Le bassin Stephanien de St-Etienne ne résulte pas d'une extension tardi-hercynienne généralisée: c'est un bassin pull-apart en relation avec un décrochement dextre

Abstract

The St-Etienne area (Stephanian coal-basin and Mt Pilat metamorphic massif) has been taken as one of the examples of late orogenic extension in the Variscan belt. The key structure was the Mt Pilat ductile normal fault which was considered as the southern boundary of the St-Etienne coal basin. We show with precise field observations and taking into account the old subsurface data obtained from coal exploitation that this structural interpretation must be modified. The Stephanian coal-basin is a pull apart basin on a NE-SW strike-slip fault, which is distinct from the Mt Pilat detachment. We compare the latter to the North Himalayan normal fault and we discuss the evolution of this part of the Hercynian belt from the Monts du Lyonnais to the Cévennes al lithospheric scale.