Development of counterscarps by flexural toppling of schist in the Bedretto valley, Swiss Alps

Abstract

Counterscarps are typical topographic features of deep-seated gravitational slope deformation (DGSD) and are commonly made by flexural toppling. However, their internal structures and chronological development are not well understood. We investigated the geological structure and development of two large flexural topplings of mica schist on the southern side of the Bedretto Valley, Swiss Alps, via field surveys, topographic analysis, trenching, and 14C dating. The flexural topplings occur on ridges between the tributary valleys and extend to a depth of 200 m without inflections; their bases are planar and exposed along the sides of the tributary valleys and expose downslope above the major slope breaks that form the rim of a U-shaped valley and are presumably made during the Lateglacial, which suggests that these two flexural topplings started before or during the Lateglacial. Two trenches suggest that the counterscarps are made by preferential shearing along brittle tectonic faults and that the displacement occurred episodically and that the total displacement reached 30 m in dip separation of the faults. A displacement event between approximately 3400 and 3000 y BP was induced by earthquake shaking, which is strongly suggested by gouge injection into the overlying deformed sediments in the trough.