ALBERT

Description: The dynamic Neogene evolution of the Western European Alps included exhumation of the external crystalline massifs, thrust propagation to the foreland, drainage network reorganization, and major climatic variations. To constrain possible interactions between those factors, accurate geomorphological and sedimentological archives are required. However, intra-orogenic areas are subject to erosion, and extensive glacial cover during the Quaternary erased most of the geomorphic markers in the Alps. For these reasons, the genesis of the main features of the modern landscape, such as the major valleys and the drainage network, remains poorly understood. This study highlights how recently discovered karstic archives from the perched paleo-karst of the Obiou peak (Dévoluy massif, SE France) record the tectonic and drainage-network evolution of this part of the Alps during the Neogene. The Obiou caves are located at 2250-2380 m elevation, ∼1600 m above the modern Drac valley; they contain fluvial deposits including sand-clay units and rounded crystalline cobbles derived from the adjacent Ecrins-Pelvoux massif. As the Dévoluy and Ecrins-Pelvoux massifs are currently separated by the axial Drac valley (a major tributary of the Isère River), these cave sediments must have been deposited by a radial drainage system before incision of the modern Drac. We report new multi-method results from these sediments, including cosmogenic-nuclide burial dating (21Ne, 10Be, 26Al in quartz), provenance analysis (clast petrography and heavy-mineral analysis), and detrital thermochronology (apatite fission-track and (U-Th)/He) combined with a paleo-environmental reconstruction from palynology. 21Ne/10Be dating of cobbles and sand constrains the burial age to 11.5 ± 1.5 Ma, providing a maximum age for the modern axial drainage system and a minimum long-term incision rate of ∼140 m/Myr for the Drac valley. Comparison of the combined data to both modern rivers and nearby Oligocene foreland-basin deposits provides evidence for two successive drainage reorganizations. Early Miocene exhumation and development of high topography in the Ecrins-Pelvoux massif, linked to localized thrusting on a crustal-scale ramp, led to initial deflection of the antecedent radial drainage network, beheading its headwaters by establishment of the axial upper Durance valley. Subsequent propagation of thrusting into the subalpine Dévoluy massif and associated uplift during the mid to late Miocene led to establishment of the modern drainage system.