Résumé
Abstract<br />THE PIEMONTESE ZONE (WESTERN ALPS)A PALEO-SUBDUCTION<br />COMPLEX : METAMORPHIC, GEOCHRONOLOGIC AND STRUCTURAL<br />EVIDENCES.<br />This PhD Thesis presents a multidisciplinary approach (petrology, tectonics, fissiontracks<br />chronometry and geophysics) of the geodynamic evolution of the Piemontese zone<br />from the western Alps (Pelvoux-Dora Maira geotraverse of GéoFrance 3D programm).<br />This zone corresponds to the tectonic juxtaposition of two main domains :<br />-The Queyras Schistes lustrés in which different metamorphic P-T paths, reflecting a<br />progressive evolution under blueschist facies conditions, have been recognized. These<br />rocks have been buried at depth of 20-35 km under paleo-gradients of 25-30°C/km.<br />-The eclogitic units (Monviso and Dora Maira massifs), characterized by heterogeneous<br />high-pressure conditions (numerous pressure gaps), reflecting contrasted conditions of<br />burial under a very low thermal gradient of 4-6°C/km.<br />These two domains are interpreted as the remnants of different structural levels of an<br />intra-oceanic subduction zone. The Queyras Schistes lustrés, with an important metasedimentary<br />component and ocean-derived detritism, are regarded as a crustal<br />accretionary wedge, while the eclogitic units corresponds to a deep-seated wedge<br />composed by serpentinized peridotites and crustal slices sampled at different depths along<br />the Benioff zone. The exhumation of blueschists and eclogites is diachronous and<br />achieved, while subduction was still active, throught contrasted mechanisms. The tectonic<br />pile of the Piemontese zone was created at 30 Ma and its exhumation achieved by the<br />combination of erosion and extension. This syn-convergence extension occured under<br />greenschist facies conditions corresponds to the tectonic denudation, toward west, of the<br />eclogitic massifs. In the Queyras Schistes lustrés, the greenschist facies related structures<br />have been tilted toward the west during the exhumation. In the Piemontese zone,<br />extensional tectonics was synchronous of thrust development and shortening of the<br />european foreland and is still active today.<br />The crustal scale geometry of the internal zones is characterized by the occurence of a<br />mantle identor. This rigid, apulian, mantle back-stop was responsible for the stacking of<br />deep crustal slices derived from the european margin. The mantle identor is divided in<br />two units by crustal scale inverse faults and promotes the present-day strain partioning of<br />the western Alps. In this context, syn-convergence extension corresponds to the<br />accomodation of the vertical identation of the HP units, in relation to the continous NW /<br />SE convergence.<br />Key words: Western Alps, Piemontese zone, HP-LT metamorphism, intra-oceanic<br />subduction, exhumation, syn-convergence extension, mantle back-stop.