The Sierre landslide: chronology, dynamics, and insights into Alpine post-glacial instabilities

Today, natural disasters in alpine valleys pose a continuous threat to urban areas and infrastructure. Investigating past mass movements and assessing their return periods enhances our ability to predict and mitigate future hazards. The Sierre landslide in the Rhone valley (Canton of Valais) is one of the largest yet least explored Alpine landslides. Spanning 12 km in length and with a pre­sent-day volume of 0.9 km 3 , the landslide’s timing was previously unknown. A minimum bound of about 9 ka was inferred indirectly from 14 C ages of wood found in channelized debris-flow deposits that incise the landslide deposit. In this study, we used detailed Quaternary geomorphological mapping, surface exposure dating, and dynamic modelling to determine when, how, and why the Sierre landslide occurred. Field investigations included mapping the landslide mass and release area in detail. To date the landslide deposits, 15 rock samples, 13 from boulders and 2 from the detachment surface, were collected and analyzed for cosmogenic 36 Cl. The pre-landslide topography was reconstructed, and the runout was modeled using DAN3D ® . We found that the Sierre landslide occurred at 10.7 ± 1.5 ka ago, during the transition from the Younger Dryas to the Early Holocene. Contrary to earlier hypotheses suggesting glacier interaction, our results demonstrate that the landslide was primarily a gravitational collapse governed by material properties and pre-failure topography. Dynamic modelling highlights the influence of low friction coefficients in the valley sediments, enabling the long runout distance. These insights challenge existing paradigms of glacier involvement in the Sierre landslide and contribute to understanding Alpine post-glacial landscape evolution. This study underscores the importance of investigating past mass movements and landscape evolution in alpine valleys to better assess and mitigate future hazards.