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Structural and metamorphic evolution of the Ambin Massif (Western Alps); toward a new alternative exhumation model for the Brianconnais Domain

Jerome Ganne, Jean-Michel Bertrand, Serge Fudral, Didier Marquer and Olivier Vidal
Structural and metamorphic evolution of the Ambin Massif (Western Alps); toward a new alternative exhumation model for the Brianconnais Domain
Bulletin de la Societe Geologique de France (November 2007) 178 (6): 437-458

Abstract

The basement domes of the central part of western Alps may result either from a multistage tectonic evolution with a dominant horizontal shortening component, an extensional behaviour, or both. The Ambin massif belongs to the "Brianconnais" domain and is located within the HP metamorphic zone. It was chosen for a reappraisal of the tectonic evolution of the Internal Alps in its western segment. Structural investigations have shown that Alpine HP rocks were exhumed in three successive stages. The D1 stage was roughly coeval with the observed peak metamorphic conditions and corresponds to a non-coaxial regime with dominant horizontal shortening and north movement direction. Petrological observations and P-T estimates show that the exhumation process was initiated during D1, the corresponding mechanism being still poorly understood. The D2 stage took place under low-blueschist facies conditions and culminated under greenschist facies conditions. It developed a retrogressive foliation and pervasive shear-zones at all scales that locally define major tectonic contacts. D2 shear zones show a top-to-east movement direction and correspond actually to large-scale detachment faults responsible for the juxtaposition of less metamorphic units above the Ambin basement and thus to a large part of the exhumation of HP rocks toward the surface. D2 shear zones were subsequently deformed by D3 open folds, large antiforms (e.g. the Ambin dome) and associated brittle-ductile D3 shear-bands. The D1 to D3 P-T conditions and P-T path of the blueschists occurring in the deepest part of the Ambin dome, was estimated by using the multi-equilibrium thermobarometric method of the Tweeq and Thermocalc softwares. Peak pressure conditions, estimated at about 14-16 Kb, 500 degrees C, are followed by a nearly-isothermal decompression that occurred concurrently with the major D1-D2 change in the ductile deformation regime. Eastwards, the Schistes Lustres units exhibit a similar geometry on top of the Gran Paradiso dome but exhibit opposite D2 movement direction. Lower-grade units are lying above higher-grade units, the shear zones occurring in between being similar to Ambin's D2 detachments. Thus at regional scale, the D2 detachments seem to form together with the Ambin shear-zones, a network of conjugate detachments. Such a pattern suggests that the exhumation history is mostly controlled by a D2+D3 crustal-scale vertical shortening resulting in the thinning of the previous tectonic pile formed during D1. The slab-break off hypothesis may explain such an extensional behaviour within the western Pennine domain. It is suggested that the thermo-mechanical rebound of the residual European slab initiated between 35 and 32 Ma the fast exhumation of the previously thickened orogenic wedge (stack of D1 HP slices). It was immediately followed by a collapse of the wedge that may correspond to the E-W Oligocene extensional event responsible for the opening of rifts in the West European platform.


ISSN: 0037-9409
EISSN: 1777-5817
Coden: BSGFAE
Serial Title: Bulletin de la Societe Geologique de France
Serial Volume: 178
Serial Issue: 6
Title: Structural and metamorphic evolution of the Ambin Massif (Western Alps); toward a new alternative exhumation model for the Brianconnais Domain
Affiliation: Universite Paul Sabatier, Laboratoire des Mecanismes et Transferts en Geologie, Toulouse, France
Pages: 437-458
Published: 200711
Text Language: English
Summary Language: French
Publisher: Societe Geologique de France, Paris, France
References: 122
Accession Number: 2008-019122
Categories: Igneous and metamorphic petrologyStructural geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., 1 table, geol. sketch maps
N45°00'00" - N45°49'60", E05°30'00" - E07°00'00"
Secondary Affiliation: Universite de Savoie, FRA, FranceUniversite de Franche-Comte, FRA, FranceUniversite Joseph Fourier, FRA, France
Country of Publication: France
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Societe Geologique de France. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200807
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