Paleo-landslides on the meridional border of the Larzac Plateau (France): recognition and predisposing/triggering factors

The southern border of the Larzac plateau, in the Occitanie region (France), has been affected by several complex landslides involving the failure of Jurassic carbonate series overlying Triassic clays. Detailed geological and geomorphological mapping, including photogrammetry, LiDAR and fracturing surveys on the upper stream of the Lergue and Laurounet valleys, show that landslide scarps correlate with fault or fracture strikes. While landslide scarps are mostly correlated with NNW-SSE fractures set direction related to the Pyrenean orogenesis and N-S faults direction, another fracturing E-W direction correlated with the Liasic extension could explain the lateral terminations of the landslide deposits. These fractures and faults are assumed to influence the initiation and spreading of the rock failures evolving into roto-translational landslides. Depth of sliding surfaces is controlled by the presence of evaporites in underlying Triassic clays. Exposure dating of two of the investigated landslide scarps using chlorine-36 nuclides suggests a triggering at the end of the Late Pleistocene/Early Holocene.