TECTONIC EVOLUTION OF THE MONTAGNE NOIRE (FRENCH MASSIF-CENTRAL) - A MODEL OF EXTENSIONAL GNEISS DOME

The tectonic interpretation of the Montagne Noire Gneiss Dome (southern French Massif Central) has been controversial for a long time. Several models have been proposed : diapiric uplift, wrenching and diapirism, compressive anticline, and metamorphic core complex. Evidence for extensional tectonics in the French Variscan Belt favours the latter interpretation. Strain and metamorphism patterns in the eastern part of the Montagne Noire result from two successive extensional deformations during Late- Carboniferous to Permian times. The occurrence of a major detachment zone along the northern edge of the Montague Noire Gneiss Dome as well the presence of sedimentary basins to the north point to the asymmetry of the Stephanian-Permian extensional system. We propose a new model of gneiss dome involving isostatic uprising and consecutive tectonic denudation of the ductile lower crust. This process results in asymmetrical extensional systems characterized by roll-under folding of the footwall and development of basins in the hangingwall as in the Montagne Noire. The model is finally discussed in comparison to previous interpretations.