Thermal and mechanical evolution of an orogenic wedge during Variscan collision: an example in the Maures–Tanneron Massif (SE France)

Description

Synthesis of structural, petrological and geochronological data for the Maures– Tanneron Massif and its integration in the framework of adjacent massifs (i.e. Sardinia and Corsica) has allowed us to propose a new model of evolution for the southern Variscan belt. After Siluro-Devonian subduction associated with high-pressure– low-temperature (HP/LT) metamorphism M 0 (c. 10– 15 8C km 21) and subsequent Carboniferous nappes stacking, the belt underwent strong reworking related to back-thrusting. Nappes stacking and back-thrusting were associated with typical Barrovian metamorphism M 1 (c. 20–30 8C km 21) starting at 360 Ma that progressively evolved to higher temperature metamorphisms M 2 (c. 40– 60 8C km 21) and M 3 (c. 60– 80 8C km 21) during 330–300 Ma in the internal part of the belt. Progressive increase of the thermal gradient is interpreted as a consequence of gravitational instabilities triggered in the partially molten orogenic root. Continuous compressive forces applied to the belt allowed vertical extrusion of the orogenic root in fold-dome structures. The mass transfer is accommodated by orogen-parallel transpressive shearing synchronous with M 3 during Late Carboniferous time. The orogenic wedge is characterized by two main tectono-metamorphic units decoupled by a major shear belt: an Internal Zone with migmatites and syntectonic granitoids, where HP relicts have been exhumed, and an External Zone that escaped the late HT event and preserved precious structures.

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