Onset and timing of deep-seated gravitational slope deformations in the eastern Alps, Italy.

A previously unrecognised complex of deep-seated gravitational slope deformations (DSGSD) extends 20 km along upper Venosta Valley in the eastern Alps, Italy. Mt. Watles exhibits spectacular features indicative of DSGSDs including double ridges, trenches, and counterscarps. The slope deformation occurs along a nappe boundary now corresponding to the Schlinig normal fault, is also conditioned by recent faults marked by shallow earthquakes, and shows glacial/paraglacial controls on slope evolution. The Mt. Watles DSGSD is a complex, deep-seated, compound slide along a basal shear zone, involving the Schlinig fault. Gravitational reactivation of NE-trending fractures formed gravitational scarps, counterscarps, and half-grabens in the upper slope, whereas the lower part of the slope partially collapsed. Although our work indicates the importance of the Schlinig fault and recent fracturing on slope failure, the trigger seems to be postglacial debuttressing of the valley walls. Radiocarbon dating of peat deposits within one of the major counterscarps indicates that slope deformation started during the Lateglacial period and continued during the Holocene in several slope sectors.