Miocene Cave Sediments Record Topographic, Erosional and Drainage Development in the Western European Alps

Several major rivers in the Western European Alps, including the Rhone, Isère and Durance Rivers, show a conspicuous axial drainage pattern, the origin of which remains poorly understood. Long-term drainage evolution can be reconstructed from the sedimentary record, but foreland-basin sediments provide an integrated view of (a large part of) the orogen that may lack spatial resolution, and very few markers remain within the mountain belt due to erosion and extensive Quaternary glaciation. However, a recently discovered karstic archive from the Obiou caves (Dévoluy massif, south-eastern France) records the tectonic and drainage-network evolution of this region since the Miocene. The Obiou caves are located at 2250-2370 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 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 reorganisations. Early Miocene exhumation and development of high topography in the Ecrins-Pelvoux massif, linked to localised 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.