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-oor 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 inde nitely. 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 nely-crushed sediments can ll 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.
main textAt the scale of a human life, mountain peaks appear to stand immovable. Yet on geological time scales, mountain peaks are ephemeral: their shape and altitude are constantly evolving in response to the competition between tectonic uplift and erosion. There are, however, very few direct observations of how the highest mountain peaks evolve. Whereas their upheaval can be considered progressive over time, we don't know if their denudation is also progressive or occur in major increments, and if it proceeds at the same rates as valley-oor 1 ? It has been postulated that a glacial buzzsaw effect places an upper limit on altitude and relief irrespective of the tectonic uplift rate 2,3 , con ning mountain heights to within about 1.5 km of the local Equilibrium Line Altitude 4 (ELA). In such conceptual model, e cient periglacial weathering, frost cracking and rock falls along steep hillslopes produce rapid retreat of the glacier cirque headwalls, limiting peak elevation at rates commensurate with rapid valley-oor downcutting by glaciers 5 . In various settings, such model has been, however, challenged 6-10 . In the Himalayas, one of the most active mountain range on Earth, the >8000m peaks exceed ~3000m above the ELA and seem to contradict the notion of glacial buzzsaw 10,11 . There, local measurements of cirque headwall retreat show rates mostly lower than regional denu...
