Pervasive distribution of extension-dominated landforms: implications for the kinematics of glacier-free rock avalanches in the Tibetan Plateau highlands

Pervasive distribution of extension-dominated landforms of rock avalanches in glacial regions has been widely reported, whereas those developed in non-glacial areas are scarce with their dynamics remain debated, particularly regarding the genetic mechanism of longitudinal ridges. To explore the emplacement mechanism of such avalanches, this study investigates a large Holocene rock avalanche characterized by the sequential distribution of extensive extension-dominated landforms. Its transition zone is marked by a suite of counter-slope platforms with steep scarps, as well as lateral ridges featuring stepped echelon-like linear structures. The accumulation zone contains a series of longitudinal ridges, with thicknesses (T) ranging mainly from 15 to 35 m and spacing (S) from 20 to 50 m - significantly larger than those reported in supraglacial cases. Meanwhile, a decreasing trend in the averaged S/T ratio versus averagedT is reached for longitudinal ridges, spanning from laboratory granular flows to natural RAs. The avalanche substrate consists of gravel-dominated fluvio-glacial deposits with a relatively steep slope and a sharp planar contact with the main avalanche body is identified. Based on a comprehensive analysis of these extensional landforms and the avalanche-substrate contact characteristics, we propose that the avalanche underwent an extension-dominated propagation process with the mass exhibiting a rapid extension-faulting in the transition zone and an active stretching in the main part of the accumulation zone. The combined action of self-excited vibration and frozen layer lubrication enhanced the high mobility of the rock avalanche and facilitated the formation of the longitudinal ridges. The frontal area of the accumulation zone was dominated by a passive deceleration process with minor extension, which was controlled by a frozen layer lubrication with the self-excited vibration absent.