Late Cenozoic erosion rates in the Western Alps revealed by detrital apatite fission-track analysis

The Western Alps represent one of the most intensely studied mountain ranges on Earth in terms of structural growth, erosion and sediment flux and have provided important datasets in support of late Cenozoic acceleration in global erosion rates in response to a changing climate. Approximations of past erosion rates using sediment volumes and regional correlations of bedrock thermochronology data have been criticized for methodological bias. We present new detrital apatite fission-track (AFT) analyses, together with a new analysis of previously published data from the Lower Miocene to modern sediments of the remnant pro-foreland basin of the Western Alps. Detrital U-Pb zircon ages are presented from the newly collected samples in order to characterize provenance. Lag times, which are used as proxies for paleo-erosion rates, are evaluated by the comparison of both the youngest-age-peak (P1) and bulk modal fission-track ages to depositional ages; the latter are introduced to approximate erosion rates over a larger portion of the palaeo-Rhône catchment relative to that represented by the P1 population. The impact of drainage reorganization, sediment recycling and dispersed sampling are considered. The data are interpreted to record relatively rapid exhumation from ca. 20 to 10 Ma, with a late to post-orogenic deceleration after ca. 10 Ma. Increased erosion rates recorded in the P1 populations of Late Pleistocene terraces and modern river sediment are interpreted as a response to localized glacial deepening of valleys.