A polished and striated pavement formed by a rock avalanche in under 90 s mimics a glacially striated pavement

A polished and striated bedrock surface resembling a classical glacial striated pavement formed in 2009 in substantially <90 s beneath a rock avalanche on Jiweishan Mountain. Opportunities to closely examine such surfaces are rare, allowing researchers to speculate widely on basal rock-avalanche processes. We constrain future speculation by discussing signatures left on a basal sliding surface at a distance of 500 m from the headwall of this 2.2 km long rock avalanche. There are scratches, chatter marks and plucking scars on a surface of dolomitic black shale. They indicate frictional drag of particles in direct contact with the bed, causing relatively mild abrasion. Striations have beginnings and ends, vary in depth along their lengths, and do not always follow straight lines. These patterns suggest that the scratching particles moved about relative to one-another while confined in a deforming basal gouge. We used rotary shear equipment to experimentally shear both dry and saturated landslide gouge against intact shale from the sliding surface to create similarly grooved pavements under a realistic normal stress (1 MPa). Our observations and experiments suggested that the base of the landslide slid rapidly across this site on Jiweishan Mountain as a slightly erosive, initially water-saturated, dense grain flow with a low dynamic friction coefficient of about 0.1. Friction heated the base to about 800 °C, decomposing talc and dolomite to produce high-pressure live steam and carbon dioxide. What was left was a polished and grooved pavement highly reminiscent of a glacially striated pavement.