Paleo-landslides in the southern France (Larzac plateau)

The Larzac carbonate plateau (France) is subject to numerous slope instabilities on its edges, ranging from toppling to landslides. Due to their extremely slow slip rates (3mm/year), these last large rotational instabilities remain poorly understood, particularly in terms of characterisation and dynamics. Our study focuses on several deep paleo-landslides of this type, located in two valleys: the Lergue and the Laurounet. These landslides evolved in sedimentary rocks including the highly fractured Jurassic carbonates overlying the Triassic sandstones and the thick Triassic clays. This work aims to study the initial phase mechanisms. In a climate change context, with extreme precipitations as in southern France (“cevenol events”), understanding paleo-landslide mechanisms has an added value in the comprehension of the future slope stability in similar geological contexts.

We used a multi-method approach to characterize the investigated landslides. Remote sensing and field surveys allowed mapping of the landslides, identification of geomorphological features, main and secondary scarps, and their associated slide blocks. Rock mass fracturing was characterized at localities in and away from the landslides. Mechanical characterization was obtained through the Rock Mass Rating (RMR)/Geological Strength Index (GSI) and laboratory tests. Finally, terrestrial cosmogenic nuclides (³⁶Cl for carbonate surfaces) were used to determine the exposure age of the landslide scarps.

The investigated million-cubic-meter landslides show upslope and secondary circular scarps with counter-slope slide blocks, signifying rotation. However, at deeper levels, the failure surface flattens within the evaporite-rich clays. Dating two paleo-landslides places their occurrence between 10 and 18 kyrs, suggesting the Late Pleistocene/Holocene transition. A directional correlation is evidenced between the dense NNW-SSE joint network that cut the carbonates and N-S faults with the landslide scarps. The study suggests that landslides exhibit a rotational-translational mechanism, influenced by lithological differences between fractured carbonate units and weak underlying clays. This reaffirms the significance of clays in landslide failure, with evaporite levels playing a role in deep rupture surface branching in certain cases. Furthermore, a major structural control is evidenced, with the faults serving for initiation or as lateral ramps of the landslides depending on their orientation relative to the slope. Dating results suggest that increasing precipitation could have led to slope failures.

These geological constraints were employed to test scenarios for the initiation of the rotational-translational landslides of the Larzac carbonate plateau using the distinct elements method 3DEC. Field data supplied geometry, while the mechanical parameters of the multi-layer rock mass were estimated based on the RMR and GSI data. The three families of discontinuities, layering planes, and the sub-vertical NNW-SSE and WSW-ENE joints were also included, as well as the in-situ pore pressure. The stability analysis revealed the significant impact of joints/faults and lithology contrast on the stability and geometry of the failure surface. This study illustrates how landslides can be related to a combination of predisposing parameters such as structural inheritance and variation of properties in the heterogeneous rock mass that control their modes of failure and geometries.