Successive shearing tectonics during the Hercynian collisional evolution of the southwestern French Massif Central

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 D (sub 1) , which occurred probably in the Lower Devonian, is responsible for the south-westward thrusting of the high metamorphic Levezou nappe which belongs to the Upper Gneiss Unit above the Lower Gneiss Unit overlying itself the Para-autochthonous Unit, (locally called the St-Sernin-sur-Rance nappe). In the late Devonian-early Carboniferous, this stack of nappes is reworked by a second event, D (sub 2) , 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 evolves into a dextral strike-slip fault from west to east. The D (sub 2) event is followed by a top-to-the south D (sub 3) thrusting dated around 330-340 Ma. The main feature of the D (sub 3) compressional stage is the emplacement of the whole stack of nappes previously structured by D1 and D2 events upon the Albigeois series. The D (sub 3) event produced south-verging recumbent folds in the Albigeois, Mont-de-Lacaune and Montagne Noire. The significance of the D (sub 2) event either as thrusting or normal faulting is discussed.