La zone piémontaise des Alpes occidentales : un paléocomplexe de subduction. Arguments métamorphiques, géochronologiques et structuraux

Abstract : Abstract
THE PIEMONTESE ZONE (WESTERN ALPS)A PALEO-SUBDUCTION
COMPLEX : METAMORPHIC, GEOCHRONOLOGIC AND STRUCTURAL
EVIDENCES.
This PhD Thesis presents a multidisciplinary approach (petrology, tectonics, fissiontracks
chronometry and geophysics) of the geodynamic evolution of the Piemontese zone
from the western Alps (Pelvoux-Dora Maira geotraverse of GéoFrance 3D programm).
This zone corresponds to the tectonic juxtaposition of two main domains :
-The Queyras Schistes lustrés in which different metamorphic P-T paths, reflecting a
progressive evolution under blueschist facies conditions, have been recognized. These
rocks have been buried at depth of 20-35 km under paleo-gradients of 25-30°C/km.
-The eclogitic units (Monviso and Dora Maira massifs), characterized by heterogeneous
high-pressure conditions (numerous pressure gaps), reflecting contrasted conditions of
burial under a very low thermal gradient of 4-6°C/km.
These two domains are interpreted as the remnants of different structural levels of an
intra-oceanic subduction zone. The Queyras Schistes lustrés, with an important metasedimentary
component and ocean-derived detritism, are regarded as a crustal
accretionary wedge, while the eclogitic units corresponds to a deep-seated wedge
composed by serpentinized peridotites and crustal slices sampled at different depths along
the Benioff zone. The exhumation of blueschists and eclogites is diachronous and
achieved, while subduction was still active, throught contrasted mechanisms. The tectonic
pile of the Piemontese zone was created at 30 Ma and its exhumation achieved by the
combination of erosion and extension. This syn-convergence extension occured under
greenschist facies conditions corresponds to the tectonic denudation, toward west, of the
eclogitic massifs. In the Queyras Schistes lustrés, the greenschist facies related structures
have been tilted toward the west during the exhumation. In the Piemontese zone,
extensional tectonics was synchronous of thrust development and shortening of the
european foreland and is still active today.
The crustal scale geometry of the internal zones is characterized by the occurence of a
mantle identor. This rigid, apulian, mantle back-stop was responsible for the stacking of
deep crustal slices derived from the european margin. The mantle identor is divided in
two units by crustal scale inverse faults and promotes the present-day strain partioning of
the western Alps. In this context, syn-convergence extension corresponds to the
accomodation of the vertical identation of the HP units, in relation to the continous NW /
SE convergence.
Key words: Western Alps, Piemontese zone, HP-LT metamorphism, intra-oceanic
subduction, exhumation, syn-convergence extension, mantle back-stop.
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Contributor : Stéphane Schwartz <>
Submitted on : Thursday, November 6, 2003 - 4:28:51 PM
Last modification on : Friday, April 5, 2019 - 8:02:57 PM
Long-term archiving on: Wednesday, November 23, 2016 - 3:41:59 PM

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Stéphane Schwartz. La zone piémontaise des Alpes occidentales : un paléocomplexe de subduction. Arguments métamorphiques, géochronologiques et structuraux. Géologie appliquée. Université Claude Bernard - Lyon I, 2000. Français. ⟨tel-00003697⟩

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