THE VARISCAN MONTAGNE NOIRE IN SOUTHERN FRANCE: A DEXTRAL SIGMA CLAST IN A TRANSPRESSIVE SUTURE

The Montagne Noire is located at the southernmost tip of the French Variscan Massif Central, the metamorphic internal zone of amalgamated continental blocks that formed during collision of multiple terranes between Laurentia and Gondwana during the Paleozoic (DEMANGE, 1998).

The Montagne Noire has been interpreted by others as a classical “mantled gneiss dome” with an ENE-trending core (Zone Axiale) covered by low-grade metamorphic nappes. To the N and S, the Zone Axiale is bounded by two Late-Variscan E-W striking shear zones. It is composed of gneisses and migmatites intruded by granites and surrounded by a thin envelope of amphibolite- to greenschist-facies metasediments (called Schistes X).

Our new mapping of the NE termination of the Zone Axiale revealed a complex tectono-metamorphic development of six different normal fault bounded units in the Schistes X. These six units preserve different deformation and folding phases that are difficult to correlate, different numbers of deformation phases, and different distributions of HT-LP metamorphic index minerals formed during two metamorphic events.

These data lead to the interpretation that the different units came together by late transitional ductile to brittle normal faults after having been ductilely deformed and metamorphosed at higher PT conditions. The dip direction of the faults rotates from the SE in the SE to the NNE in the NE of the Zone Axiale. Top-to-NE dipping ecc structures indicate the same late extensional tectonics. Both structures seem to be continuous from the NE of the Zone Axiale in the Schistes X up to the boundary of the Stephanian sedimentary half graben of Graissessac N of the northern shear zone.

Based on the internal structural development and the overall shape of the Zone Axiale, we interpret the Montagne Noire as a sigma-shaped piece of crust bounded by dextral shear zones in the suture between the internal zone of the Massif Central and Gondwana in Late Variscan stages. The late extensional brittle tectonics in the NE of the Zone Axiale may reflect their position in the releasing bend of the northern shear zone. We thus favor the lateral transpressive convergence model from DEMANGE (1998) with Early Variscan contraction dominating ductile phases and localized simple shear tectonics causing brittle structures at the end of the Variscan orogeny.