Demise of a Himalayan giant and the dramatic fate of the highest peaks on Earth

Abstract

Although the topographic evolution and erosion dynamics of the Himalayan range have been extensively documented, it is not known how the very high Himalayan peaks erode. Some conceptual models assume that intense periglacial processes involve regressive erosion of high peak headwall at rates dictated by valley-floor downcutting of glaciers. However, recent data indicate that frost-cracking intensity decreases with elevation, suggesting instead that highest Himalayan peaks are free of erosion, raising the question of their long-term evolution. Here, we report geological evidence for a giant rockslide that occurred around 1190 AD in the Annapurna Massif (central Nepal), involving a total rock volume of ~23km3, and that decapitated a paleosummit culminating most probably above 8000m of altitude. Our data demonstrate that the main mode of high-altitude erosion could be catastrophic mega-rockslides, leading to the sudden reduction of the high peaks elevation by several hundred meters and ultimately preventing the high Himalayan peaks from growing indefinitely. This erosion mode, associated to steep slopes and high relief, arises from a higher mechanical strength of the high-peak substratum, probably due to the presence of permafrost at high altitude and the absence of bedrock weathering. Giant rockslides also have major implications for landscape evolution: the massive supply of finely-crushed sediments can fill the valleys over >150km further downstream and saturate the Himalayan rivers with sediments for a century or longer time periods. This raises the crucial question of natural hazards associated with such high-magnitude events in the Himalayan regions.