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Les dômes de socles HP-BT dans le domaine Pennique des Alpes nord-occidentales (massifs d'Ambin et de Vanoise Sud) : modalités de leur exhumation

Abstract : In Western Alps, most evolutionary models admitted that oceanic to European and Apulian crust-derived nappes were progressively exhumed during W to NW-verging overthrusting events, synthetic to the subduction. This memoir deals with the exhumation dynamics of the previously subducted HP units in the southern "Alpes Graies" area. The structural and metamorphic behaviour of the latest exhumation stages was specially investigated. The tectonic and metamorphic evolution of basement domes of the Briançonnais (=Penninic) domain - Ambin and South Vanoise massifs - was studied in details. Used tools were structural mapping, petrologic analysis of microstructures, thermobarometry of alpine mineral assemblages and geochronology (Rb/Sr and Ar-Ar on phengites from shear zones). The most important results show : - A North to North-West verging, syn-blueschist facies, deformation stage is poorly preserved in the upper units but well-preserved in the dome's cores. The so-defined D1 stage corresponds to the pressure and temperature peak (garnet-jadeite assemblage - 15 Kb, 500 °C). - An East verging, post HP shearing deformation, the D2 stage, is responsible for the main structural pattern observed in both basement and cover units of Ambin and South Vanoise. Syn-metamorphic deformation occurred under low blueschists conditions (glaucophane-chloritoid assemblage - 7 to 9 Kb, 480 °C) down to greenschist facies conditions (chlorite-albite assemblage - 3 to 5 Kb, 300 °C). At regional scale, the post HP, D2 shear zones show a complex pattern: from East-verging shear zones in Ambin and South Vanoise, to West-verging shear zones in Gran Paradiso and Dora Maira, and to conjugated East and West-verging shear zones in the Schistes Lustrés located in between. Together, the D2 shear zones define a thinning deformation regime with vertical shortening that probably contributed to a large part of the exhumation of HP alpine units. In the proposed scenario, the D2 thinning event occurred near to the Upper Eocene-Lower Oligocene boundary and was the result of a thermal and mechanical disturbance of the European lithosphere (slab breakoff?). The D2 stage outlines a major geodynamic change. On one hand, the ductile evolution of the Internal domains corresponds to an early subduction-type dynamics and related HP unit exhumation and, on the other hand, the post-Oligocene collision dominant in the External domains corresponds to a late collision-type dynamics linked to the filling of forebelt molassic basins.
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https://tel.archives-ouvertes.fr/tel-00006944
Contributor : Jerome Ganne <>
Submitted on : Wednesday, September 22, 2004 - 1:08:09 PM
Last modification on : Thursday, November 19, 2020 - 3:54:26 PM
Long-term archiving on: : Friday, April 2, 2010 - 8:41:16 PM

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Jérôme Ganne. Les dômes de socles HP-BT dans le domaine Pennique des Alpes nord-occidentales (massifs d'Ambin et de Vanoise Sud) : modalités de leur exhumation. Géologie appliquée. Université de Savoie, 2003. Français. ⟨tel-00006944⟩

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