The Alasu rock avalanche in the Tianshan Mountains, China: fragmentation, landforms, and kinematics

Geomorphological and sedimentological characteristics are key evidences for understanding the emplacement mechanisms of rock avalanches. Based on remote sensing, photogrammetry, and field surveys, the depositional characteristics of the Alasu rock avalanche (ARA) in southern Tianshan, China, were investigated in detail. It is reached that the ARA has a detached rock mass volume of ~ 93 × 10 6 m 3 and propagated 3874 m on rugged terrain with a drop height of 1030 m. Large rock blocks (95,598 rock blocks with sizes ≥ 0.5 m) are widely distributed on the deposit surface. There is no decreasing trend in block size with travel distance for these rock blocks, while jigsaw and sibling structures of megablocks commonly occur in the carapace facies. The size and structural features of megablocks demonstrate that the carapace facies underwent a passive emplacement process with low degrees of fragmentation and disturbance. Furthermore, a series of surficial landforms, including irregular platforms, minor scarps, lateral levees, run-ups, ridges, and troughs, are preserved in the rock avalanche deposit. The most remarkable features are the large, subparallel, curved longitudinal ridges with continuous directional variations and three run-up traces. Extensions of these ridges indicate that the avalanche mass experienced a continuous extension-dominated process in emplacement due to its momentum transfer effect and high energy propagation. These observations and analyses may elucidate the emplacement mechanisms of rock avalanches in rugged landscapes.