A new perception in the development, failure, and hyper-mobility of a giant rock avalanche in the suture zone

Giant rock avalanche is extremely rare worldwide, while giant rock avalanche developed in suture zone has presented unique development characteristics. The suture zone is a product of plate moving and strong tectonic activity, where the appearance of a giant avalanche not only plays a barometer role for the regional disaster development environment but an indicator role for the complicated geological environment. In 2019-2021, the author has found a giant paleo-rock avalanche (name Basu avalanche) in the Bangonghu-Nujiang suture zone of the Tibetan Plateau. Some infrequent characteristics such as huge volume, development in nappe structure, and hyper-mobility (debris impact height > 600m) appeared for this giant rock avalanche. In this paper, based on the detailed investigation, ³⁶Cl dating, and reconstructing the pre-avalanche terrain methods, the development, failure, and hyper-mobility of this giant rock avalanche have been analyzed. The result shows that: (1) The volume of the Basu avalanche is about 3.5×10⁹m³, the residue is about 1.4×10⁹m³ now. The avalanche occurred at 205.70±7.71ka B.P.(ka: millennium), subsequent the accumulation body occurred two times secondary landslides (name Duolasi landslide) at 17.57±0.72ka B.P. and 7.01±0.32ka B.P., respectively; (2) The nappe structure, formed from the uplift and orogeny process of the suture zone, controls the development and volume size of the Basu avalanche, while the strong earthquake is the biggest likely to trigger the avalanche finally failure because of the dense active faults distribution; (3) Because of the rich Ultrabasic clasts derived from the F2 fault and fine particles produced by cataclastic rock mass, the Basu avalanche formed the slide belt that thickness from centimeters to meters during the motion. The lubrication effect of the slide belt has dominated the avalanche debris's high-speed motion and hyper-mobility, the mechanism is that: due to the huge avalanche volume and induced the high pressure and closed slide belt environment, the slide belt fine-grain formed the lubrication layer with certain water involved, and the friction force sharply decreased; (4) Because of the Basu rock avalanche and the debris flow successive blocked the Leng River, the Leng River valley has experienced diversions process and the river valley from the ’S’ shape to approximate straight-line shape.