The Montagne Noire Hercynian Axial Zone is not an extensional metamorphic core complex but a compressional post-nappe anticline with an anatectic core

Recently, the metamorphic domes of the Hercynian belt and particularly the Montagne Noire Axial Zone have been interpreted as Basin and Range type extensional structures. These domes would have result from a general gravitational collapse of the Hercynian belt during Stephano-Permian times. The Montagne Noire dome would have been formed by roll under and denudation of the lower crust by the help of a relational normal fault separating the axial zone from the Graissessac Stephanian basin. In this hypothesis, the ductile deformation of the metamorphic Axial Zone and especially the lineations would have been formed during an extensional event synchronous with the deposition of the Stephanian Graissessac and the Permian St-Affrique and Lodeve basins. In our opinion this interpretation is in agreement neither with the structural data nor with the geochronological data obtained in the eastern part of the Zone Axiale. If the dome had been formed by an extensional event, the ductile lineations related to the main north-dipping detachement fault should be roughly normal to the long axis of the dome with top to the north shearing. They are in fact parallel to the long axis of the dome and related to an eastward and westward shearing respectively toward the periclinal extremities of the gneissic core. Most of the ductile deformation including the E-W to ENE-WSW trending lineations are older than the 327 +/- 5 Ma Vialais anatectic granite dated by U/Pb method on monazites and zircons. Thus the age of the deformation is not Stephano-Permian.