Late-Neogene to Quaternary uplift, relief and drainage evolution in the western French Alps: new insights from surface and underground karst archives in the subalpine massifs

The late-Neogene to Quaternary evolution of the western European Alps has been marked by major changes in geodynamic, tectonic and climatic forcing. The complex interplay between endogenic and exogenic processes has resulted in rock-uplift changes, topographic relief development and major drainage pattern reorganizations. However, quantitative estimates on these interrelated mechanisms have remained scarce due to the poor preservation on surface geological archives in the alpine massifs. This is the case for the frontal part of western Alps, i.e. the subalpine massifs, for which the overall tectonic architecture and total deformation/uplift are well constrained but the timing and rates remained poorly known.Here, we focus on the Vercors subalpine massif and specifically target karstic systems in the upper Bourne catchment that have been developed and potentially preserved over million-year timescales. Our study combines karst network analysis with the investigation of surface geomorphological markers (abandoned canyons) to provide an incision history and integrated geomorphic evolution of the Bourne catchment from the late Neogene to Quaternary. We develop an innovative multi-method approach with 3D mapping of both surface and underground markers associated to geochronological investigation of preserved detrital sediments (26Al/10Be burial dating, U/Pb dating, paleomagnetism) and speleothems (U-Th dating). Our results show first changes in the surface drainage pattern at ~10 Ma for the Bourne catchment, in agreement with tectonic deformation and topographic uplift at that time. The Bourne incision history reveals a multi-stage complex evolution, with a late Neogene incision phase followed by relative quiescence during the Pliocene. The late-stage history of the Bourne is marked by a second incision phase since ca. 2 Ma that could be linked to isostatic response to relief development during major Alpine glaciations. Our new results nicely complement recent data from the nearby Devoluy massif, and indicate a late-Neogene structuration and uplift of the subalpine massifs which has been relatively contemporaneous with the exhumation of the external crystalline massifs (Belledonne, Ecrins-Pelvoux). This tectonic structuring resulted in the present-day plateau configuration of the subalpine massifs, and their uplift led to a major change from an earlier radial into the modern orogen-parallel drainage system, which was then marked by the Quaternary alpine glaciations.