Formation mechanism and motion processes of the Aizigou giant paleolandslide，Jinshajiang river

The Emeishan basalt group widely distributed in southwestern China is often considered as an ideal place for building large hydropower dams due to its deep canyon landforms. Historically, the large-scale high-position and long-distance landslides in Emeishan basalt have caused a large number of casualties, property damage and far-reaching environmental effects. Currently, there is no deep and systematic research for the development process of such landslides over the world, and it is still difficult to objectively access the risk of large-scale high-position landslides in southwestern China. Therefore, the study on the formation mechanism of large-scale high-position and long-distance landslides in Emeishan basalt has significant scientific and practical implication.　　The Aizigou paleolandslide, which represents a typical large basalt high-speed remote landslide, is located in the Liucheng section of the Lower Jinshajiang 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 mechanism and motion process 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 dynamic behavior of the landslide is divided into four stages as follows. ① Seismic waves in the upper part of the slope were significantly amplified by the topography of the mountain. When the accumulation of vibrational energy over a short time period exceeded the strength of the rock mass, the landslide was formed easily. Accordingly, the deformation and failure mechanism was sliding and fracturing. ② Landslide materials with a volume of approximately 3.82×10⁸ m³ slid down from a high position at a high velocity, after which they disintegrated into a debris flow after colliding with the mountainside on the right bank of Aizigou valley. ③ The high-speed debris flow moved approximately 3 km downstream within Aizigou valley. ④ Landslide materials blocked the Jinshajiang river, thereby forming a large barrier dam with a volume of 2.73×10⁸ m³ and establishing a dammed lake behind the barrier dam; its backwater region reached 75 km upstream. An analysis of the landslide-dammed lake deposit samples via optically stimulated luminescence reveals that the landslide occurred approximately 25 thousand years ago.