Gigantic rockslides induced by fluvial incision in the Diexi area along the eastern margin of the Tibetan Plateau.

A number of large landslides, which have strongly affected fluvial processes in the upstream catchment of the Minjiang River along the eastern margin of the Tibetan Plateau, Sichuan, were investigated with regard to both geological structure and topographic development. Topographic analysis suggested that there is a major knickpoint that formed not as a result of landslides but via tectonic activity and that the knickpoint propagated upstream, forming an inner gorge and undercutting and destabilizing nearby slopes. The effects of the knickpoint propagation and inner gorge formation on slope stability are dependent on the geometrical relationships between the river and geological structures. When the geological trend is normal or highly oblique to the river axis, landslides generally do not occur, but gigantic rockslides have occurred on one side of the Minjiang valley, which can be attributed to a wedge structure consisting of bedding planes and joints with intersections dipping valleyward and tight folds with hinges plunging valleyward. When the beds are planar, moderately dipping, and strike nearly parallel to the river, buckling deformation commonly occurs on cataclinal slopes, often transforming into catastrophic failure. Landslide dams form another type of knickpoint on the river channel that gradually disappear from the downstream to upstream as a result of river erosion. Our findings strongly suggest that the study of slope development by river incision must consider geological structures and that an understanding of geological structures and river incision history can provide a conceptual model for the prediction and mitigation of geohazards in tectonically active drainage basins. [ABSTRACT FROM AUTHOR]