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Original Article |
1 1Laboratoire de Géodynamique des Chaînes Alpines, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex, France (e-mail: ptricart@ujf-grenoble.fr)
2 2Laboratoire Interdisciplinaire de Recherche Impliquant la Géologie et la Mécanique, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex, France
We present new apatite fission-track (AFT) data from the central western Alps that confirm the synchronicity and high cooling rates during Latest Miocene–Pliocene final exhumation of the External Crystalline Massifs but also provide evidence for diachronous Neogene evolution along and across the internal arc. To the SE of the Pelvoux Massif, across the front of the internal arc (Penninic Frontal Thrust), the jump in AFT ages (c. 22 Ma) and in final cooling rates is significantly larger than further north. This difference results from reversal of movement along a major Oligocene thrust. In its hanging wall, the western Briançonnais Zone provides a mean AFT age of c. 27 Ma, which is older than further north. Early cooling in the southern Briançonnais Zone would result from rapid erosion of the compressional fan structure built during the Oligocene. Across the entire Briançonnais and Piémont nappe stack, with the exception of the Dora-Maira Massif, AFT ages young eastward and span the entire Miocene, a period during which this structure underwent extension. Further north a reverse gradient with ages younging northwestward has been described, prompting the question of the asymmetry of the internal western Alpine arc during its late-stage tectonic and morphological evolution.
