Giant non-catastrophic landslides and the long-term exhumation of the European Alps

Landslides influence local slope morphology, affect sediment flux from hillslopes to rivers, and mass wasting in response to tectonics and climate forcing. However, the links between giant, noncatastrophic landslides known as Deep-Seated Gravitational Slope Deformations (DSGSDs) and the long-term evolution of orogenic landscapes are almost unknown. We explore these links in the European Alps using the first orogen-scale inventory of DSGSDs (>900 over an area >105 km2) and a dataset of published apatite fission-track ages (>1000) that provides an estimate of the long-term exhumation patterns of the orogen. We show that DSGSDs are more widespread than previously considered, and exhibit an orogen-scale distribution not explained by well-known local lithological and structural controls. We test the hypothesis that this orogen-scale distribution correlates to the longterm evolution of the Alps by subdividing the study area into 37 square sub-areas (50x 50 km), classified according to combinations of long-term exhumation and mean annual rainfall. On each subarea we perform a morphometric analysis of topography (hypsometry, relief, slope). Excluding local and regional controls due to rock type and structure, DSGSDs tend to cluster in areas with intermediate exhumation rates (fission-track age between 10 and 40 Ma), where large-scale topography is less dissected and incision is localised along major valleys. Here DSGSD abundance correlates positively with the degree of valley incision and related relief. Instead, DSGSDs lack in areas which underwent either low exhumation rates, resulting in insufficient relief production, or high exhumation rates associated to rapid uplift or higher erosional dissection of topography. Negative correlation between DSGSD abundance and mean annual rainfall suggests that effective hydrological surface processes contribute, on the long-term timescale, to the development of large-scale topography unfavourable to DSGSDs, especially in areas of high exhumation rates. Where DSGSDs are abundant, long-lasting slope deformations effectively adjust post-glacial relief by reducing slope inclination values, and are thus expected to significantly contribute to the long-term denudation of active orogens.