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Carboniferous tectono-metamorphic evolution of the Variscan crust in the Belledonne-Pelvoux area.

Évolution tectono-métamorphique Carbonifère-Permien du massif du Pelvoux (Massif cristallin externe, Alpes occidentales), avec discussion sur la cinématique de fluage de la zone de cisaillement Est Varisque.

Abstract : Based on new structural, petrological and U-Th-Pb data, a reappraisal of the Variscan tectono-metamorphic history of the SW Belledonne-Pelvoux (Eastern Cristalline Massifs, French Alps) mid-lower crust is proposed. These results are keys to better understand the flow pattern of the variscan partially molten crust and the geodynamic evolution of the sub-meridional branch of the variscan belt. The SW Belledonne area exposes the suprastructure where the medium grade metamorphic rocks record a middle Carboniferous evolution, with a westward gently dipping S1 foliation refolded by east-verging inclined folds (F2) with a subvertical to west-dipping axial plane cleavage S2. Further east, in the Pelvoux area, the high-grade metamorphic rocks of the infrastructure, mostly migmatites, record a prominent D2 deformation with a penetrative NE-SW steeply dipping S2 foliation and a network of NS and NW-SE trending shear zones the kinematics of which indicate a bulk sinistral transpression. D1 and D2 features are interpreted as reflecting a NW–SE contraction, first marked by dominant nappe stacking and subsequently overprinted by transpressional shearing. The supra/infrastructure boundary is typified by a D3 deformation zone that consists of a flat laying S3 foliation with a NW-SE stretching lineation along which top to NW sense of shear is observed. The D1 event corresponds to mid-carboniferous crustal thickening. Exhumation of deep-seated rocks during the transpressional D2 event followed a near isothermal (ca. 700 °C) evolution down to pressure of ca. 5kbar in the period 325-306 Ma. From ca. 306 Ma, D2 and D3 acted synchronously while the mid-lower crust recorded a near isobaric temperature increase up to 850 °C before final cooling and retrogression. The P-T-t history of the Pelvoux area is very similar to the nearby Velay dome and suggest that the Belledonne-Pelvoux area may belong to the Western Moldanubian zone of the Variscan Belt. In that vision, we suggest that coupling between D2 and D3 deformation enable the south-eastward migration, i.e. from the orogenic plateau to the foreland, of the hot and low-viscosity partially-molten crust.