Mise en évidence d'une tectonique tangentielle antérieure au plissenent majeur dans la chaîne hercynienne de l'Anti-Atlas occidental, Maroc

Variscan deformation in the Palæozoic foreland of the Moroccan Anti-Atlas is characterised by its heterogeneous character and its dampening effect towards the Tindouf Basin. Structural analyses shows three sequential episodes of deformation, D1, D2 and D3, displayed at different levels of observation. The first episode D1, which was observed for the first time in the western Anti-Atlas Hercynian Belt provides clear evidence for tangential tectonic deformation operating before the major phase of folding. It corresponds to an early event of ‘thin-skin tectonics’, which develops a composite schistosity related to ductile horizontal pure shearing. It is considered that D1 was responsible for the first folding of the sedimentary sequences associated with several imbricated thrust sheets with vergence to the east. The second episode, D2, represents the major phase of north-northeast-southwest Variscan folding. The D3 episode was more discrete, of brittle type and marked by subequatorial sinistral kink-bands.This D1 tangential event, in the western Anti-Atlas, Morocco, represents a typical structural domain as part of the tectonic evolution during Variscan-Appalachian collision.