Giant collapse of a high Himalayan peak and its major consequences downstream
Authors: Lavé Jerome , Guérin Cyrielle , Valla Pierre , France Lanord Christian , Benedetti Lucilla , Morin Guillaume , Galy Valier 
Published: February 2023

Slope failures and deep seated landslides are usually considered as the most efficient processes for hillslope erosion in active orogens. Erosion in the central Himalaya (Nepal) confirms such assertion (Morin et al., 2018), with in addition the probable predominance of the very large landslides in the erosive budget of the range (Marc et al., 2019).In this contribution, we report geological evidence for a giant rockslide that occurred around 1190 AD (14C burial age, 36Cl exposure age, and IRSL dating) in the Annapurna massif (central Nepal), and filled a wide glacial cirque (Sabche cirque) and the Seti valley farther downstream with a rock-avalanche deposit up to 1km thick, made of finely-crushed breccia. This rockslide, which involves a total rock volume of ~23 km3, decapitated the paleo-Annapurna IV, a paleo-summit culminating likely above 8000 m of altitude.Such giant rockslides are rare but not uncommon in central Himalaya with former description of pluri-kilometric rockslides involving up to ten cubic kilometres mass wasting (e.g. Weidinger et al., 2002). They have major implications for landscape evolution since they could represent the main mode of erosion of the high glaciated peaks, leading to the sudden reduction of ridge crest elevation by several hundred meters.They also have major implications on the fluvial network and how it responds to such massive and sudden supply of sediments. Downstream of the Seti river, the Pokhara Basin is filled by ~5 km3 of mostly conglomeratic sediment emplaced by fluvial, hyperconcentrated, or turbulent, sediment-laden flows. Extensive 14C dating of organic fragments found in the fine-grained units of the Pokhara sediments (Schwanghart et al., 2016; this study) provide robust constraints on the timing of aggradation. Onset of aggradation around 1200 AD (as well as the calcareous clasts whose only possible origin is the Sabche cirque) indicates that the conglomerates are the result of the active erosion of the Sabche rock-avalanche breccia. The aggradation lasted for about a century at an average rate of 1m/yr. During that period, the erosion of the fine-grained breccia has been extremely efficient and rapid: of the initial 27 km3 of rockslide debris, less than 10% can still be observed today in the Sabche cirque. During that period, the rate of sediment yield delivered by the Seti river was comparable to the highest measured values of post-volcanic eruption sediment transport. It led to an overwhelming content of carbonate-rich material in Narayani river sediments up to 300 km downstream, in the Narayani megafan in the middle of the Gangetic plain. Compared to the Narayani basin (30,000 km2), which exports annually ~0.05km3/yr of sediment, post-collapse erosion of the Sabche breccias (only 0.2% of the Narayani total basin area) would have increased this annual flux by a factor of 3 over 100-150 years.This particular example illustrates how the erosional/sediment routing system in active mountain range, as well as the landscapes, can be durably affected by one single extreme event.