Influence of structural geology on rock slope failure in a paraglacial environment: insights from the Southern Swiss Alps

Abstract

The onset and development of large rock slope failures in alpine environments are influenced by a combination of multiple factors, including lithology, inherited structural features on different scales, and the morpho-climatic history of the region. In the Southern Swiss Alps, seven large rock slope failure accumulations can be recognized along the five valleys north of Bellinzona, (Riviera, Leventina and Blenio in Canton Ticino, Calanca and Mesolcina in Canton Graubünden).  The region exposes a predominance of crystalline rocks as orthogneiss and paragneiss with similar mechanical characteristics, an aspect that limits the lithological control on the rock slope failures. In addition, the availability of detailed geochronological documentation of both glacial retreat following the Last Glacial Maximum LGM and the major slope collapses motivated the search for a potential correlation, which, however, has not been found (De Pedrini et al. 2023). For this reason, slope failures in this region are potentially controlled by the peculiar structural setting. In this work, we aim at investigating the impact of structural geology on style of activity and timing of the rock avalanches and deep rockslides of the region. We rely on a catalog of the instabilities (Ambrosi and Czerski, 2016 and De Pedrini et al. 2023) and lineament mapping based on the visual interpretation on 0.5 to 2 m resolution hillshade (swissALTI3D multidirectional Hillshade, Federal Office of Topography swisstopo) and stereo-photogrammetry of aerial strips (Image strips swisstopo, Federal Office of Topography swisstopo). The manual tracing of lineaments is compared with an automatic lineaments tracing performed on surface models of Switzerland in the form of a classified point cloud (swissSURFACE3D, Federal Office of Topography swisstopo). Knowledge on structural lineaments and site-specific field surveys allow us to identify the proper structural setting for each large rock slope failure (already collapsed, active or dormant), and to study structural patterns that may promote slope response after deglaciation at regional scale.The results of this analysis, aimed at the definition of the influence of glacial retreat plus the influence of structural geology, could provide an additional instrument to the comprehension of the paraglacial slope response in crystalline rocks and could thus represent an added value for long-term hazard assessment.