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The ca. 80 km long trans-Himalayan highway between Gangtok and Yumthang has experienced at least three large rock slope failures (RSF) within the past 40 years and tens of smaller RSF related to the 2011 Sikkim earthquake. More than 30 conspicuous boulder deposits suggest that similar failures happened in the past. Since the largest of these deposits are located within the shallowest sections of otherwise 60 – 75° steep slopes, they are often the location of settlements. We have used Terrestrial Cosmogenic Nuclide (TCN) dating to understand better where and how often these events are likely to occur.The trans-Himalayan highway connects the Lesser Himalaya, with a tropical to subtropical climate, with the cold-temperate climate in the Higher Himalaya north of the Main Central Thrust (MCT). This highway also crosses the orographic barrier, with rainfalls exceeding 3000 mm/yr in the south and less than 500 mm/yr in the north. On September 10th, 1983, a large RSF was triggered by “exceptional” rainfall and impacted the settlement of Manul, with an estimated life loss of 200 persons. Today, the deposit is covered by a dense tropical forest 30-m high that restricts detailed analysis. However, boulder size and boulder density on the surface suggest that it was a rock avalanche.The second reported RSF is a rock avalanche with a volume of 12 million m3 that occurred close to the village of Yumthang on March 11th, 2015. This deposit overlies two generations of prehistoric rock-avalanche deposits. No trigger was reported.The last reported RSF involved a volume of 8.7 million m3, occurred on August 13th, 2016 at Dzongu, NW of Mangan. While no trigger for the collapse was reported, satellite footage indicates at least ten years of pre-failure rock-slope deformation. The deposit has the typical carapace of a rock avalanche, but videos posted on social media instead suggest that it was a collapse that took place over several hours.RSF deposits are found in similar numbers in both the Higher and Lesser Himalaya, with the highest concentration in the vicinity of the MCT and a second cluster close to the village of Yumthang. We sampled ten of the deposits for TCN dating, including two of the historic events. Both historic events returned zero ages. The two older deposits overlain by the 2015 Yumthang rock avalanche returned equally young ages, suggesting multiple recent events at that site within a short time. The zero ages of both historical events suggest that inheritance of nuclides prior to failure in the samples can be ruled out. The ages of the remaining deposits range from 0.2 to ~12 kyr. Several deposits have bimodal age distributions. Others have three different ages in different sectors of the deposit. These results show that multiple RSF similar to the Yumthang site often can affect the same slope sector, leaving deposits on the same slope sections. Thus, the 30 identified deposits by far are the lower limit of RSF failures in the study area and that the threat of RSF is high. |
