Emplacement mechanisms of the Tagarma rock avalanche on the Pamir-western Himalayan syntaxis of the Tibetan Plateau, China

Abstract

To determine the emplacement mechanisms of rock avalanches, the surficial and internal sedimentological characteristics of avalanche deposits are attracting increased scientific interest. In this study, deposits of the massive paleoseismic Tagarma rock avalanche in the Tarim Basin of the Pamir-western Himalayan syntaxis, China, are investigated to enrich the knowledge on rock avalanche dynamics. Based on a combination of remote sensing data and in situ surveys, a series of typical surficial landforms, including a Toreva block, arc-shaped transverse ridges, aligned megablocks, and longitudinal lineament, are studied in detail. Clusters of superimposed gneiss megablocks preserving the bedrock structures of the source scar are widely distributed along the travel path. Moreover, inverse grading in the deposit and a substrate characterized by clast-supported pebbles with imbricate structures are also observed. Cracks and fissures are common in most pebbles, indicating intense interactions between the avalanche mass and the substrate. Through analysis of these surficial and internal characteristics, we propose that the avalanche mass should have propagated rapidly as a single unit with slight internal mixing and high internal pressure fluctuations. These internal pressure fluctuations obviously exceeded the overburden pressure and contributed to the intense shattering of the main deposit, especially the basal facies. The basal facies was mainly emplaced by a frictional, simple shear process and controlled the avalanche hypermobility. From the proximal to the distal parts, the transport of the avalanche mass can be classified as an extension-dominated sliding process in the transition zone and a compression-dominated sliding process in the accumulation zone.