How to interpret Cosmogenic Radionuclides Exposure (CRE) ages on river-incised gorges undergoing rock falls: application to Southern French Alps and Provence
Résumé
River channels and hillslopes are shaped by the joint action of localized, vertical fluvial incision along the channel and of diffuse surface creep, landslides or rock falls on the adjacent slopes, the latter being often gathered under the generic term of hillslope processes. The interplay between river incision and hillslope processes is responsible for various landscape forms, from smooth, low-relief areas to sharp and deeply incised domains. In areas where river incision is the dominant erosive process, determining the gradual exposure of river gorge walls by cosmogenic radionuclide dating permits to estimate the long-term (several ka) local incision rate. However, strongly cohesive rocks like massive limestones or sandstones may be prone, from time to time, to abrupt and localized degradation by rock falls. On a gorge wall, besides resetting the exposure age signal on the area where a block has been detached, rock fall events also produce debris that transiently protect the bedrock from river incision and the bottom of the gorge wall from cosmic radiations. In some extreme cases, rock fall events can even lead to the formation of epigenetic gorges. Although gorge walls appear as good markers of river incision, the random occurrence of rock falls may therefore add complexity to the interpretation of exposure ages, to the point where the actual river incision imprint is barely discernable. In this presentation, we simulate the 1D evolution of topography and Cosmogenic Radionuclides Exposure (CRE) ages on a gorge wall progressively formed by river incision and randomly reshaped by rock falls, in order to evaluate the imprint of these events on the CRE signal. We then discuss the implications of these models on the sampling strategy and on the interpretation of previously dated river gorges in the Southern French Alps and Provence.