Repeated failures of the giant Beshkiol landslide and its impact on the long-term Naryn Basin flooding, Kyrgyz Tien Shan

Earthquake-triggered landslides pose significant hazards and their long-term effects can radically reshape the local physiography but also may generate cascading risks. Indeed, a landslide could dam the river, having for consequence the formation of an upstream lake, which in turn makes the dam unstable, leading to cataclysmic flooding in the case of sudden failure. The Naryn River is one of the most important rivers in the Western Tien Shan, and major changes in its dynamics could have a significant economic impact in Central Asia and lead to political tensions over water management. Our study focuses on the Beshkiol paleo-landslide (>10km3), one of the largest in Central Asia, an overlooked hazard along the Naryn River.Through a multi-disciplinary approach that combines detailed geomorphological, sedimentological and chronological (luminescence, cosmogenic and radiocarbon) analysis over a study area more than 130 km-long, we determined the different phases that affected the evolution of this landslide from the late Pleistocene to the late Holocene. First of all, two lacustrine sequences have been identified in the Naryn Basin, illustrating two successive periods of river damming and a lake outburst flooding. The triggering of the Beshkiol landslide occurred ~52 ka ago, led to the damming of the Naryn River and the formation of an 80 km-long lake upstream. Our chronological constraints highlight a residence time of 36,000 years, one of the longest ever documented in the world for a natural dammed-lake. This lake then drained in a cataclysmic event around 15 ka, which most likely led to the flash flooding of the downstream basin of the Naryn River (Kazarman Basin), as evidenced by very high energy deposits identified upstream of the landslide. However, shortly afterwards (less than 1,500 years), the foot of landslide was reactivated, causing the formation of a second lake, with a residence time estimated at ~7,600 years. This period was followed by a gradual emptying, and a phase of erosion that shaped the present landscape. Our results highlight that cascading events took place over the last 50,000 years and show complex interactions between the Naryn River and the largest landslide in Central Asia. Today, this landslide is categorized as inactive, but in view of the large volumes of material that can be reactivated by earthquakes or changes in precipitation, it is necessary to take this hazard into account as several thousand people living in the region could be impacted