Formation mechanism and evolution process of the Chada rock avalanche in Southeast Tibet, China

The Chada rock avalanche is a prehistoric high-elevation giant rock landslide located in the Boshula Mountains, Lhorong County, Southeast Tibet. It is composed of conglomerates with a volume of 6.62 × 106 m3 and has a height difference of 1450 m and a transport distance of 3155 m. The accumulational landform shows characteristics indicating rock avalanches. With a unique red conglomerate as the marker of landslide movement, we combined the results of geological surveys, aerial surveys, and engineering geological drilling to determine the entrainment and geomorphic features of the rock avalanche. The rock avalanche was divided into the main scarp, entrainment zone (residual deposit, mixed deposit, and impact fragmentation areas), transport zone (compressed, local landslide, and longitudinal ridge areas), and deposit zone. The sequence of deposits in the valley indicates that the rock avalanche formed before the first-stage terrace and after the second-stage terrace. Combined with 3D numerical simulation, four movement stages were obtained: (1) the rock mass was broken and disintegrated due to progressive failure, initiating high-speed sliding; (2) the sliding mass scraped the thick previous slope material and formed oblique ridges by forward extrusion and lateral friction; (3) the 4.95 × 106 m3 sliding mass was compressed and decelerated to form bending ridges, and the 1.67 × 106 m3 sliding mass continued to move through the channel; and (4) the sliding mass extended to form longitudinal ridges in the channel and hummocks in the valley. The rock avalanche accelerated three times and decelerated three times during its motion.