Interplay of fluvial incision and rockfalls in shaping periglacial mountain gorges

Fluvial incision is the consequence of landscape readjustment to combined tectonic and climatic processes. In the southwestern Alps (Haute Provence Geopark), deep gorges incised by the Bes River attest of efficient erosional processes at the front of the Alpine mountain range. This catchment stands in a peripheral Alpine position, out of the glaciated domain during the last glacial periods, which makes it suitable to quantify fluvial incision and related erosional processes in a glacier-free environment. In this paper, we combine high resolution 3D topographic mapping and in situ produced cosmogenic Cl-36 dating of a mountain gorge (the Clue de Barles Gorge). First, the very high-resolution 3D topographic modeling from aerial and drone surveys permits to map the erosion markers on the gorge walls and to accurately determine the topographic shielding factor for Cosmic Ray Exposure (CRE) dating. Secondly, Cl-36 CRE age distribution highlights two distinct geomorphic domains along the vertical profile: i) the higher section is characterized by clusters of similar CRE ages, interpreted as related to paraglacial rockfall events; ii) the lower section shows increasing ages with height, which are ascribed to fluvial incision occurring at a rate of 0.15 mm/yr since 25 ka, and of 2 mm/yr since 2 ka. Our results for the Clue de Barles, compared to other sites in the South French Alps highlight that: i) the gorge morphology is the result of the combination of both vertical fluvial incision and lateral gravitational processes, ii) the mean Quaternary fluvial incision rate in the Bes River catchment is at least twice lower than further east in the formerly glaciated Alps. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This study used high-resolution 3D topographic mapping and cosmogenic Cl-36 dating to investigate a mountain gorge in the southwestern Alps, revealing that the gorge morphology is the result of both vertical fluvial incision and lateral gravitational processes, with fluvial incision rates in the catchment area at least twice lower than in the glaciated Alps to the east.