Diachronous late-stage exhumation across the western Alpine arc: constraints from 2 apatite fission-track thermochronology between the Pelvoux and Dora–Maira Massifs
Résumé
We present new apatite fission-track (AFT) data from the central western Alps that confirm the 10 synchronicity and high cooling rates during Latest Miocene–Pliocene final exhumation of the External 11 Crystalline Massifs but also provide evidence for diachronous Neogene evolution along and across the internal 12 arc. To the SE of the Pelvoux Massif, across the front of the internal arc (Penninic Frontal Thrust), the jump 13 in AFT ages (c. 22 Ma) and in final cooling rates is significantly larger than further north. This difference 14 results from reversal of movement along a major Oligocene thrust. In its hanging wall, the western 15 Brianc ̧onnais Zone provides a mean AFT age of c. 27 Ma, which is older than further north. Early cooling in 16 the southern Brianc ̧onnais Zone would result from rapid erosion of the compressional fan structure built 17 during the Oligocene. Across the entire Brianc ̧onnais and Pie ́mont nappe stack, with the exception of the 18 Dora–Maira Massif, AFT ages young eastward and span the entire Miocene, a period during which this 19 structure underwent extension. Further north a reverse gradient with ages younging northwestward has been 20 described, prompting the question of the asymmetry of the internal western Alpine arc during its late-stage 21 tectonic and morphological evolution.