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

Kévin Fréville 1, 2, 3, 4, * Pierre Trap 2 Jonas Vanardois 2 Jérémie Melleton 3 Michel Faure 1, 4 Olivier Bruguier 5 Marc Poujol 6 Phillipe Lach 3 
* Corresponding author
4 Géodynamique - UMR7327
BRGM - Bureau de Recherches Géologiques et Minières (BRGM), ISTO - Institut des Sciences de la Terre d'Orléans - UMR7327 : UMR7327, INSU - CNRS - Institut national des sciences de l'Univers, UO - Université d'Orléans : UMR7327
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.
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Submitted on : Wednesday, May 25, 2022 - 12:42:13 PM
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Kévin Fréville, Pierre Trap, Jonas Vanardois, Jérémie Melleton, Michel Faure, et al.. Carboniferous tectono-metamorphic evolution of the Variscan crust in the Belledonne-Pelvoux area.. Bulletin de la Société Géologique de France, Société géologique de France, In press, ⟨10.1051/bsgf/2022008⟩. ⟨insu-03678397⟩

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