The largest rock avalanche in China at Iymek, Eastern Pamir, and its spectacular emplacement landscape

Abstract

Catastrophic rock avalanches are widely distributed on Earth and share a series of common diagnostic features that play a vital role in revealing their emplacement mechanisms. Combined with satellite imagery, field investigation, and statistical analysis, a comprehensive investigation is conducted on the gigantic Iymek rock avalanche (IRA) in eastern Pamir, China. The IRA with a volume of ~0.98 km3 detached mass advanced 16.3 km on an unconfined terrain, covered an area of 48.5 km2 with a depth of 10–90 m, and deposited an estimated volume of 1.37 km3 deposits, making it the largest rock avalanche known in China. Its spectacular emplacement landscape provides an excellent laboratory for studying rock avalanches. Morphologically, the IRA deposit exhibits characteristic undulating features, including trimlines, longitudinal ridges, hummocks, transverse ridges, raised distal edge, and raised lateral levees. On the profile, numerous representative landslide-tectonic structures, principally composed of the preserved stratigraphic sequence, pervasive shear bands, spectacular clastic dikes, diapiric intrusion, folds, and normal/thrust faults, are revealed. The sequential assemblage of these morphological landforms and internal structures demonstrates that the propagation of the IRA should behave like a laminar flow with distributed shearing and obey three major stress processes, i.e. extension-dominated process in the proximal area, compression-dominated process in the distal area, and slightly lateral spreading in the central area and the areas adjacent to the margin of the deposit sheet. During its propagation, the strong substrate, mainly composed of gravel-dominated coarse material, should play a decisive role in casting most of the depositional landscapes. Restricted by the strong substrate, prominent lateral levees are well-developed in the accumulation zone with a series of rolling flame structures. This powerfully indicates that the formation of the lateral levees should be attributed to a continuously sideward movement of the frontal mass due to the push from the rear part instead of a particle self-segregation mechanism. Furthermore, the cutting relationship between the clastic dikes and subhorizontal shear bands in local positions indicates that the localized liquefaction of the substrate is not a contributor for the high mobility of the IRA and is only a product of the strong interaction between the sliding mass and the substrate at the final stage. This comprehensive study not only provides important information that contributes to an interpretation of similar features on Earth, but also yields insight into the underlying mechanisms responsible for the emplacement of rock avalanches.