Human settlements lie on paleolandslide deposits:Risk, evidence and formation of an ancient river-blocking landslide in the Jinsha River tributary, SE Tibetan Plateau

Abstract

The upper reaches of the Jinsha River, known as the “Three Parallel Rivers” region, situated on the southeastern margin of the Qinghai-Tibet Plateau, are susceptible to frequent large-scale river-blocking landslides. The Dingqu River, a tributary of the Jinsha River, accommodates four paleolandslides that serve as crucial land resources for the survival and development of the local population. The county seat of Derong is situated on the largest paleolandslide deposits among them. However, this fact has not been verified or disclosed in prior engineering geological investigations, resulting in inadequate long-term management and prevention of geological hazards that pose a threat to the county seat. This study aims to clarify the mechanisms of regional geological hazards using field investigations, optically stimulated luminescence (OSL) dating, stratigraphic drilling, and river profile analysis. The key objectives are to identify crucial evidence of paleolandslide deposits, determine their ages, and explore the role of river geomorphology in their formation. The results indicate that the age of the paleolandslide is 1.41 ± 0.12 ka, with a minimum deposit thickness of 100 m and an estimated volume of 250 million m³. The headward migration of knickpoint from the Dingqu River outlet caused rapid river incision at a high rate of 5.25 ± 0.38 mm/yr, providing the geomorphic conditions for the formation of the landslide. The lack of evidence for dammed landforms, such as lake deposits, vertical-step knickpoints, and outburst flood deposits, except for landslide deposits, can be primarily attributed to the limited lifespan of landslide dams and the erosion of flood deposits caused by the high river incision rate. It is recommended to prioritize measures to prevent river incision and manage water usage above the paleolandslide deposit to ensure the safety of towns located on these paleolandslide deposits. This study enriches the database of damming rockslide in the Jinsha River tributaries and provides a detailed case for exploring the coevolution of active orogenic rivers and slopes, reflecting potential risks faced by human settlements on paleolandslide deposits.