Thrust-related, diapiric, and extensional doming in a frontal orogenic wedge: example of the Montagne Noire, Southern French Hercynian Belt

The Montagne Noire, which is situated at the toe of the orogenic wedge of t he French Massif Central South European Variscides, appears to be a well-su ited area for studying the origin and evolution of middle to upper crustal domes adjacent to foreland basins. The data reported in the present paper s how that the Montague Noire dome is a particular type of basement-involved frontal culmination in an orogenic wedge and foreland basin system. This fr ontal culmination is characterized by a syn-contractional HT decompression recorded by clockwise PTt paths and widespread strata overturning in thrust and fold structures, which controlled the sedimentation in the adjacent fo reland basin. These unusual characteristics are interpreted to be a result of the succession of thrusting, diapirism and extensional collapse. Antifor mal stacking of syn-metamorphic thrust sheets controlled the first stages o f the foreland basin development. Diapirism was essentially responsible for the HT decompression and widespread strata overturning. Extensional doming was a result of late- to post-metamorphic collapse acting on the preexisti ng high-amplitude dome. Diapirism and associated isothermal decompression m etamorphism, which constitute the essential difference between the Montagne Noire and 'ordinary' frontal ridges in orogenic wedges, were probably enha nced by a local partial melting of the upper to middle crust. It is suggest ed that the occurrence of these phenomena in front of an orogenic wedge was related to local over-thickening due to the superposition of an upper crus tal antiformal stack on top of a lower crustal ramp anticline. (C) 2001 Els evier Science Ltd. All rights reserved.