Geomorphology and Sedimentology of the Nyixoi Chongco Rock Avalanche and Implications for Emplacement Mechanisms

Abstract

Large rock avalanches are ubiquitous surface hazards on Earth and are characterized by long runout distances and high velocities. These extreme mobility features are regarded as the key causes of catastrophic damage. Commonly, these rock avalanches are characterized by a complicated set of geological settings and behaviors. Although many hypotheses have been proposed to explain this phenomenon, a comprehensive explanation of its geological features is lacking. To precisely identify the extreme mobility mechanisms of large rock avalanches, we examined data collected from a deposit of the Nyixoi Chongco rock avalanche (NCRA) (Tibetan Plateau, China). Through a combination of preliminary observations and analyses of the morphology and sedimentology of the deposits, we reconstructed the kinematic process and recognized that the formation of superficial structures is related to mass flow emplacement dynamics driven by high-speed avalanche debris impacting a liquefiable substrate. One mechanism to explain the extreme mobility of the NCRA is the reduction in the basal layer resistance owing to contraction-induced excess pore pressure. To further validate and quantify this long runout mechanism, numerical simulations were conducted using a multiphase model to precisely determine how deformable granular mixtures in the basal layer led to excess pore pressure and underwent liquefaction when the avalanche was emplaced, demonstrating that this process dominated the behavior of the Nyixoi Chongco rock avalanche. The present study provides an improved method and understanding of the kinematic processes and runout mechanisms of the extreme mobility of similar rock avalanches.