Successive shearing tectonics during the Hercynian collisional evolution of the southwestern French Massif Central.
Résumé
In the French Massif Central, the Devonian-Carboniferous tectonic evolution of the Rouergue-Albigeois area is characterized by three phases of low-angle ductile shearing. The first event D1, which occurred probably in the Lower Devonian, is responsible for the south-westward thrusting of the high metamorphic Lévézou nappe which belongs to the Upper Gneiss Unit above the Lower Gneiss Unit overlying itself the Para-autochthonous Unit, (locally called the St-Serninsur-Rance nappe). In the late Devonian-early Carboniferous, this stack of nappes is reworked by a second event, D2, characterized by a top-to-the-NW shearing of the Para-autochthonous Unit upon the Lower Gneiss Unit developed under medium pressure/medium temperature metamorphism. The contact between the Lower Gneiss Unit and the Para-autochthonous Unit is a top-to-the NW low-angle fault that progressively evolues into a dextral strike-slip fault from west to east. The D2 event is followed by a top-to-the south D3 thrusting dated around 330–340 Ma. The main feature of the D3 compressional stage is the emplacement of the whole stack of nappes previously structured by D1 and D2 events upon the Albigeois series. The D3 event produced south-verging recumbent folds in the Albigeois, Mont-de-Lacaune and Montagne Noire. The significance of the D2 event either as thrusting or normal faulting is discussed.