Long-term fluvial incision rates and postglacial river relaxation time in the French Western Alps from 10Be dating of alluvial terraces with assessment of inheritance, soil development and wind ablation effects.

摘要

Studying river long-profile development as a response to tectonic and climatic controls requires reliable age-dating of paleo-profile remnants preserved as river terraces. Cosmic Ray Exposure (CRE) dating often represents the only method available to date river terraces, but the interpretation of cosmogenic nuclide concentrations is complicated by pre-depositional inheritance and post-depositional disturbance of the terrace deposits through pedogenesis and surface inflation or deflation. Here, we use cosmogenic 10Be measurements to date alluvial terraces in the French western Alps, in order to estimate river incision rates and to infer river response to climatic fluctuations and tectonic forcing. We assess inheritance by constructing 10Be concentration vs. pebble depth profiles and use a Monte-Carlo technique to estimate terrace ages. We find that inheritance is negligible on all terraces, enabling us to date terraces as young as 5 ky. Terraces that predate the last glaciation experienced intense pedogenesis and wind ablation which led to significant scatter in the 10Be concentrations of surface samples. We assess these effects using a model of 10Be ingrowth and show that the oldest CRE ages of surface clasts are close to the probable terrace age. We study two catchments which have undergone varying degrees of glacial disruption. The Buëch River experienced variations in runoff and sediment flux during the last glaciation, as well as occasional ice-dammings of its outlet. Its upper terraces record incision rates averaged over 190 ky of ~0.8 mm/yr, consistent with denudation rates estimated in the surrounding areas and suggesting long-term stability of river incision rates. Climatic forcing is well documented for the Drac River, which was repeatedly dammed by glaciers during cold periods. Its postglacial incision history was triggered by an 800-m drop in base level following ice-dam disappearance. Long-profile development by knickpoint propagation explains the time-lag of 2-5 ky between this base-level drop and terrace abandonment upstream, as well as subsequent peak incision rates of >6 cm/yr followed by a gradual decrease in incision rates. The present knickpoint location, ~55 km upstream from the glacial damming site, enables us to calculate a fluvial response time of 15-20 ky, controlled by knickpoint propagation rates of several meters per year, within the Drac River.