A new interpretation of Stephanian deformation in the Decazeville basin (Massif Central, France); consequences on late Variscan tectonism.
Résumé
Five stages of faulting were observed in and around the Stephanian Decazeville basin, in the SW French Massif Central, at the southern edge of the Sillon houiller fault. The older stage ends during middle Stephanian time, and corresponds to a strike-slip regime with N-S shortening and E-W extension. Before the end of the middle Stephanian, three other stages were recorded: two strike-slip regimes with NW-SE, then E-W compression and NE-SW, then N-S extension; and finally a NNE-SSW extensional regime during the main subsidence of the basin from the end of the middle Stephanian to late Stephanian. Based on mining documents, a new interpretation of the N-S striking folds of the Decazeville basin is proposed. Folding may not be associated with E-W compression but with diapirism of coal seams along syn-sedimentary normal faults during the extensional phase. A last strike-slip regime with N-S compression and E-W extension may be related to Cainozoic Pyrenean orogeny. At a regional scale, it is suggested that from the end of the middle Stephanian to the late Stephanian, the main faults in the Decazeville basin may represent a horsetail splay structure at the southern termination of the Sillon houiller fault.