Relationships between tectonics, slope instability and climate change: Cosmic ray exposure dating of active faults, landslides and glacial surfaces in the SW Alps

International audienceIn the Argentera massif (French Southern Alps), large active landslides develop along strike of an active corridor of dextral strike-slip faults revealed by shallow ongoing seismicity. Glacially polished bedrock outcrops are offset by right-lateral strike-slip faults. Gravitational structures appear to be spatially connected to these active faults. Dating using the in situ-produced 10Be cosmogenic nuclide performed on glacial, tectonic and gravity surfaces. The late glacial–interglacial Holocene transition is constrained by 10Be ages between 12 and 15 ka obtained on glacially polished surfaces. The main tectonic activity closely post-dates the main deglaciation event and is constrained by 10Be ages of 11 and 7–8 ka obtained on fault scarps. Three successive periods of landsliding are recognized, at 11–12, 7–9 and 2.5–5.5 ka. These Holocene ages were obtained on right-lateral strike-slip fault scarps indicating that recent Alpine tectonics are expressed by transcurrent movements. The discussed close age relationship between deglaciation and a tectonic pulse may suggest that post-glacial rebound and enhanced pore water pressure do influence seismogenic tectonic activity. Gravitational destabilizations at 11–12 and 7–9 ka are coincidental with the main tectonic activity, and suggest tectonic shaking as a landslide trigger. The third gravitational destabilization at 2.5–5.5 ka could be attributed either to slope weakness resulting from multiple low-magnitude earthquake events, as currently revealed by the seismic activity or to climatic causes during the wetter optimum climatic period. These early and middle Holocene ages coincide with a phase of large landslide throughout the Alps scale which suggests that these large gravitational mass movements could be related to combined effects of intense tectonic activity and transitions form cold and dry period to warm and wetter phase