Formation mechanism and movement processes of the Aizigou paleolandslide, Jinsha River, China

The Aizigou paleolandslide is a large basalt landslide in the Liucheng section of the Lower Jinsha River, southwestern Sichuan Province, China. Geological investigation and interpretation of remote sensing imagery, in addition to experiments, numerical simulation, and geochronological dating were used to determine the formation and movement of the landslide. Four factors were critical: a substantial height difference between the landslide shearing surface and the foot of the slope; multiple structural defects within the rock mass; a tuff intercalation in the basaltic series with long-term softening due to surface water and groundwater; and seismic activity. The kinematic behavior of the landslide was simulated using a 3-D discrete element model (3DEC), with four stages: (1) Seismic waves in the upper part of the slope were amplified by the mountain topography. When the vibrational energy exceeded the strength of the rock mass, the landslide formed, with deformation and failure by sliding and fracture. (2) Landslide materials with a volume of approximately 3.82 × 108 m3 slid at high speed from a high position, before colliding with the mountainside on the right bank of Aizi Valley. (3) The high-speed debris flow then moved 3 km downstream in Aizi Valley. (4) Landslide materials with a volume of 2.72 × 108 m3 blocked Jinsha River. A lake formed behind the barrier dam; its backwater region reached 75 km upstream. Optically stimulated luminescence dating of the lake sediments suggested that the landslide occurred about 25,000 years ago.