COLLAPSE OF THE THICKENED VARISCAN CRUST IN THE FRENCH MASSIF-CENTRAL - MONT PILAT EXTENSIONAL SHEAR ZONE AND ST-ETIENNE LATE CARBONIFEROUS BASIN

A large-scale extensional shear zone is described from the Mont Pilat area in the northeastern part of the French Massif Central. This shear zone is about 2 km thick and is characterized by well-developed N-dipping foliation and stretching lineations. Deformation occurred under conditions of retrograde metamorphism from high-temperature conditions to greenschist facies. It is superimposed on a composite foliation related to the earlier Variscan (380-350 Ma) compressional events associated with southward emplacement of nappes and HP-MT metamorphism. Regional N-S lineations and shear criteria observed at a regional scale indicate a northward displacement of the hanging wall during extension. Large strains locally recorded in thick mylonitic zones suggest that displacement was substantial (some tens of kilometres). Late superimposed extensional brittle deformation and pseudotachylites show that northward displacement on normal faults continued at a shallower depth. A large granitic pluton (Granite du Velay), intruded late in the the Variscan orogeny, was associated with extension. Some small leucogranitic syntectonic melts were emplaced within the shear zone and are strongly deformed. Geochronological (Rb/Sr) data on these leucogranite bodies give an age of 320 Ma and 40Ar 39Ar dating of synkinematic biotites from gneissose rocks in the shear zone yields a plateau age of 313 Ma. These data show that extensional ductile deformation and magmatism were still active during the latest Late Carboniferous. Thus, extension occurred immediately after the well-known late Palaeozoic collision-type compressional tectonics which had significantly thickened the crust. We propose that the extensional tectonism described here is due to the gravitational collapse of the initially thick Variscan crust. The Stephanian intracontinental sedimentary basin of St. Etienne was formed along the hanging wall of the normal shear zone during the late stages of this post-orogenic crustal extension. © 1990.