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.
Résumé
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.
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