Impact of deep-seated gravitational slope deformations on topographic relief evolution in the European Alps

Landslides are known to affect the coupling between hillslopes and rivers, and evacuation of sediment in response to tectonic forcing, climate, and related changes. Nevertheless, very little is known about the distribution and geomorphic impact of giant non-catastrophic landslides, known as Deep-Seated Gravitational Slope Deformations (DSGSD) on active orogenic landscapes. These slope instability phenomena involve entire high-relief alpine slopes with relatively low displacement rates, and are constrained by rock lithology and geological structures on different scales. We compiled the first original orogen-scale inventory of DSGSD, covering the entire European Alps and including more than 900 individual phenomena, and showed that these slope instability phenomena are more widespread than previously recognised. Moreover, our large dataset suggests that, besides local lithological and structural constraints, the orogen-scale distribution of DSGSD undergoes higher-order controls. We explored the relationships between DSGSD distribution and the long-term evolution of the orogen by combining comprehensive apatite fission-track (FT) data, climate, and geomorphometry. We characterized topography (relief, slope angles, hypsometry) and DSGSD distribution along swath profiles and within sub-areas, classified according to combinations of long-term exhumation rates and rainfall patterns. We found a link between DSGSD occurrence and morpho-climatic signatures associated to the long-term exhumation history of orogen sectors. DSGSD lack in areas characterised by very high or very low exhumation rates. They tend to cluster in areas with intermediate exhumation rates, where topography is poorly dissected and maximum incision to the base levels is localized along major valleys. In this setting, DSGSG play an efficient role in adjusting post-glacial relief by the reduction of slope inclinations to values well below those typical of fluvial threshold hillslopes. Since DSGSD are long-lasting and often active phenomena, we suggest that their contribution to denudation may be significant and worth to be accounted for in geomorphological models of landscape evolution .