The giant mid-Holocene Linka rock avalanche with long-runout river blockage in the southeastern Tibetan Plateau

Abstract

As a response to rapid tectonic uplift and intense river incision, giant river-damming landslides have frequently occurred in the mountain and canyon region of the southeastern Tibetan Plateau. These landslides in turn can transiently affect the longitudinal profile evolution of rivers. The ca. 440 Mm³ channelized Linka rock avalanche initiated as a giant translational rockslide on the south-facing dip slope of the NW-trending divide of the Waqu catchment. The detached rock mass involved the thick and resistant Wada limestone, and the underlying Basu red-bed unit, comprising sandstone, conglomerate, and siltstone. The NWW-striking bedding planes and the SE-dipping joint set bounded the landslide source area at its base and sides, respectively, and a buckling failure controlled by the high-angle SW-dipping joints at the slope toe induced this giant mass-movement event. The rock slope failure was likely triggered by an earthquake that occurred at ca. 5.5 ka before present, probably corresponding to a prehistorical rupture on the active Bianba-Luolong Fault Zone. The confined rock avalanche deposits, covering ca. 5.5 km², rode up both of the constraining ridges that laterally constrained the motion path. The instantaneous velocity as the rock avalanche traveled through the distal bend of the motion path was calculated to be ca. 34 m/s using a superelevation-based equation. The rock avalanche traveled a horizontal distance of ca. 5970 m with a vertical drop of 1380 m, displaying a high mobility (H/L index value of 0.231), which can be attributed primarily to dynamic rock fragmentation. This giant mass-movement event completely blocked the Waqu River with a ca. 170-m high dam. This natural dam has experienced breaches at least 6 times (including 1 seepage failure and 5 subsequent overtopping failures), and the largest peak discharge from the 1st seepage failure was estimated to be ~ 19,500 m³/s. The formation and breaching of the natural dam have caused a transient but extremely profound influence on the longitudinal profile evolution of the Waqu River. This research involving landslide initiation, displacement, deposition, and the formation and breaching of the landslide dam can help enhance the understanding of effective risk assessments of landslide-induced disaster chains in high-relief regions.