections (see location on Fig. 2) show the offset of the tectonic and lithologie contacts due to the D2 mylonites and D4 faults. The area in the rectangle of the upper cross-section is enlarged to emphasize the D2/D4 geometric relationships on the lower cross-section where heavy lines represent D2 mylonite zones and D4 normal faults. 390 R.K. HUBER AND D. MARQUER The Alpine nappe pile was created in a sub- duction zone environment during the closure of the Piemontais and Valaisan oceans. The Aus- troalpine nappes were thrusted toward the west during the upper Cretaceous, whereas the Pen- ninic units were emplaced by thrusting toward the northwest in the early Tertiary (see review in Froitzheim et al., 1994).The upper Penninic units are considered to be an orogenic wedge consisting of underplated basement and sedimentary slices during the Valaisan subduction (see Marquer et al., 1994). After the onset of continental collision, E-W extension took place along major ductile displacement zones (e.g. Turba Mylonite Zone: Liniger, 1992; Nievergelt et al., 1996). During late folding, which overprints and steepens the previous structures, the Bergell granites intruded (Rosenberg et al., 1995). The latest structures are brittle normal faults cross-cutting all the previous structures (e.g. the Forcola fault: Marquer, 1991 ) and may be coeval with displacement along the Engadine line and the Iorio-Tonale line, which corresponds to the late stage of the Insubric line (Schmid et al., 1987, 1989; Heitzmann, 1987; ZiNGG et al., 1990). Recent attempts to constrain the timing of the Alpine deformation events have been published by several authors for the Suretta nappe (Hurford, 1986; Hurford et al., 1989), the Tambo nappe (Marquer et al., 1994) and the Bergell intrusion (von Blanckenburg, 1992; Davidson et al., 1996; Oberu et al., 1996; Hans- mann, 1996). Alpine structures and geometry We propose a relative timing of deformation phases based on the interference between the dif- ferent Alpine structures. Structural maps contain- ing schistosity (S) and mineral stretching lineation (L) for each deformation phase and trajectory maps (Fig. 3), as well as E-W and N-S trending cross-sections (Fig. 4) were constructed. The schis- tosity plane S and the stretching lineation L rep- resent the XY plane and the X axis, respectively, of the finite strain ellipsoid (Ramsay, 1967). The N-S and E-W profiles depict the geometry of the structures of the syn-collision deformation phases (from D2 to D4) which affect different rocks types and early tectonic contacts (Fig. 4a and Fig. 4b). The structures can be observed at all scales, from the microscopic to the regional scale. The main results are described as follows: First deformation phase (Dl): The first lin- eation (Ll) plunges to NNW and the schistosity (Sl) dips gently to the NNW (Fig. 3a and Fig. 3b). The lineation is defined by mineral and aggregate preferred orientations and long axes of quartz pebbles in Permo-Triassic conglomerates. This ductile deformation phase shows heterogeneous deformation at all scales. In the basement rocks, this behaviour leads to deformation gradients and shear zones surrounding weakly deformed do- mains where pre-Alpine structures and mineral relics are preserved (Marquer, 1991). In the Mesozoic cover, isoclinal folds have an axial pla- nar schistosity parallel to Sl.The fold axes related to Dl scatter mainly around an average N80 di- rection, which is parallel to the average orienta- tion of second phase lineations (see below). Be- cause of the strong D2 overprint, only a few loca- tions in large granite bodies, e.g. orthogneisses and the Truzzo granite, have preserved Dl structures. These domains are characterized by local occur- rences of Sl and Ll trajectories between zones of well developed D2 schistosity (Fig. 3a and Fig. 3b). For example, the best preserved Dl domain is lo- cated near coordinates 755/135 (Fig. 3b). Along the nappe contacts and in the south-eastern part of the study area, Dl structures have been reori- ented during later deformation phases (Fig. 3a and Fig. 3b). Second deformation phase (D2): The second ductile phase D2 is heterogeneous and creates the dominant penetrative schistosity S2 in the study area. Strong deformation gradients exist at loca- tions where rheological differences occur, for ex- ample along cover-basement contacts. The S2 schistosity is moderately dipping towards NE and bears a sub-horizontal, roughly E-W trending stretching lineation L2 (Fig. 3b and Fig. 3c) in ar- eas not affected by D3 deformation (see below). The mineral lineation L2 is characterized by ori- ented micas, plagioclase and polymineral aggre- gates. The L2 orientation scatters between N20-N130 with a clustering around N80-N90 (Fig. 3c).This scattering is due, in part, to the over- print by the latest deformation phases (D3 and D4). The development of D2 mylonite zones and local heterogeneities in the rocks are responsible only for a slight scattering of L2. Associated SE vergent isoclinal folds have S2 parallel axial planes and mainly E to NE trending moderately inclined axes (bulk average around N80) in the strongly D2-deformed zones. In weakly deformed areas, fold axes, associated with this S2 axial plane schistosity and E-W trending lineation, show a wide scattering around the average N80 direction with values ranging from NO to N100. This scat- tering of the F2 fold axes can be explained by fold- ing of inhomogeneously oriented, early foliations (Dl or pre-Alpine) and reorientation of fold axes TECTONIC EVOLUTION OFTAMBO AND SURETTA NAPPES, VAL BREGAGLIA (BERGELL) 391 during progressive D2 deformation, which tends to bring them into a parallel orientation with the E-W stretching lineation. Baudin et al. (1993) de- scribed the same phenomena in the middle and northern part of the Tambo nappe. Third deformation phase (D3): The third lin- eation L3 is moderately (45-50°' to steeply (80-90°) south-plunging (Fig. 3d) and is mostly ob- served on oriented chlorites and quartz pressure shadows around feldspars. The S3 schistosity also dips moderately to steeply south (45-85°) and is axial planar with non cylindrical, stair-case-like, north-verging folds with E-NE trending, moder- ately inclined axes (Fig. 4a, stereogram 1). On the basis of field geometry, the S3 schistosity and the stair-case-like folds are considered to be coeval with a set of steeply, south dipping, but north di- rected, brittle-ductile shear zones localized in basement rocks. This dominant set of shear zones is conjugated with a north dipping, south thrust- ing, minor set (Fig. 4a, stereogram 2). These brit- tle-ductile structures only appear in restricted E-W trending belts where older schistosities be- come folded and steep (Fig. 3d). In the N-S trend- ing cross-sections, these belts appear periodically with a wave-length of about 5 km (Fig. 3d and Fig. 4a, AP3).The steep fold limbs and the local thrusts are responsible for the steepening of pre-existing structures such as nappe contacts (Fig. 4a). Fourth deformation phase (D4): These late structures are brittle and correspond to localized normal faults with a NW-SE orientation (Fig. 3d). The northeastern side is down thrown (Fig. 4b). ITiese faults are well developed in the central and north-eastern part of the studied area (Fig. 3d) but cause only minor reorientation of pre-existing structures. Kinematics For each deformation phase, the kinematics have been investigated using shear sense indicators such as schistosity-shear plane relationships (C/S: Berthé et al., 1979), extensional crenulation cleavage (ECC: Platt and Vissers, 1980) and asymmetric microstructures related to non-coaxi- al deformation of mineral aggregates (see review in Hanmer and Passchier, 1991). Special atten- tion was given to the analysis of shear zone pat- terns. For every deformation phase defined by its schistosity and lineation, the distribution, geome- try and asymmetry of the shear zone pattern gives information about the bulk sense of shear (Gapais et al., 1987). The shear zone patterns in metagranites were analyzed and compared with the kinematics described for the late Variscan Truzzo granite in the western part of the study area (Marquer, 1991). The characterization of the kinematics of the different deformation phas- es was carried out in the basement as well as in the cover. Observation of asymmetric schistosities and lineation trajectories in map view (Fig. 3 b and c), in cross-section (Fig. 4b), at the outcrop (Fig. 5), and in thinsection are coherent for each deforma- tion stage. Dl shear sense indicators are scarce, however, a few preserved domains (Fig. 3b, e.g. coordinate: 755/135) can be found in the metagranites (e.g. Truzzo granite). Non-coaxial ductile shear bands indicate mainly a top to NW thrusting (Fig. 5a). These results are consistent with recent studies of the strain partitioning and the kinematics in the Roffna intrusive complex in the northern part of the Suretta nappe (Marquer et al., 1996). D2 deformation is dominant in the study area and leads to greenschist facies mylonites in the strongly deformed part of the nappes. In the base- ment, these mylonite zones are grey and fine- grained with a strong schistosity. This penetrative schistosity is mainly defined by quartz, phengites, chlorite and albite. Mylonite zones and non-coax- ial shear bands associated with the D2 E-W ex- tension, indicate a top to the E sense of shear as shown, for example, by the angular relationship between the schistosity and the shear zones (Fig. 5b). The non-coaxiality can be demonstrated by the dominance of mylonitic shear zones with top to the E movement. Conjugate shear bands with a top to the W shear sense are less frequent.The my- lonites displace lithologie and tectonic contacts with an identical sense of shear lowering the east- ern domains (e.g. Permo-Triassic cover and Por- phyrie gneisses on Fig. 4b). These mylonites are highly strained zones preferentially which are lo- cated close to the contacts between Mesozoic sed- iments and the underlying basement of the nappes. The heterogeneous mylonite zones with top to the E movement cause the undulation of the trajectories of S2 and L2, leading to large- scale asymmetric structures (Fig. 3 b and c), and induce a reduction of the thickness of the nappe pile towards the SE. This decrease of thickness is particularly well observed in W-E cross-sections: the Suretta nappe, for example, exhibits a thick- ness of more than 3 km in the western part and less than 2 km in the eastern part (Fig. 4b). In map view, the initial top to the E shearing yields an ap- parent dextral strike slip component of the shear zones, indicated by the undulating D2 trajectories (Fig. 3 b and c) and the offset of the Tambo sedi- mentary cover (Fig. 4b). This apparent sense of 392 RK. HUBER AND D. MARQUER F/g. 5 (a) Dl shear zones in the Truzzo granite with a top to the NVV movement (Baccino di Truzzo: 744.000/136.500): (b) D2 shear BOfttfl in paragneisses wilh a top to the E movement as it can be shown by the relationship between the schistosity and the shear zones (Lan Pensa da Rutic: 768.500/136.700); (e) D3 thrust in the Chiavenna unit near the contact to the Tambo nappe (S. Croce: coor- dinates 756.000/132.600); (d) D4 normal fault in the Avers schistes lustrés near the contact with the Suretta nappe (Nambrun: coordinates 768.700/137.400). S: schistosity; SZ: shear zone or brittle-ductile fault. TECTONIC EVOLUTION OFTAMBO AND SURETTA NAPPES, VAL BREGAGLIA (BERGELL) 393 SOUTHERNALPS Fig. 6 Schematic N-S cross-section showing the type, geometry, and kinematics of the D3 structures at a regional scale: The differential uplift with its highest elevation is created above the Gruf unit. During D3 deformation, low angle normal faults develop as collapse structures in the northern part at the top of the Tambo and Suretta nappes (extensional structures: from Nussbaum [1995] at the top of the Suretta nappe; Baudin et al. [1993] at the top of the Tambo nappe). D2 deformation: Turba MZ corresponds to the Turba Mylonite Zone after Nievergelt et al. (1996). The position of the Bergell intrusion corresponds to the syn-D3 deformation (modified after Schmid et al., 1990). shear is due to a rotation of about 60° around a sub horizontal E-W trending axis and corre- sponds to the steepening of the previously flat- laying D2 structures during D3 deformation (Fig. 4a).The kinematics deduced in this region is com- patible with observations made in the north of the Tambo and Suretta nappes (Baudin et al., 1993; Marquer et al., 1996) and in the overlying units (Liniger, 1992; Nievergelt et al., 1996). In areas of strong D3 deformation in the base- ment rocks, south dipping narrow shear zones formed near the brittle-ductile transition are dominant. On these steep shear zones a down-dip lineation is present due to oriented chlorites (Fig. 4a, stereogram 2). The C/S geometrical relation- ships observed in these shear zones indicate a top to the N movement (Fig. 5c). These localised thrusts are associated with a minor set of steeply north-dipping thrusts. The S3 schistosity and the major fault set (south dipping thrusts) enclose a narrow angle (25-30°), whereas a wide angle (45°) is found between the S3 schistosity and the minor fault set (north-dipping thrusts) (compare Fig. 3d and Fig. 4a, stereogram 2). These geometrical re- lationships associated with the dominance of south-dipping thrust planes are interpreted as a non-coaxial deformation with a component of simple shear towards the north. These thrusts be- come more abundant in the southern part of the studied area. In the middle and the northern part of the Suretta nappe (e.g. Lago di Lei), the D3 de- formation at the top of the basement, based on relative chronology between D2 folds and D4 faulting (NUSSBAUM, 1995), leads to the appear- ance of extensional structures corresponding to E-W trending and north dipping low angle nor- mal faults which down throw the hangingwall to- wards the north (Fig. 6). In summary, the D3 de- formation is progressive and partitioned across the strike of the mountain belt, so that the geom- etry and the kinematics of the D3 deformation in the southern domain indicate a bulk vertical stretching with asymmetric conjugate thrust faults and E-W trending folds, while the D3 deforma- tion in the northern part of the nappe pile is marked by top down to the north extension and E-W north vergent trending folds (Baudin et al., 1993; Mayerat, 1994). (These different types of structures will be interpreted in a tectonic model in the last part of this paper.) The D4 deformation is marked by brittle NW- SE trending, north-east dipping high angle normal faults with down-dip striae on their fault planes. On the basis of shear criteria, such as asymmetric deformation of older cleavages and crystallization on the lee side of asperities (Petit, 1987), these brittle faults show a lowering of the NE part of the units (Fig. 4b and Fig. 5d). Being the latest struc- ture they crosscut all the previous ones. For ex- ample at Nambrun (coordinates: 768.700/137.400) (Fig. 5d), the Turba mylonite has been reoriented along the D4 fault planes, which are steeper than the mylonite. 394 RK. HUBER AND D. MARQUER Discussion and conclusions Before the first ductile Dl deformation of the Tanibo and Suretta nappes, thin-skinned tectonics caused the formation of a sedimentary nappe, the Starlera nappe, which covers the Tambo and the Suretta nappes (Baudin et al., 1995). At the scale of the Tambo and Suretta nappe, no deformations corresponding to this early tectonic event were recognized in the basement rocks. The emplace- ment of this nappe at shallow levels leads to strong deformations localized close to the basis of the Starlera nappe (e.g. carnieules, Baudin et al., 1995) and at the top of the autochthonous sedi- mentary cover of Tambo and Suretta nappes, of- ten constituted by layers of calcschists and brec- cias. Similar early décollement nappes have been described in the western French Briançonnais zone (e.g. "Quatrième écaille", Barféty et al., 1992). The first ductile deformation (Dl) present in the Tambo and Suretta nappes, is interpreted as the main nappe stacking event and was directed towards NW corresponding to Eocene subduc- tion of the Briançonnais basement during the clo- sure of the Valais trough. The second deformation (D2) is presumably due to syn-collision ductile ex- tension of the nappe pile parallel to the orogenic belt. The third deformation D3 formed during late continental collision implying differential uplift from north to south with the strongest exhuma- tion in the southern parts of the studied area. This deformation event creates stair-case like folds as- sociated with the south dipping local thrusts, which may be interpreted as a conjugate set with respect to the vertical movement along the Insub- ric mylonites (Heitzmann, 1987; Schmid et al., 1987, 1989). North of the area of greatest ex- humation, the steepened topography collapsed along local E-W trending normal faults with N-S extension during the latest stages of this deforma- tion phase. This fourth deformation involves late orogenic normal faulting compatible with the movements along the SSW-NNE striking sinistral Engadine Line and the E-W striking dextral In- subric Lines (Tonale Line s.s.) (Heitzmann, 1987; Schmid et al., 1987, 1989). The strike-slip bulk kinematics associated with the two lines imply a major extension towards the NE and a NW-SE compression which is compatible to the offset di- rection of the D4 normal faults. The maximum age of the Dl phase in the Tam- bo and Suretta nappes is constrained by the sedi- mentation of theArblatschfiysch (Ziegler, 1956; Eiermann, 1988) and radiometric data at about 50-35 Ma (for review of isotopie data, see Mar- quer et al., 1994).TheTurba mylonite.an E-W ex- tensional structure (Liniger, 1992; Nievergelt et al., 1996), related to the D2 deformation, is crosscut at Lavinair Cruse (coordinates: 772/138) by the Bergell granodiorite, which intruded at about 30 Ma (von Blanckenburg, 1992). There- fore D2 must have occurred roughly between 40 and 30 Ma. This Eocene to Oligocene extension D2 is interpreted as the result of collapse of over- thickened crust due to the Dl nappe stacking. The D3 deformation is probably syn- to post-Beigell granodiorite intrusion. Submagmatic deformation in the Tertiary intrusion, and folding of the west- ern intrusive contact (Davidson et al.,1996),point to kinematics in deeper levels of the continental crust which are compatible with the previously de- scribed D3 phase. Cooling associated with uplift and exhumation of the southern part of the study area began about 30 Ma (Hurford et al., 1989). D3 deformation during cooling is supported by the south dipping and north vergent thrusts which crosscut the intrusion in its solid state (Rosen- berg et al., 1994). Rapid cooling corresponding to the D3 uplift is also reported from the Suretta and Tambo nappes from 30 Ma until about 20 Ma (Hurford et al, 1989;Marquer et al., 1994).The timing of the D4 phase is not well constrained. We assume the D4 phase may be around or younger than 20 Ma,probably coeval with the dextral strike slip along the lnsubric line, which post-dates the uplift of the whole region. The normal faults, asso- ciated with D4 could be the symmetric structural equivalents to the brittle component of the Sim- plon Fault zone (Manktelow, 1985; Steck, 1990) in the western part of the Penninic zone, but they may not be contemporaneous, because of the younging of the cooling ages towards the Simplon area (Hurford et al., 1989; for review: Hunziker et al., 1992). On the basis of the previously described data and assumptions, a model for the tectonic evolu- tion of the nappe pile can be proposed (Fig. 6). This interpretation is mainly based on the geome- try of the structures and the kinematics recorded by the Tambo and Suretta nappes and it leads to an alternative model for the evolution of the D2 and D3 deformations with respect to recent inter- pretations derived from structural investigations in the Bergell area (Rosenberg et al., 1995; Davidson et al., 1996). In these recent models, the first stage of the intrusion history of the Bergell pluton implies coeval different shear senses at the top of the Suretta nappe (top to the south) and the bottom of the Tambo nappe (top to the north) leading to an horizontal intrusion of the pluton before a regional N-S compression. But only the second stage proposed for the emplacement of the Bergell pluton, the regional N-S compression, is compatible with the structural observations of TECTONIC EVOLUTION OF TAMBO AND SURETTA NAPPES, VAL BREGAGLIA (BERGELL) 395 D3 deformation described in the Tambo and Suretta nappes. On the other hand, for the previ- ous stages, a continuous syn-collision lithospheric extension from D2 to D4 can be proposed on the basis of the structural investigations in the Tambo and Suretta nappes. This progressive extension is defined by eastward escaping extensional struc- tures due to relaxation of a buoyancy disequilibri- um in an abnormally thickened crust (see review of exhumation processes in Platt, 1993).The duc- tile D2 mylonites and shear zones were first cre- ated at a deep-crustal level. The progressively shallower tectonic setting of the Tambo and Suret- ta nappes during D2 deformation, corresponding to isothermal decompression from 1.0 GPa to 0.5 GPa (Baudin and Marquer, 1993), is consid- ered as a syn-collision extension process due to the D2 ductile vertical shortening, low-angle detach- ments toward the East (Fig. 6) (e.g. Turba My- lonite Zone, Nievergelt et al, 1996) and associ- ated erosion. The subsequently cooled units were subjected to brittle-ductile deformations during D3 and D4 under Barrowian metamorphic condi- tions (Marquer et al., 1994). At these shallow crustal levels, the D4 normal faults were formed, corresponding to NE-SW extension, parallel to the belt axis. With this interpretation, the ongoing syn-collision extension, leading to D2 and D4 structures, is interrupted by a double event corre- sponding to the Bergell intrusion and the D3 up- lift. The progressive deformation during D3 leads to a succession from ductile to brittle-ductile structures. Isoclinal folds and thrusts, described at the base of the Bergell intrusion (Rosenberg et al., 1994,1995; Davidson et al., 1996), and north vergent staircase-like folds in the southern part of the Tambo and Suretta nappes were accompanied by the formation of steep brittle-ductile thrusts in the basement rocks (Fig. 6). The thrust and fold zones might be considered as antithetical to the Insubric mylonites and allow a differential uplift of the southern part of the nappe pile. The fold and thrust zones steepen the pre-existing schis- tosity in the southern part of the Tambo and Suretta nappes. In the northern, more external ar- eas, the southern highly uplifted area collapses into northwards directed normal faults at the top of the Suretta nappe while D3 north vergent folds are created in the upper sedimentary units, such as the Schams nappes for example (D3 in Schreurs, 1993) or in the frontal part of the Tambo nappe (D4 in Mayerat, 1994) (Fig. 6). The geometry of these D3 structures were previously described by Baudin et al. (1993) at the top of the Tambo nappe, where these authors interpreted systemat- ic northward vergence of D3 folds and extension- al crenulation cleavage as a result of a bulk top to the North shearing. At the scale of the nappe pile, these structures could accommodate the bulk gravitational disequilibrium associated with the vertical extrusion in the most internal part, close to the Insubric line (Fig. 6). In summary, four deformation phases are de- scribed in the southern parts of the Tambo and Suretta nappes:The Dl deformation is associated with nappe stacking towards the NW in a subduc- tion environment. Syn-collision ductile east-west directed extension (D2) creates important my- lonite zones leading to a bulk top to the east sense of shear. Syn-collision brittle-ductile uplift (D3) is syn- to post-Bergell granodiorite intrusion and forms steep folds and conjugate thrusts. Later, brittle normal faults indicate NE-SW extension (D4). Taking into account previous works in the Bergell area, a rough timing can be proposed. Constrained by mineral ages and the overthrust- ing on the Arblatsch flysch, the Dl deformation of Tambo and Suretta took place between 50 and 35 Ma. The Turba mylonite has been crosscut by the granodiorite intrusion (30 Ma, von Blanckenburg, 1992; Nievergelt et al., 1996) and is linked to the D2 structures observed in the studied area. This restricts the lower age of D2 to 30 Ma probably indicating a pre- to syn-Bergell tonalité intrusion age for the D2 deformation. The Oligocene high cooling rates are most plausibly associated with the D3-uplift. The rapid cooling in the Tambo and Suretta nappes ends at 20 Ma. This age could be attributed to the begin- ning of the predominance of D4 deformation, as D4 brittle extension started when the uplift de- creased. The main schistosity (D2) is contemporaneous with and due to an E-W extension strongly over- printing almost all previous structures. The reduc- tion of the thickness of the nappe pile, as observed on the tectonic map, is not caused by the intersec- tion of geological structures with the topography but is rather created by the large scale D2 ductile extension structures, which appears to be strongest in the southeastern parts of the Tambo and Suretta nappes. 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Zürich, (HF), 57., 248 pp. Wenk, H.R. (1970): Geologische Beobachtungen im Bergell. I. Gedanken zur Genese des Bergeller Gra- nits. Rückblick und Ausblick. Schweiz. Mineral. Pe- trogr.Mitt.50,321-348. Wenk. H.R. (1973): The structure of the Bergell Alps. III. Eclogae geol. HeIv. 66/2,255-291. Wenk, H.R. (1974): Two episodes of high-grade meta- morphism in the Northern Bergell Alps. Schweiz. Mineral. Petrogr. Mitt. 54,555-565. Ziegler, W (1956): Geologische Studien in den Flysch- gebieten des Oberhalbsteins (Graubünden). Eclo- gae geol. HeIv., 49/1,1-78. Zingg, A. and Hunziker, XC. (1990):The age of move- ments along the Insubric Line west of Locarno (northern Italy and southern Switzerland). Eclogae geol. HeIv. 83/3,629-644. Manuscript received March 1,1996; revised manu- script accepted July 15,1996. Reprinted from TECTONOPHYSICS INTERNATIONAL JOURNAL OF GEOTECTONICS AND THE GEOLOGY AND PHYSICS OF THE INTERIOR OF THE EARTH Tectonophysics 296 (1998) 205-223 tie tectonometamorphic history of the peridotitic Chiavenna unit from [esozoic to Tertiary tectonics: a restoration controlled by melt polarity indicators (Eastern Swiss Alps) R.K. Huber*, D. Marquer Geological Institute, Neuchâtel University, E.-Argand 11, 2007 Neuchatel, Switzerland Accepted 24 June 1998 ELSEVIER TECTONOPHYSICS Edltors-In-Chief J.-P. BURG T. ENGELDER K.P. FURLONG F. WENZEL Honorary Editors: Editorial Board ETH-Zentrum, Institut für Mineralogie und Pétrographie, Sonneggstraße 5, CH-8092, Zürich, Switzerland. Phone: +41.1.632 6027; FAX: +41.1.632 1080; e-mail: jpb@erdw.ethz.cn Pennsylvania State University, College of Earth & Mineral Sciences, 336 Deike Building, University Park, PA 16802, USA. 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PRINTED IN THE NETHERLANDS TECTONOPHYSICS ELSEVIER Tectonophysics 296 (1998) 205-223 The tectonometamorphic history of the peridotitic Chiavenna unit from Mesozoic to Tertiary tectonics: a restoration controlled by melt polarity indicators (Eastern Swiss Alps) R.K. Huber*, D. Marquer Geological Institute, Neuchâtel University, E.-Argand 11, 2007 Neuchatel, Switzerland Accepted 24 June 1998 Abstract Occurrences of mantle and lower crustal rocks are often preserved in mountain ranges. When they occur in the internal part of collision belts, their initial geometry and palaeogeography are sources of debate. The mafic-ultramafic Chiavenna unit is one example in the Pennine units located in the Eastern Swiss Alps. Because of strong deformation and metamorphism during Tertiary collision tectonics, the pre-collision geometry of this unit can only be restored using a qualitative geometrical analysis of die successive ductile deformation phases (D1-D4). In-situ melting and melt migration during die main Alpine deformation phase (D2) is a way-up indicator and is of special importance to this restoration. The restored Prealpine geometry of die Chiavenna unit indicates that a sub-continental mantle close to a thinned continental margin becomes denuded during die Mesozoic extension and subsequently covered by carbonate sediments. Taking into account die position of the Chiavenna unit in the nappe pile and its P!T-path, a northern Briançonnais situation during Mesozoic time becomes most probable. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Chiavenna unit; Eastern Switzerland; Pennine unit; subcontinental mantle; thinned continental margin; melt way-up criterion 1. Introduction Petrology and structures of upper mantle rocks and their relationships with the lower crust lead to a better understanding of the geometry at the mantle- crust boundary, the tectonic evolution of the man- tle at the lower crustal level and their subsequent exhumation. Mantle peridotites occur as xenoliths in volcanic rocks, at the boundary of thinned conti- nental margins (Boillot et al., 1995) or in ophiolites * Corresponding author. Fax: rachel.huber@geol.unine.ch +41 (32) 718 2601; E-mail: involved in collision belts. Only the later two allow the structural studies that are necessary to define the geometrical relationships of the crust-mantle boundary and the tectonic evolution. Mantle rocks are often involved in orogenic processes as small peridotite bodies. Such ultramafic bodies of diverse origin occur in the Alps. They are recognized as sub- continental mantle, like the Malenco unit (Tromms- dorff et al., 1993; Müntener, 1997; Hermann, 1997), or as ophiolite sequences, like the Zermatt zone or the Piatta nappe (Triimpy, 1980; Pfeifer et al., 1989; Manatschal and Nievergelt, 1997). This work focuses on the origin and significance 0040-1951/98/$ 19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0040-1951(98)00143-7 206 R.K. Huber, D. Marquer/Tectonophysics 296 (1998) 205-223 of the Chiavenna unit in the Eastern Swiss Alps. The Chiavenna unit is a mafic-ultramafic body of unresolved origin with a poorly established Alpine collision history. The aim of this work is to de- fine the tectonic evolution during collision tectonics, to deduce the initial geometry at the mantle-crust boundary and to propose a Prealpine scenario for the Chiavenna unit. Because the Chiavenna unit under- went strong deformation and recrystallization during Alpine tectonics, the Alpine tectono-metamorphic history needs to be established first. Four main Alpine deformation phases have been distinguished on the basis of structural and micro- structural field work and deformation-metamorphism analysis. Huber and Marquer (1996) correlated the four deformation phases to the overlying tectonic units confirming their regional relevance. In the peri- dotite, P7-conditions are estimated using the stable mineral assemblages in the microstructures of each deformation phase. The structural phases and the /T-path in the peridotites are compared with those in the overlying Tambo and Suretta nappes and the underlying Adula nappe. The similarities and differ- ences in /T-history in the different units limit the proposed Alpine tectonic evolution of the Chiavenna unit. Qualitative, step-by-step restoration of the struc- tures of each deformation phase leads to a recon- struction of the Prealpine initial geometry. Of partic- ular aid to the geometrical reconstruction are polarity indicators. Local partial melting forms leucosomes which may exhibit way-up criteria (Burg, 1991). Therefore, special attention was paid to melt pro- duced in the meta-gabbros and intruding the overly- ing peridotites during D2-deformation. These melt- related structures were used to restore these rocks in their original position. The initial Prealpine geome- try of the Chiavenna unit is discussed in terms of palaeotectonics and palaeoenvironment. 2. Geological setting The Chiavenna and adjacent units belong to the Eastern Pennine units situated in the Eastern Swiss Alps and northern Italy (Figs. 1 and 2) (Triimpy, 120 Fig. 1. Location of the study area on a sketch map of the Eastern Swiss Alps. Numbers refer to the Swiss coordinate grid (E-W coordinates: 745-768; N-S coordinates: 127-137). R.K. Huber, D. Marquer/Tectonophysics 296 {1998) 205-223 207 208 R.K. Huber, D. Marquer/ Tectonophysics 296 (1998) 205-223 1980). The investigated area extends from the coor- dinates 745/128 to 762/131 according to the Swiss coordinate system (Fig. 1). The Chiavenna unit (Dal Vesco, 1953; Schmutz, 1976) is located between the Tambo nappe in the north and the Graf unit in the south (Fig. 2). The Gruf unit has granulite relics de- void of retrogression (Droop and Bucher-Nurminen, 1984). The imprint of granulite metamorphic facies is considered by them as a metamorphic event of Alpine age. The Gruf unit has been classically corre- lated with the Adula nappe (Blanc, 1965; Pfiffner et al., 1990a,b). In the south, the Gruf unit is cross-cut by the Tertiary Bergell intrusion (Wenk, 1970, 1973; Moticska, 1970; Gulson, 1973; Wagner et al., 1979; Trommsdorff and Nievergelt, 1983; Rosenberg et al., 1994, 1995; Davidson et al., 1996). The Tambo and Suretta nappes are composed of polycyclic and polymetamorphic basement rocks (paragneiss, layers of amphibolite and orthogneiss) cross-cut by Permian acidic monometamorphic in- trusions (Grünenfelder, 1956; Blanc, 1965; We- ber, 1966; Marquer, 1991; Marquer et al., 1996). Their unconformably overlying Permo-Mesozoic autochthonous and allochthonous sedimentary cov- ers show a typical stratigraphy of internal Briançon- nais sediments (Staub, 1919, 1924; Baudin et al., 1995) (Fig. 2). At the top of the Suretta nappe, the Stadera nappe is tectonically overlain by the Schams nappe (Schmid et al., 1990; Schreurs, 1993) and the Avers schistes lustrés and is covered by the orogenic lid of the Austroalpine units (Triimpy, 1969; Milnes and Schmutz, 1978; Liniger and Guntli, 1988; Guntli and Liniger, 1989) (Figs. 1 and 3). The Chiavenna unit consists of meta-peridotites, amphibolites, and carbonates and is regarded as an 'ophiolitic sequence' by Schmutz (1976). Because the peridotites overlie the amphibolites, the sequence was interpreted as an overturned unit (Schmutz, 1976). The Chiavenna ophiolites are addressed as the Chiavenna unit in this paper because of the lack of a typical ophiolitic sequence (Anonymous, 1972) (see Geometrical restoration). The main lithologies of the unit are meta-peridotites and amphibolites present in equal amounts (estimated surface of 50% for each lithology) (Fig. 2). Carbonate occurrences are rare and consist of discontinuous layers with centimetre to metre thicknesses (Figs. 2 and 4). Meta-peri- dotites have various metamorphic mineral assem- blages which recrystallized during the Alpine oroge- nesis. The mineralogy of the meta-peridotites shows magnetite, chlorite, talc, amphibole, olivine, pyrox- ene with different degrees of serpentinisation. Essen- tially monomineralic rocks formed of talc, antigorite, chlorite or amphibole occur in narrow deformation zones. Clinopyroxene and spinel are rare. Amphibolites are classified according to textural and mineralogical criteria at mappable scales. Fine- grained greenish amphibolites, sometimes layered, show a rich mineralogy with plagioclase, quartz, am- phibole, biotite, chlorite, epidote/zoisite, titanite and oxides. Coarse-grained dark massive amphibolites with meta-gabbroic texture, show 'flaser' texture and poor mineral association (plagioclase, amphibole and biotite). Multicoloured, centimetre-scale layered amphibolites associated with small centimetre- to decimetre-scale cale-silicate boudins are in minor abundance and always occur close to the meta-peri- dotites. Their mineralogy is distinguished from the other amphibolites by the presence of diopside and calcite. Carbonates are located in or close to the meta- peridotite as veins, discordant to the mantle lay- ering. These calc-silicate veins have undergone at least four deformation phases. In the amphibolites, they occur as calc-silicate boudins parallel to the compositional layering. The third type of carbon- ates are almost pure calcite-marble. They are layered at a centimetre to decimetre scale. They separate coarse-grained from the fine-grained amphibolites. This layered marble is the only mappable carbonate lithology. 3. Tectono-metamorphic setting The Austroalpine nappes underwent deformation under greenschist facies metamorphic conditions during Late Cretaceous time (Liniger, 1992; Man- atschal and Nievergelt, 1997). During Eocene sub- duction, very high-pressure relics from the eclogite facies occur only locally within of the underlying Adula nappe (Fig. 9) (Heinrich, 1986; Meyre and Puschnig, 1993; Meyre et al., 1997). Other eclogite facies rocks of the upper Pennine region (Tambo and Suretta nappes) are described as Prealpine relics, whereas Alpine pressure-dominated metamorphism R.K. Huber, D. Marquer/ Tectonophysics 296(1998) 205-223 209 $ SOUTHERNALPS 10 km Fig. 3. Interpretative N-S crustal section (modified after Schmid et al., 1990) showing the main Alpine deformations: Dl corresponds to the Tertiary south-directed subduction (black arrow) and is responsible for the stacking of the nappes. D2 represents the ductile E-W extension of the nappe pile. During this stage, major mylonite zones developed along the nappe contacts (e.g. Turba-MZ). The compressive D3-phase shows a differential vertical uplift with the highest elevation created above the Gruf unit. This deformation phase is restricted to the zone north of the Insubric line (Insubric mylonites). reached conditions not higher than blueschist facies (Biino et al., 1997). The Oligocene metamorphism is of Barrovian type, increasing through the nappe pile in pressure and temperature from greenschist facies in the upper- most Pennine unit to upper amphibolite facies in the lowermost Gruf and the Chiavenna units (Trornms- dorff and Evans, 1969; Niggli, 1970; Wenk, 1974; Todd and Engi, 1997). Only a few granulites exist in the Gruf unit and are suggested to be of Alpine age by Bucher-Nurminen and Droop (1983) and Droop and Bucher-Nurminen (1984). In the Alps, in tecton- ically higher units, other granulites are present and are interpreted as being of Prealpine age (Gardien et al., 1994; Hermann, 1997; Müntener, 1997). In the underlying Pennine units, Prealpine HT evidence exists in the form of migmatites (Hänny et al., 1975; Romer et al., 1996). Therefore, without geochrono- logical evidence, the age of Gruf unit granulites is still a subject of debate (Romer et al., 1996). Contact metamorphism due to the Bergell intru- sion has been well-documented for the higher units (Gieré, 1985: Austroalpine and Suretta nappes) and recently studied in the Gruf unit (Davidson et al., 1996), but is structurally not well defined in the Chiavenna unit (Schmutz, 1976). In the upper eastern Pennine units, we have recog- nized four main Tertiary Alpine deformation phases (Dl, D2, D3 and D4) corresponding to four main tectonic events which overprint Prealpine structures (Marquer, 1991; Huber and Marquer, 1996). The four phases are also recognized in our study area and are as follows. The Pennine units were thrust northwestward dur- ing the Eocene (Froitzheim et al., 1994). The Pen- nine nappes were in a subduction zone environ- ment during the closure of the Valais Pennine basin (Fig. 3: Dl). The upper Pennine units which consist of underplated basement and sedimentary slices, an accretionary prism, formed during the Valais subduc- tion (see Marquer et al., 1994). The D2-structures are the most penetrative structures and related to a duc- tile, syn-collisional E-W extension. Major ductile detachment zones (Fig. 3; D2, e.g. Turba mylonite zone, Liniger, 1992; Nievergelt et al., 1996) cross-cut the tectonic contact of the nappes. Recent attempts to define the timing of Alpine deformation events (Hunziker et al., 1992) have been published by sev- eral authors for the Suretta nappe (Hurford, 1986; Hurford et al., 1989), and the Tambo nappe (Mar- quer et al., 1994), suggesting an Eocene subduction (Dl) followed by Oligo-Miocene collision (D2). The D3- and D4-structures are not penetrative. They are linked to vertical extrusion of the crustal block situated to the north of the Insubric line (D3) and brittle-ductile E-W extension parallel to the Forcola line (D4). During late folding (D3) which overprinted and steepened the previous structures in 210 R.K. Huber, D. Marquer/ Tectonophysics 296 (1998) 205-223 the southern part of the studied area, the Bergell granite intruded (Fig. 3; D3) (Rosenberg et al., 1995). This intrusion took place between 32 and 30 Ma (von Blankenburg, 1992; Davidson et al., 1996; Oberli et al., 1996). The latest structures (D4), corresponding to a NE-SW extension, are brittle normal faults cross-cutting all previous structures (Fig. 1) (e.g. the Forcola fault: Marquer, 1991). They may have been contemporaneous with displacements along the Engadine line and the late dextral Iorio- Tonale line (Schmid et al., 1987, 1989; Heitzmann, 1987; Zingg and Hunziker, 1990). The Novate intru- sion was affected by the D4 normal faults. Therefore, the intrusion age of 25 Ma (von Blankenburg, 1992) limits the maximum age of the D4-phase onset. 4. Alpine structures and geometry in the Chiavenna unit The Alpine deformation history in the study area can be described by four major deformation phases (D1-D4) which correspond to the major structures and tectonic events described above. Only D2 and D3 were well observed in the Chiavenna unit. Prealpine mantle relics in the peridotites and Dl Alpine structures from the subduction event are rare. Despite this, it is possible to reconstruct the ge- ometry of the Chiavenna unit for each phase. The first (Dl) phase's schistosity and lineation have been preserved in only a few outcrops. Where there was limited D2-deformation, for example in the neck of large Dl-boudins and close to D2-fold hinges, there is weak penetrative second axial-planar schistosity. The average regional Dl-orientation (Sl: 340/30 in azimuth and dip, Ll: 340/30) best shown in the late Variscan Truzzo orthogneiss (Marquer, 1991) cannot be followed into the Chiavenna unit. The most pen- etrative D2-phase was responsible for the rotation of the Sl and Ll relic into a position sub-parallel to the D2-schistosity. Other preserved Dl-structures are large-scale isoclinal folds, which, because of fold style similarity, can only be distinguished from the D2-folds by superimposed fold geometry (Figs. 4 and 5b). After restoring younger phases, Dl-folds show isoclinal fold style on a large scale with two main axial planes in the study area (Fig. 5c). The first phase folds affected preexisting structures and the compositional layering of the ultramafic and mafic rocks as well as their lithological contacts. The D2-deformation is the most penetrative on the regional scale and produced the dominant schistosity. This schistosity and the corresponding lineation can be correlated to zones where they are not affected by younger deformation. At these locations, for example the Tambo and the Suretta nappes in the middle and northern parts, the S2 shows a general N to NE dip of about 20°, while the L2 trends sub-horizontally E- W (Marquer, 1991; Baudin et al., 1993; Huber and Marquer, 1996). Large-scale isoclinal folds show an axial planar schistosity parallel to S2 and affect the entire nappe pile (Fig. 4). They show a general SE vergence, which becomes even more evident after restoring the D3 phase (Fig. 5). Large-scale boud- inage during this phase created the actual shape of the Chiavenna unit (Fig. 2). This boudinage was also responsible for the horizontal cut off of the Chi- avenna unit demonstrated in the N-S cross-sections (Fig. 4; top of section 1). A tectonic breccia asso- ciated with in-situ melting was formed at the base of the mega-boudins (see Section 5 and Figs. 6 and 7). The mega-boudins developed in major mylonite zones. These zones form the contacts to the upper unit, the Tambo nappe and the lower unit, the Graf unit. Similar zones exist in the whole nappe pile, for example along the Tambo-Suretta contact (Hu- ber and Marquer, 1996) and the greenschist facies Turba mylonite on top of the Suretta nappe (Fig. 3; Liniger, 1992). These mylonites are the last struc- tures in the continuum of the deformation regime of the D2-phase and show bulk top-to-the-east ductile detachments (Fig. 8). The third deformation phase D3 had less influ- ence than the D2 phase and developed only locally Fig. 4. Four N-S cross-sections across the Chiavenna unit showing the general geometry (see location in Fig. 1). The contact to the surrounding Tambo nappe and Gruf unit is discordantly cross-cut by the second schistosity (black lines in the Tambo nappe and the Gruf unit). Steep D3-folds (AP3: axial planes) and D3-shear zones (SZ3) (see thrusts in Fig. 1) represent the last ductile to brittle-ductile structures. The circle on cross-section 4 locates Fig. 6. Scale: no vertical exaggeration. R.K. Huber, D. Marquer/ Tectonophysics 296 (1998) 205-223 211 No.l Peridotite ÜMaficrocks /finegrained ¦iïiïil \ coarse grained Carbonate :¾¾¾¾¾¾! Tambo nappe .....Gruf unit S No. 3 SZ3 212 K.K. Huber, D. Marquer/Tectonophysics 296 (1998) 205-223 Fig. 5. (a. b) Schematic N-S cross-sections through the Chiavenna unit, structurally restored to before the D3-phase. The D2-folds (AP2: axial planes) have deformed an already complex geometry and pre-existing fold axial planes (AP]). Cross-section (a): compilation of the sections 1 and 2 (Fig. 4: eastern Chiavenna unit); cross-section (b): compilation of sections 3 and 4 (Fig. 4: western Chiavenna unit), (c) After unfolding the D2-folds, Dl-folds (API) are shown in a schematic N-S cross-section through the Chiavenna unit. The major thrust plane represents the subduction zone. The figure is not to scale, (d) Palacogeographic interpretation after restoration of the Dl-phase shows discordant contacts between the topping basalt and the carbonate and the carbonate and the gabbro-peridotite complex. Note the lack of the carbonate in the north and the irregular shape of the gabbro. Basalt extruded from dikes, following Mesozoic normal faults. The black frame surrounds today's outcrop area. The figure is not to scale. in an E-W-trending 20 km wide belt just north of axial planes are the main geometrical structures of the Insubric line (Fig. 3). In the study area, stair- this phase (Fig. 4). There is a deformation gradi- case-like E-W-trending folds with steeply S-dipping ent increasing towards the south (Figs. 3 and 4). R.K. Huber, D. Marquer/'Tectonophysics 296 (1998) 205-223 213 Peridotite Metagabbro Leucosome (granitic melt) Breccia Tremolite vein (Fiber growth parallel to pattern) Melt migration 1 m Fig. 6. Schematic map of lhe outcrop in the lower Val Schiesone (see location in Fig. 4). Melt formation in D2-shear zones (C) in the gabbro and around breccia components (C/C). The melt intrudes in the peridotite along an extension vein. The !ittiologica! contacts are rotated by the D3-deformation and subvertical actually. The fold wavelength decreases while their amplitude increases. An axial planar schistosity with a down dip lineation developed in the strongest deformation zones. Late-D3 brittle-ductile thrusts, still with the same deformation axes, are N-directed and show steeply S-dipping thrust planes. Besides several mi- nor fault zones, there is a major zone in the west which follows the contact of the Chiavenna and Gruf units (Fig. 4; cross-section 2, 3 and 4) and, in the east, cross-cuts the Tambo nappe in the north of the Chiavenna unit (Fig. 4, cross-section 1). All these steep, but not dominant structures are responsible for the steepening of previous structures in this E- W-trending belt north of the Insubric line (Fig. 3, Huber and Marquer, 1996). The fourth deformation phase is linked to the Oligo-Miocene major faults, the Forcola fault (Mar- quer, 1991), the Engadine and the Insubric lines (Zingg and Hunziker, 1990) (Fig. 1). They do not affect the preexisting geometry in the N-S cross-sec- tions (Fig. 4), even where the Forcola fault is just underlying the Chiavenna unit and where the brittle Engadine line just ends at its eastern contact. 5. Way-up indicators of partial melting In the southern part of the Chiavenna unit close to the Gruf contact (Fig. 4) is a zone strongly af- fected by the D3 deformation. Therefore, all the 214 RK. Huber, D. Marquer/Tectonophysics 296 (1998) 205-223 .5 a 1 •e 8. 2 X _ R.K. Huber, D. Marquer/Tectonophysics 296 (1998) 205-223 215 previously created structures are steeply inclined. How much and with which sense of rotation the steepened pre-D3 structures have to be restored to obtain the D2-geometry? With the following obser- vations it is possible to answer this question and to reconstruct the initial geometry and the orientation of the structures associated with the D2 phase. The present-day geometry of the contact between the ul- tramafic and mafic rocks belonging to the Chiavenna unit in the lower Val Schiesone (Fig. 4, cross-section 4) is schematically represented in Fig. 6. The con- tact itself is a sub-vertical dextral shear zone with sub-horizontal E-W-trending stretching lineation (C plane defined after Berthe et al., 1979) occurred dur- ing the D2-phase. The D2-mineral lineations trend sub-horizontally E-W on the shear planes and on the schistosity. The meta-gabbro is composed of amphibole, pla- gioclase, biotite, epidote, ± quartz-ilmenite-mag- netite-titanite. In this meta-gabbro, in-situ granitic melt (30% quartz, 30% K-feldspar, 30% plagioclase and small amounts of biotite and white mica) fills D2 secondary shear zones (C plane after Berthe et al., 1979) (Figs. 6 and 7c). The shearing also caused an asymmetric foliation boudinage with melt col- lected in the interboudin areas (Fig. 6). The melt is an in-situ product, because it fills extension veins and occurs only in narrow zones located in the main shear zones (Fig. 6). This relationships between the structures and the location of melt is an argument against an injected melt coming from deeper levels. The latter would suppose to show cross-cut relation- ships with respect to pre-existing structures which are not present in the studied out-crops. The same granitic melt intruded in the meta- peridotite along E-W-opened extension veins in re- sponse to the D2 stretching axis which corresponds to the D2 stretching lineation (Fig. 7b). The first stage of the extension vein development is indicated by tremolite fiber growth orientation parallel to the D2 stretching lineations (X axis) (Figs. 6 and 7b). A tectonic breccia made up of components from both lithologies occurs between ultramafic and mafic rocks (Figs. 6 and 7a). The components were ro- tated in the sense of the shear and form secondary shear zones (Fig. 6; C). They are surrounded by granitic melt (Fig. 7a). The melt sampled at the immediate contact with the breccia-clasts shows a different composition: almost 100% plagioclase and relic (restitic) biotite and amphibole. In the clasts at the contact with the melt, a poikilitic 'post-kine- matic' hornblende overgrowing the S2-schistosity is present. The melt was formed in structures compatible with deformation D2 in meta-gabbro, but it cuts across the S2-schistosity in the meta-peridotite. This indicates that the melt was formed during the latest stages of D2 high-temperature deformation. This ge- ometric relationship supports a stage of melting prior to the Bergell intrusion, which occurred at the begin- ning of the D3-deformation (32-30 Ma years after von Blankenburg (1992) and Davidson et al. (1996)). Assuming it is in situ, the occurrence of granitic melt in high-strain shear zones of biotite bearing meta-gabbro implies high-temperature deformation. The melting temperature of mafic rocks could sub- stantially decrease under water-saturated conditions, creating peraluminous granitic melt and amphibole- rich restite (Beard and Lofgren, 1990; Rushmer, 1991). Different melting curves of wet mafic rocks are summarised in Rushmer (1991), indicating a minimum temperature for the melting around 7000C ± 500C for pressures between 5 and 10 kbar. This range of pressure-temperature conditions cor- responds to the decompression recorded by the Chi- avenna unit during D2 (see Section 7 and Fig. 9). Wet conditions may be governed by a fluid supply that was localised in the referenced narrow shear zones. The two-mica leucosome and the poikilitic hornblende could indicate a peraluminous melt and restitic hydrous mineral phase, respectively (Beard and Lofgren, 1990; Rushmer, 1991). While the meta-gabbro exhibits ductile structures, the peridotite shows brittle behaviour under the same deformation temperature conditions, emphasised by the tension gash openings and the breccia formation (Fig. 6). During shearing, mylonitisation enhances the fluid circulations along shear zones that can lead to wet melting conditions. The breccia at the interface between the gabbro and the peridotite is interpreted as an hydraulic breccia which, as well as local melting in the gabbro shear zones, occurs because of the presence of a free-fluid phase. The distribution and geometry of melt, that was formed in the meta-gabbro and was collected in the peridotite, suggest that the peridotite was overlying 216 R.K. Huber, D. Marquer/Teclonophysics 296 (1998) 205-223 the gabbro unit at the time of the melt emplacement. The final geometry of this outcrop shows a strong overprint of the D3-phase, reorienting all previous structures (Fig. 6). Therefore, the S2-schistosity and the D2-shear zone are now in a sub-vertical position (Fig. 4, cross-section 4, close to the contact with the Gruf unit). The rotation sense and the amount that occurred during the D3-phase can be estimated by knowing that S2 and SZ2 are gently inclined towards the northeast in regions with little or no D3-overprint (middle and northern Tambo and Suretta nappes in Marquer (1991) and Huber and Marquer (1996)) and the polarity of the D2-phase (Fig. 6). The D3 rotation axes correspond to the D3-fold axes and trend E-W, slightly dipping to the east. Therefore, restoring the D3-phase implies a clockwise rotation around the E- W-trending axes. The final geometry and D3-rotation sense locate the Chiavenna unit on the northern limb of a D3-antiformal structure, which is most probably the Gruf antiform (Fig. 3). Once restoring the S2, SZ2 and the melt into their original position during D2, it becomes apparent that the mafic-ultramafic contact (Fig. 6) is located on the normal limb of a large-scale D2-antiform (Fig. 5b). In other words, the position of the meta- peridotite, overlying the meta-gabbro, is an inherited geometry, older than the D2-phase. 6. Geometrical restoration The first step in the reconstruction leading to the possible geometry before Alpine tectonics is to re- store the youngest deformation phase. Because the D4-phase does not change the present-day geome- try, the D3-structures have been restored as a first step. After unfolding the D3-phase, the geometry of the D2-structures becomes apparent. Four N-S cross-sections point out the D3-deformation in the Chiavenna unit (Fig. 4). The D3 thrust zones lie discordant to the Chiavenna-Gruf contact (Fig. 2). They caused no major offset, leading to a discor- dance at map or cross-section scale (Figs. 2 and 4). In contrast, the D3-folds with steep south-dip- ping axial planes are responsible for the steepening of all previous structures. The main thrust zone, with a top-to-the-north shear sense, is located close to the southern contact of the western Chiavenna unit (Fig. 4, cross-sections l-4).In the eastern Chi- avenna unit, the zone is located in the north of the unit (Fig. 4, cross-section 1). Because the D2-axial planes are subparallel to the D3-axial planes in the present-day geometry of the steep zones, they could be misinterpreted (Fig. 4). The recognition of the two phases is based on field observations such as su- perimposed folding, intersection of schistosities and different directions of stretching lineations. The large-scale effect of the D3-phase was a dif- ferential uplift of the zone north of the Insubric line, which creates an antiform-like structure if the previ- ously flat-lying nappe contacts are regarded as an en- veloping surface of the antiform (Fig. 3). The frontal part of the Tambo and Suretta nappes preserved the original flat-lying orientation of the D2-structures and the nappe contacts (Fig. 3). In the Chiavenna unit, restoring the northern limb of the D3-fold in- volves clockwise rotation around a sub-horizontal E-W-trending axis. This effect was confirmed us- ing the previously described polarity indicator. After this rotation, the northern units (Tambo and Suretta nappes) lie on top of the southern units (Chiavenna and Gruf units) and in that case, large-scale D2- folds (AP2) with a general south-vergence and two major fold planes (API) refolded by D2-deforma- tion become apparent. These are represented in two schematic N-S cross-sections of the Chiavenna unit (Fig. 5a,b). The tectonic contacts of all units appear to have been folded by the D2-phase (Fig. 5a,b). Unfolding the D2-folds leads to the geometry of the Dl-phase. The intense Dl-folds (API) show S-vergence (Fig. 5c). No continuity of the Dl-folds could be found in the Tambo and Gruf units (Fig. 4). Therefore, the overlying Tambo nappe and the under- lying Gruf unit where separated from the Chiavenna unit by a tectonically active contact during the Dl- phase (Fig. 5c). Finally, unfolding the Dl-folds shows the initial geometry of the lithology sequence of the Chiavenna unit (Fig. 5d). At this stage, the peridotite is partially overlain by coarse-grained basic rock, interpreted as gabbro. Several reasons might account for the vary- ing gabbro thicknesses. It might be due to the initial form of an intruded gabbro, or to tectonics, as normal faults or ductile deformation during the Dl and D2 phases (Fig. 5d). Normal faults could have occurred because combined strike-slip and extension events RK. Huber, D. Marquer/Tectonophysics 296 (1998) 205-223 217 formed the Valais basin and the northern margin of the Briançonnais terrane (Fig. 5d and Fig. 10a) (Stampfli et al., 1998). The high deformation un- dergone during Alpine tectonics obliterated these previous structures. On the other hand, the gabbro is covered directly by a thin carbonate layer (Fig. 5d). The carbonates could represent sediments, deposited on an exposed sub-continental mantle intruded by gabbros at a thinned continental margin, as is found in other parts of the Alps (Tasna nappe: Florineth and Froitzheim, 1994; Malenco unit: Hermann, 1997; Müntener, 1997) or in the Galicia margin (Boillot et al., 1995). With this reconstruction, fine-grained basic rocks, interpreted as basalts (MORB; Talerico, 1997), lie discordantly on the carbonate and the peri- dotite (Fig. 5d). A possible interpretation for this unconformable meta-basalt layer on the top of the carbonate rocks is the occurrence of local basic volcanism during the Mesozoic extension leading to an abnormal sequence of oceanic rocks (Fig. 5d). 7. Metamorphism Mineral assemblages for each deformation phase were determined based on textural/microstructural relationships and compared with the T-peak meta- morphism in this area (Niggli, 1970; Schmutz, 1976). The main mineral crystallization in all litholo- gies took place during the D2 deformation phase. Therefore, most often the older mineral assemblages as well as the corresponding structures show relic textures. In ultramafic rocks, the oldest mineral relic is brown spinel. This indicates a Prealpine mantle ori- gin from the spinel-lherzolite stability field (Fig. 8). Few relic mineral phases are preserved from the Dl-phase. A magnetite corona around the spinel, magnetite relic schistosity in enstatite grown prior to the S2 and antigorite with a static overgrowth of olivine and talc (Schmutz, 1976; C. Talerico, pers. commun., 1997) all indicate stable conditions 500 600 700 800 900 1000 T (C°) Fig. 8. The metamorphic history of the Chiavenna unit represented in a petrogrid (HCMAS) for meta-peridotites after Jenkins (1980). The garnet stability field for mafic rocks (after Bûcher and Frey, 1994) and the enstatite and antigorite stability field for ultramafic rocks (after Raymond, 1995) are shown. Prealpine and Alpine mineral assemblages (J-5) are placed in their corresponding stability field and correlated with the Alpine deformation phases (D1-D3). 218 R.K. Huber, D. Marquer/Tcctonophysics 296 (1998) 205-223 in the chlorite-antigorite-clinopyroxene peridotite and the chlorite-antigorite-amphibole peridotite sta- bility fields (Fig. 8). The relic schistosity may be interpreted as Sl. The transition between the Dl- and the D2-phase is represented by the previously mentioned enstatite and olivine, as well as the first growth of chlorite and amphibole. These minerals are texturally older than the S2, but younger than the Dl described mineral phases. The D2-schistosity in the ultramafic rocks is man- ifest by talc, amphibole, sometimes chlorite or/and talc and a second generation of small equigranu- lar olivine. During D2-mylonitization, olivine and amphibole and later antigorite recrystallized. The D2-minerals indicate conditions of the amphibole- chlorite-talc peridotite stability field (Fig. 8). The dominant mineral of the D3-phase is antig- orite, but in early D3-structures some amphibole and chlorite are still abundant. This indicates sta- ble conditions in the amphibole-chlorite-serpentine peridotite field. In Mg-rich amphibolites, the absence of garnet or garnet pseudomorphs limits the maximum pressure to 14 kbar (Fig. 8; gr-out stability curve; after Bûcher and Frey, 1994). Therefore the Pf-conditions for the Chiavenna unit during Dl are estimated at 550- 6500C and 12-14 kbar (Figs. 8 and 9). The Chiavenna unit came in contact with the over- laying Tambo nappe during the Dl-phase. Later, they followed the same tectonometamorphic history. The early Alpine history (Dl-phase) of the Chiavenna unit can be related to the tectonometamorphic evolu- tion of the Valais subduction by taking into account the structural restoration (Fig. 5) and the PT-path of the Tambo nappe (Fig. 9) (Huber, 1997). Therefore, the fT-conditions at the base of the Tambo nappe are a minimum for the Chiavenna unit. This esti- mate is justified, especially taking into account the small thickness of the Chiavenna unit (Fig. 4). The fT-conditions for Dl at the base of the Tambo nappe are about 13 kbar and 500-6000C (Fig. 9) (Baudin and Marquer, 1993). The growth of enstatite occurred between the development of the Sl and the S2. The estimated minimum temperature is around 7000C (Fig. 8; en-in stability curve; after Raymond, 1995). This growth of enstatite suggests a temperature increase from Dl 200 400 600 800 1000C0 Fig. 9. Compiled /T-paths of the northern (Tb-N, Su-N) and southern (Tb-S, Su-S) Tambo and Suretta nappes, the Chiavenna unit and the Adula nappe (Ad, after Meyre et al., 1997). The deformation phases are indicated with Dl, D2 and D3. Note the increase of temperature during the D2-phase in Tb-S, Su-S and the Chiavenna unit, whereas Tb-N and Su-N are submitted to isothermal decompression during D2. Although at higher temperature, the iT-path from the Chiavenna unit shows more affinity to the P7"-paths from the Tambo and Suretta nappes than to the one from the Adula nappe. Prealpine mantle relics of the Chiavenna unit are represented by the spinel-lherzolite stability field. to D2 and during the early D2 (Figs. 8 and 9). The above-described melt-structure relationships in the mafic rocks, forming melt in locally hydrated shear zones during early D2, indicate melting conditions at about 700-7500C. This observation confirms the temperature estimates based on the enstatite crystal- lization (Trommsdorff and Evans, 1969). The further /T-development during D2, based on recrystalliza- tion in successive structures, indicates cooling and decompression in the amphibole-chlorite-talc peri- dotite stability field (Fig. 8). The dominant mineral of the D3-phase is antig- orite, but in early D3-structures some amphibole and chlorite are also abundant (Fig. 8; antg-in stabil- ity curve; Raymond, 1995). Because the first oc- currence of antigorite is related to the latest D2- R.K. Huber, D. Marquer/Tectonophysics 296 (1998) 205-223 219 mylonites, the PT-path enters into the amphibole- chlorite-serpentine peridotite stability field (Fig. 8) at about 5500C and 5 kbar just before the onset of the D3-deformation phase. The PT-path during D3 shows decompression and cooling (Fig. 9). The PT-path for the Chiavenna unit (Fig. 9) has a similar shape to the PJ-paths of the southern part of the Tambo and the Suretta nappes (Tb-S and Su-S in Fig. 9), although with higher temperature. The temperature increases during the early D2-phase, the same pattern as observed in the southern parts of the Tambo and Suretta nappes. In the Briançonnais and Chiavenna units no high-pressure relics were found like in the Adula nappe (Ad in Fig. 9) (Heinrich, 1986; Meyre et al., 1997; Brenker and Brey, 1997). In that case, the tectonometamorphic history of the Chiavenna unit seems to be related more to the overlying Tambo and Suretta nappes than to the underlying Adula nappe. 8. Palaeogeographic interpretation Based on the unfolded Prealpine palaeogeome- try of the Chiavenna unit (Fig. 5c), the position in the nappe pile (Fig. 3) and the described pressure- temperature-paths (Fig. 9), the following palaeogeo- graphic interpretation is proposed, assuming that the main carbonate layer corresponds to Meso- zoic sediments (Fig. 5c). Sediments directly over- lying the peridotite and discordant to the peridotite- gabbro contact indicate a common geometry for a thinned continental margin in a lithospheric exten- sion regime, where the subcontinental mantle rocks become exposed due to deep seated normal faults, subsequently covered by sediments (Florineth and Froitzheim, 1994; Boillot et al., 1995; Hermann, 1997; Miintener, 1997). Such a scenario can be imagined at the southern or northern rim of the Valais basin. As the high-pressure metamorphic con- ditions in the Chiavenna unit are not as high as in the Adula nappe, we attribute the Chiavenna unit to the northern margin of the Briançonnais terranes (Fig. 10a). Therefore, the gabbros may be inter- preted as intrusions near the mantle-crust boundary, intruded before the Mesozoic extension. Two expla- nations are possible for the variable thickness of the gabbros (Fig. 5c): an initial heterogeneity due to the intrusion shape or a change of the thickness due to the effect of normal faulting. The overlying basalts may be explained by local extrusion of basic volcanic rocks in the vicinity of these normal faults during the first stages of the Valais basin opening. These basalt extrusions, cross-cutting the denuded subcontinental mantle at the northern margin of the Briançonnais terranes, could represent an early stage in the de- velopment of the formation of Valais oceanic crust (see review of the geological evolution of Briançon- nais terranes in Stampili et al., 1998). During the subduction of the Valais basin, corresponding to the first Alpine deformation phase (Dl), in sequence thrusting tectonics leads to the emplacement of the Chiavenna unit into the nappe pile (Fig. 10b). 9. Conclusions Assuming that the carbonates are of Mesozoic age, the unravelling of the geometry of the Chi- avenna unit shows peridotites originating from a subcontinental mantle. Peridotites were intruded by gabbros located at the crust-mantle boundary at pre-Mesozoic time. During the Mesozoic extension, mantle denudation led to development of the Valais basin between the north of the Briançonnais ter- ranes and the south European margin. This extension is responsible for the characteristic geometry of a denuded subcontinental mantle in the vicinity of a thinned continental margin, as well as its exposure at the seafloor resulting in the deposit of an uncon- formable carbonate cover. The position of the Chi- avenna unit in the nappe pile and the similar shape of its pressure-temperature-path, compared to those of the Tambo and the Suretta nappes, favour the origin of the Chiavenna unit at the northern thinned margin of the Briançonnais terranes. During the subduction of the Valais basin, thrusts at mid-crustal level initi- ated the nappe geometry of the Tambo and Suretta nappes, whereas the lower parts of the crust disap- peared in the subduction zone (Fig. 10). The initial geometry and location of the Chiavenna unit and movement along the mantle-crust décollement led to the emplacement of the Chiavenna unit in the nappe pile (Fig. 5c, Fig. 10b). During this deformation phase (Dl), the first isoclinal folds were formed. The second isoclinal folds (D2) affecting the contacts 220 /?. K. Huber, D. Marquer/Teclonophysics 296 (1998) 205-223 N Valais Basin South Valais Transform Margin ©! 0 Subcontinental mantle a) km 0-) 10 20' 30- 40- b) Fig. 10. (a) A model for the palaeogeographic reconstruction of the Chiavenna unit interpreted as denuded subcontinental mantle at the thinned northern margin of the Briançonnais terranes (Tb: Tambo nappe; Su: Suretta nappe; Eu: European margin). Late Mesozoic sinistral transtension in the Valais basin (Val) forms the geometry of the northern Briançonnais margin with steep normal faults and assists to the exhumation of the Chiavenna unit. Tertiary thrusts (Dl) in the Briançonnais nappes are indicated with black arrows, (b) Nappe stacking during the subduction event (Dl). Most of the lower crust (grey pattern) gets subducted, while the Briançonnais domain is overriding the Valais basin (Val) and the European margin (Eu) and Adula nappe (Ad). of the stacked tectonic units were formed during syn-collisional extension. Associated thermal equili- bration led to the thermal peak during the D2 Alpine metamorphism locally creating a melt that intruded into the overlying peridotite. Late Alpine uplift along the Insubric mylonites led to the final geometry of the Chiavenna unit, shown by the steepening of the nappe pile and N-directed local thrust. Acknowledgements The research was supported by the Swiss National Founds, FSNRS No. 20.45 405.95. Caterina Talerico and Othmar Miintener from the ETH-Ziirich are thanked for stimulating discussions. Ch. Teyssier and other reviewers are acknowledged for suggesting significant improvements to this paper. RX. Huber, D. Marquer/ Tectonophysics 296 (1998) 205-223 221 References Anonymous, 1972. 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EUG 1995 THE RELATIONSHIP BETWEEN EXTENSION AND UPLIFT-EROSION PROCESSES IN THE ROOT ZONE OF THE EASTERN PENNINE NAPPES (VAL BREGAGLIA) R.K. HUBER. T. BAUDIN (Institut de Géologie, rue E.-Argand 11, 2007 Neuchâtel, Suisse) Exhumation can be caused by two main processes, either by normal faulting or by uplift-erosion processes. A distinction can be made by taking into account structural and metamorphic data, which is the aim of this work. The region of interest is the pennine nappes of the Briançonnais zone located in the eastern Central Swiss Alps. It has been chosen because of its structures of the late alpine orogenic evolution. This work is based on detailed structural and microstructural mapping. The evaluation of field data gives the following sequence of structural phases: A first gently N dipping schistosity and stretching lineation has been overprinted by a second schistosity whitch dips moderately to the NE. Its associated stretching lineation dips to the E and is linked to shear zones showing a top-to-E movement. They are responsible for the offset of lithological units in normal faults. The third schistosity and lineation are both steeply S dipping. They are contemporaneous with steeply inclined N dipping and S thrusting shear zones. In very limited zones, a fourth phase is formed by E dipping normal faults with a NE dipping lineation. Late lithological and structural offsets can be linked to it. The first phase is due to nappe stacking during the subduction of the Briançonnais basement. The second phase is due to syn-orogenic ductile extension in the nappe pile. This extension is due to a buoyancy desequilibrium caused by the abnormal thickness of the crust. The third phase formed during late continental collision which caused a major uplift accompanied by erosional exhumation. The fourth phase involves late orogenic normal faulting due to the movements along the Engadine and Insubric Lines. 2. TMIDSR 1995 ALPINE TECTONIC AND METAMORPHIC EVOLUTION OF THE ROOT ZONE OF THE EASTERN PENNINE NAPPES (VAL BREGAGLIA, CENTRAL SWISS ALPS) RACHEL K. HUBER, THIERRY BAUDIN & DIDIER MARQUER Institut de géologie, Université Neuchâtel, Switzerland Tertiary deformation and kinematics, as well as the associated metamorphic assemblages were defined in the root zone of the eastern pennine nappes. This data base allows to propose an interpretation about the tectonic evolution during Tertiary collision of the studied area and a correlation with the regional tectono-metamorphic history. The region of interest is formed by the southern parts of the Tambo and Suretta nappes, located in the Val Bregaglia, Central Swiss Alps. Those are in the position of the two upper most eastern pennine units in the nappe pile and are derived paleogeographically from to the Briançonnais zone. These heterogeneously deformed nappes are mainly build up by a polycyclic and polymetamorphic basement consisting in metasediments, amphibolites and metagranitoids. They are crosscut by late Variscan intrusives and overlain by a thin autochthoneous Permio-Mesozoic cover. The upper part of the Mesozoic cover is formed by an allochthoneous unit, the Starlera nappe. The Tambo and Suretta nappe are underlain by the "Ophiolites of Chiavenna" and the Adula nappe and overlain by the Avers schistes lustrés. This structural data, acquired by structural and microstructural fieldwork, allow to draw schistosity and lineation trajectory maps and to define the kinematics of each deformation phase. Finally, microprobe and microscope analysis permit to determinate the metamorphic evolution. Based on this survey the following phases can be distinguished: The first phase shows a ductile heterogeneous deformation with localized top to the N moving shear zones. Its schistosity and adjacent lineation are gently NNW dipping. The corresponding mineral phases recrystallized under HP conditions (eg. Tambo nappe: -13-10 kb, ~500°C). This first phase is overprinted by a ductile second phase, which creates the main gently NNE dipping schistosity. Its adjacent sub horizontal lineation trends EW. Contemporaneous shear zones with a top to the E movement can be observed at all scale. They are responsible for the offset of the lithological contacts and the thinning of the nappes in the southeast. The second phase was set under decompressional but constant thermal conditions (eg. Tambo nappe: -11-6 kb, -5000C). The third phase is observable only in restricted areas. Its schistosity and lineation are both steeply S dipping and are associated to E-W striking open folds. They are contemporaneous with a set of steeply inclined S dipping N thrusting shear zones and its conjugated N dipping and S thrusting minor set. This phase is responsible for the steepening and reorientation of the second schistosity in the root zone and the pop up of the Bergell area. It took place under lower greenschist facies conditions close to the brittle- ductile transitions. The forth phase is formed by major NE dipping normal faults. The first phase is interpreted as the main nappe stacking towards NW during the Eocene subduction of the Briançonnais basement during the closure of the Valais trough. The second phase is due to syn-orogenic ductile extension in the nappe pile parallel to the orogenic belt axe. This Eocene to Oligocene extension is due to the relaxation of a buoyancy desequilibrium caused by the abnormal thickness of the crust and led to the observed isothermal decompression. The third phase formed during late continental collision implied a differential uplift with the major elevation in the southern most parts. The uplift forces erosional exhumation and induces the starting of the cooling. This Oligocene phase is syn-post Bergell intrusion. The fourth phase involves late orogenic normal faulting contemporaneous to the movements along the Engadine and Insubric Lines (Ss. Tonale Line) and is a symmetric equivalent of the brittle Simplon phase in the western part of the Pennine zone. 3. EUG 1997 The Tectonometamorfic Evolution of the Continental Crust and the Upper Mantle during the Alpine Tertiary Continental Collision shown at the example of the Eastern Penninic Nappes (Val Bregaglia, Switzerland) Rachel K. Huberl (huber@geol.unine.ch) 1 Institute of Geology, Neuchâtel University, Neuchâtel, Switzerland The tectonometamorphic evolution during alpine Tertiary continental collision of several adjacent tectonic units from the upper crust to the upper mantle has been established taking into account metamorphic and structural data. Different processes are proposed to explain the diverging, respectively converging PT-paths of the units. The worked out area is situated in Southeastern Switzerland and contains the eastern penninic nappes. Today the units are stacked from the bottom to the top as following: Gruf unit (lower crust, no sediment cover) and Chiavenna unit (ultramafic and mafic rocks, no sediments), Tambo and Suretta nappes (upper crust up to sediment cover). They are overridden by the Austroalpine units and crosscut by the Tertiary Bergell intrusion. Unraveling the suite of structural deformation phases allows indications about the kinematic evolution of the tectonic units, whereas the mineral assemblages linked to each particular deformation phase describe a time relative PT-path. The PT-paths of the Suretta and Tambo nappes shows a similar path from HP to isothermal decompression and Barrovian cooling (Suretta: Pmax. 9-12 kb, Tmax. 500- 560°; Tambo: Pmax. >12 kb, Tmax. 580-650°). The Chiavenna unit shows constant cooling and decompression from subcontinental mantle depth to exhumation (Pmax. >10 kb, Tmax. 700-800°). So only the second part of the PT-path for the Tambo and Surtta nappe is similar to the path for the Chiavenna unit. The PT-path for the Gruf unit indicates isothermal decompression and later Barrovian cooling from HP-HT conditions to exhumation. The highest condition's mineral assemblage is related to prealpineroxene, Garnet, Sillimanite: Pmax. >10 kb, Tmax. 750-800°). It represents a mixed PT-path, which shows only a similar form to the paths from Tambo and Suretta during the cooling/decompression history. The Tambo and the overlaying Suretta nappe had the same tectonic history through the Tertiary orogenesis; they passed through subduction, and after the continent-continent collision, to exhumation. No LP-LT relicts of an oceanic stage can be found in the Chiavenna unit. They show only an exhumation, but no subduction history. This fact is interpreted as a deeply situated origin of the ultramafic and mafic rocks, joining the general PT-path trend after the continental collision. A possible subduction event does not modify the original P-indicators. The Gruf units inherited dray HP-HT mineral assemblage may be responsible for the lack of registration of an HP-LT assemblage during the alpine subduction event. But it shows distinctly the general PT-path trend after the continental collision during exhumation. The long lasting HT condition could be maintained by the near melt formation close to the Bergell intrusion. Extension in the crust and the upper mantle, preceding subduction, may explain the emplacement of the originally deep seated Gruf and Chiavenna units at upper crustal level. 4. Padova 1998 TECTONOMETAMORPHIC EVOLUTION OF THE EASTERN PENNINE ALPS DURING TERTIARY CONTINENTAL COLLISION: STRUCTURAL AND PETROLOGICAL RELATIONSHIPS BETWEEN SURETTA-, TAMBO-, CHIAVENNA AND GRUF UNITS (SWITZERLAND/ITALY). Rachel K. Huber & D. Marquer Institut de Géologie, Université de Neuchâtel, E.-Argand 11, 2009 Neuchâtel, Suisse The Suretta and Tambo nappes, as well as the Chiavenna and Gruf units belong to the Eastern Pennine nappe pile. The region of interest is formed by the southern parts of the Tambo and Suretta nappes, located in the Val Bregaglia, Eastern Swiss Alps. Those are in the position of the two upper most eastern pennine units in the nappe pile and are derived paleogeographically from to the Briançonnais zone. These heterogeneously deformed nappes are mainly build up by a polycyclic and polymetamorphic basement consisting in metasediments, amphibolites and metagranitoids. They are crosscut by late Variscan intrusives and overlain by a thin autochthonous Permo-Mesozoic cover. The Tambo and Suretta nappe are underlain by the Chiavenna and Gruf units. The Chiavenna unit is built up mostly by ultramafic and mafic rocks with a subcontinental to oceanic origin. The Gruf unit consists of metagranitoids and granulites of presumably pre-alpine age and migmatites of alpine and pre-alpine age. Tertiary deformation and kinematics, as well as the associated metamorphic assemblages were defined in the four tectonic units. This data base allows to propose an interpretation about the tectonic evolution during Tertiary collision of the studied area and a correlation with the regional tectono- metamorphic history. Based on this survey the following phases can be distinguished: The first phase shows a ductile heterogeneous deformation with localized top to the N-moving shear zones. Its schistosity and adjacent lineation are gently NNW dipping. The corresponding mineral phases recrystallized under HP-conditions (eg. Tambo nappe: -13-10 kb, ~500°C). This first phase is overprinted by a ductile second phase, which creates the main gently NNE-dipping schistosity. Its adjacent sub-horizontal lineation trends EW. Contemporaneous shear zones with a top to the E- movement can be observed at all scale. They are responsible for the offset of the lithological contacts and the thinning of the nappes in the southeast. The second phase underwent decompression with a thermal maximum at the beginning of D2 (eg. Tambo nappe: -11-6 kb, -6100C), whereas in the northern Tambo and Suretta nappes almost isothermal decompression took place. The PT-conditions during the D2-phase increase through the nappe pile from top to bottom (Suretta nappe: -10-5 kb, -55O0C and Gruf unit: -10-4 kb, -73O0C). In the north, the third phase is observable only in restricted areas, whereas in the south, the whole area is isoclinally folded. Its schistosity and lineation are both steeply S-dipping and are associated to folds with E-W striking fold axes. They are contemporaneous with a set of steeply inclined S-dipping N-thrusting shear zones and their conjugated N-dipping and S-thrusting minor set. This phase is responsible for the steepening and reorientation of the second schistosity in the root zone and the pop up of the Bergell area. In the north, this deformation took place under lower greenschist facies conditions close to the brittle-ductile transition and in the south in the Gruf unit, it started at -4 kb, ~550°C. The forth phase is formed by major NE-dipping normal faults sub-parallel to the Forcola fault. The first phase is interpreted as the main nappe stacking towards NW during the Eocene subduction of the Briançonnais basement during the closure of the Valais trough. The second phase is due to syn-orogenic ductile E-W extension in the nappe pile parallel to the orogenic belt axes. This Eocene to Oligocene extension is due to the relaxation of a buoyancy desequilibrium caused by the abnormal thickness of the crust and led to the observed isothermal decompression. The third phase formed during late continental collision implied a differential uplift with the major elevation in the southern most parts. The uplift forces erosional exhumation and induces the starting of the cooling. This Oligocene phase is syn-post Bergell intrusion. The fourth phase involves late orogenic normal faulting contemporaneous to the movements along the Engadine, Insubric Lines (Ss. Tonale Line) and the Forcola fault and is a symmetric equivalent of the brittle Simplon phase in the western part of the Pennine zone.