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Helg, Urs

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Helg, Urs
Affiliation principale
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Ancien.ne collaborateur.trice
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https://libra.unine.ch/handle/123456789/34642

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  • Publication
    Accès libre
    Illite crystallinity patterns in the Anti-Atlas of Morocco
    (2008)
    Ruiz, Geoffrey M. H.
    ;
    Helg, Urs 
    ;
    Negro, François 
    ;
    Adatte, Thierry 
    ;
    Burkhard, Martin 
    The low-grade metamorphism of the sedimentary cover of the Moroccan Anti-Atlas is investigated using Illite crystallinity (IC) method. More than 200 samples from three key areas (southwestern, central and eastern Anti-Atlas) have been taken from a maximum of different stratigraphic levels and have been analysed. The metamorphism is of low to very low degree throughout the southern flank of the Anti-Atlas. It increases from northeast to southwest. Whereas in the eastern Anti-Atlas diagenetic and anchizonal IC-values are predominant, in the western and central Anti-Atlas also epizonal IC-values are found. In every respective area the IC improves with stratigraphic age. At the scale of the entire Palaeozoic Anti-Atlas basin the IC correlates best with estimated paleo-overburden. However, burial metamorphism cannot be the cause even though considering missing sedimentary pile of Late Carboniferous age. The ‘abnormal’ paleo-geothermal gradient of 43–35 °C/km we evidenced for the Carboniferous is a true one, and has to be related to a basement sequence enriched in heat producing elements such as series of the West African Craton.
  • Publication
    Accès libre
    Tectonics of the Anti-Atlas of Morocco
    (2006)
    Burkhard, Martin 
    ;
    Caritg, Séverine
    ;
    Helg, Urs 
    ;
    Robert-Charrue, Charles
    ;
    Soulaimani, Abderrahmane
    Tectonique de l'Anti-Atlas marocain. L'Anti-Atlas est revu et examiné sous l'angle de sa signification géodynamique comme bassin paléozoïque et comme chaîne plissée paléozoïque. Le raccourcissement est accommodé par le plissement polyharmonique de la couverture, avec une nette implication du socle. Aucun système significatif de chevauchement ni duplex ne s'est développé. L'Anti-Atlas est un bassin intracratonique fortement inversé plutôt qu'une partie de la marge passive de la Paléotéthys. L'inversion doit dater du Carbonifère tardif/Permien précoce. La direction du raccourcissement a changé au cours du temps depuis une direction NW–SE vers une direction nord–sud et peut-être même NE–SW, ce qui conduit à la formation de figures d'interférences de plis en dômes et bassins aux échelles allant de 100 m à 10 km., The Anti-Atlas is reviewed and examined in the light of its geodynamic significance as a Palaeozoic basin and fold belt. Shortening is accommodated by polyharmonic buckle folding of the cover in a thick-skinned fashion without the development of any significant thrust/duplex systems. The Anti-Atlas is heavily inverted deep intracratonic basin, rather than a former passive margin of the Palaeo-Tethys Ocean. Inversion took place in Late Carboniferous to Early Permian times. Main shortening directions changed from NW–SE to north–south and maybe NE–SW through time, leading to the development of dome and basin patterns on scales from 100 m to 10 km.
  • Publication
    Accès libre
    Fold interference patterns in the Late Palaeozoic Anti-Atlas belt of Morocco
    (2004)
    Burkhard, Martin 
    ;
    Caritg, Séverine
    ;
    Ducommun, Romain
    ;
    Helg, Urs 
    ;
    Kopp, Lionel
    ;
    Sue, Christian
    We document two phases of folding within the central part of the Late Palaeozoic Anti-Atlas chain of Morocco. A first generation of SW-NE folds involve a horizontal shortening of 10-20%, accommodated by polyharmonic buckle folding of contrasting wavelengths in Ordovician Jbel Bani quartzites and Devonian Jbel Rich carbonates. A second generation of folds with similar style and wavelengths in an E-W direction lead to complex interference patterns. Dome and basins are developed within the Jbel Rich and within Lower Cambrian dolomites. Both folding phases are related to thick-skinned uplift of Precambrian basement in a Laramide style. In contrast to the typical Rocky Mountain foreland style, however, cover deformation in the Anti-Atlas is mostly decoupled from the undying basement along thick incompetent horizons such as the Lower Cambrian Lie-de-Vin and Silurian shales.
  • Publication
    Accès libre
    Folding and inversion tectonics in the Anti-Atlas of Morocco
    (2004)
    Helg, Urs 
    ;
    Burkhard, Martin 
    ;
    Caritg, Séverine
    ;
    Robert-Charrue, Charles
    The late Variscan Anti-Atlas of Morocco shows some conspicuous deviations from the standard anatomy of foreland fold-and-thrust belts. Large basement inliers crop out at a very short distance of less than 50 km behind the southeastern front of the fold belt, reminiscent of Windriver-style basement uplifts. In contrast to the Rocky Mountain foreland, however, the Anti-Atlas basement uplifts punctuate tightly folded Paleozoic cover series similar in tectonic style to the Appalachian Valley and Ridge province. Cover shortening is exclusively accommodated by buckle folding, and the Anti-Atlas fold belt lacks any evidence for duplexing or thrust faults other than the occasional steep reverse fault found near basement inliers. Basement domes have classically been considered as the result of vertical tectonics in a horst and graben fashion, or, alternatively, as large “plis de fond” [ Argand, 1924 ], basement folds. Unfolding of a large portion of an Ordovician quartzite marker bed reveals a minimum shortening of 17% (30 km). Balancing this section at the crustal scale indicates a lower crustal detachment level at 18 to 25 km depth. Basement shortening is inferred to be accommodated through massive inversion of former extensional faults, inherited from a Late Proterozoic-Lower Cambrian rifting phase.