An Early Holocene river blockage event on the western boundary of the Namche Barwa Syntaxis, southeastern Tibetan Plateau

Abstract

Late Quaternary river blockage events in the mountainous regions of the southeastern Tibetan Plateau, characterized by steep topographies and deeply-incised valleys, have commonly been triggered by tectonic and glacial activity. In this study, we described the geomorphological and sedimentological characteristics of a fluvial-lacustrine sedimentary sequence belonging to a dammed paleolake that formed along the Dongjiu River at the terminus of the Lulang Fault, on the western boundary of the Namche Barwa Syntaxis. Eight main lithofacies associations are proposed that represent four main paleoenvironments: a distal lacustrine environment, i.e. representing a lake distal environment; a lake margin environment; an ice-contact lacustrine environment; and a fluvial environment. 14C and OSL dating results indicate that the paleolake formed at least between ~7.1–3.6 ka. The paleolake covered ~2.27 km2 and contained a water volume of ~0.09 km3, with the water surface lying at an altitude of 2590 m above sea level (asl). The soft-sediment deformations induced by a paleoseismic event in the environs of the Lulang River nearby during the early Holocene, synchronous with the rock avalanche which dammed the Dongjiu River. We would therefore suggest that such a damming event might have been triggered by a paleoearthquake. Steepness index analysis of longitudinal profiles indicates that the paleodam caused by a landslide across the Dongjiu River was unable to preserve the stability of knickpoint, a migration that can potentially be related to a high river erosion rate.