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The late extension in the inner western Alps: a synthesis along the south-Pelvoux transect

¹ Laboratoire de Géodynamique des Chaînes Alpines (CNRS, UMR 5025), Observatoire des Sciences de l’Univers de Grenoble, Université Joseph Fourier, BP53, F-38041 Grenoble cedex 9. ptricart{at}ujf-grenoble.fr
² Géosciences Azur, Institut de Géodynamique, 250 rue A. Einstein, Sophia-Antipolis, F-06560 Valbonne. lardeaux{at}wanadoo.fr
³ Laboratoire Interdisciplinaire de Recherche Impliquant la Géologie et la Mécanique, Université Joseph Fourier, BP53, F-38041 Grenoble cedex 9. Stephane.Schwartz{at}ujf-grenoble.fr
⁴ Institut de Géologie, Case Postale 2, CH-2007 Neuchâtel, Suisse. Christian.Sue{at}unine.ch

During the Oligocene, in the central western Alps, tectonic accretion of the external domain to the internal orogenic wedge along the Briançonnais frontal thrust (BFT) was followed by backfolding, resulting in the Alpine fanning structure. The Briançonnais fan axis was rapidly exhumed by erosion. This growing wedge at the scale of the entire Alpine structure was a short-lived situation that ended with the onset of extension in its internal part, before the end Oligocene. To the east, in the Queyras Piedmont Schistes lustrés, extension in ductile then brittle conditions accommodated the tectonic denudation of the Dora-Maira crystalline massif below the Monviso ophiolites, themselves exhumed below the Queyras Schistes lustrés. Consistently, the final cooling of these Schistes lustrés becomes younger eastwards during the Miocene. To the west, inversion of the BFT was directly associated with dense normal faulting in the Briançonnais-Piedmont nappe stack. Local reactivation of thrust surfaces resulted in spectacular trains of tilted blocks oriented parallel and normal to the orogen. When considered at the scale of the entire internal zones, the brittle extension developed during the Neogene globally displays a multitrend character. It is a close to radial spreading that strongly suggests the gravitational collapse of an overthickened crust. Extensional movement along the BFT and multitrend normal faulting in its hangingwall continue at present, resulting in shallow depth seismic activity. From the Neogene onwards, the Alpine structure underwent contrasting tectonic regimes. Extension limited the growth of the internal wedge or accompanied its thinning at least in its upper part. Concurrently the external wedge continued growing through successive folding-thrusting phases.

Key Words: Western Alps • Exhumation • Denudation • Extension • Boudinage • Normal faulting • Seismicity • Geodesy • Thermochronology

This article has been cited by other articles:

				P. TRICART, P. VAN DER BEEK, S. SCHWARTZ, and E. LABRIN Diachronous late-stage exhumation across the western Alpine arc: constraints from apatite fission-track thermochronology between the Pelvoux and Dora-Maira Massifs Journal of the Geological Society, January 1, 2007; 164(1): 163 - 174. [Abstract] [Full Text] [PDF]