Medieval demise of a Himalayan giant summit induced by mega-landslide.

Despite numerous studies on Himalayan erosion, it is not known how the very high Himalayan peaks erode. Although valley floors are efficiently eroded by glaciers, the intensity of periglacial processes, which erode the headwalls extending from glacial cirques to crest lines, seems to decrease sharply with altitude1,2. This contrast suggests that erosion is muted and much lower than regional rock uplift rates for the highest Himalayan peaks, raising questions about their long-term evolution3,4. 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 about 23 km3. This event collapsed a palaeo-summit, probably culminating above 8,000 m in altitude. Our data suggest that a mode of high-altitude erosion could be mega-rockslides, leading to the sudden reduction of ridge-crest elevation by several hundred metres and ultimately preventing the disproportionate growth of the Himalayan peaks. This erosion mode, associated with steep slopes and high relief, arises from a greater mechanical strength of the peak substratum, probably because of the presence of permafrost at high altitude. Giant rockslides also have implications for landscape evolution and natural hazards: the massive supply of finely crushed sediments can fill valleys more than 150 km farther downstream and overwhelm the sediment load in Himalayan rivers for a century or more.

SAR and optical images correlation illuminates post-seismic landslide motion after the Mw 7.8 Gorkha earthquake (Nepal).

In the days to weeks following an earthquake, landslides can display specific post-seismic motions, including delayed initiations and post-seismic relaxations. These motions have an uncertain origin, sometimes attributed to specificities of the landslide basal interface or to fluid transports in the landslide basal shear zone. Here we address this question, by documenting the co- and post-seismic motions of slow-moving landslides accelerated by the Gorkha earthquake (Mw 7.8, 25/04/2015, Nepal). We detect 11 slow-moving landslides over an area of 750 km[Formula: see text] in the near field of the earthquake, and monitor their motions thanks to a time-series of Pléiades optical satellite images and SAR Sentinel-1 images. The post-seismic landslide motions are much larger than the co-seismic ones, reaching up to [Formula: see text] m accommodated over 2 months. A delayed initiation of several days (> 4 days) is also measured for at least two of the landslides. We analyze our findings in regards with all the previous observations on slow-moving landslides accelerated by earthquakes, and propose that the post-seismic motions are caused by diffusion of groundwater from co-seismic material contraction up to the landslide basal shear zone or from internal landslide reconfiguration. Our observations strongly suggest the main control of the hydrology in the landslide processes under seismic forcings.

Bedload transport in rivers, size matters but so does shape.

Bedload transport modelling in rivers takes into account the size and density of pebbles to estimate particle mobility, but does not formally consider particle shape. To address this issue and to compare the relative roles of the density and shape of particles, we performed original sediment transport experiments in an annular flume using molded artificial pebbles equipped with a radio frequency identification tracking system. The particles were designed with four distinct shapes and four different densities while having the same volume, and their speeds and distances traveled under constant hydraulic conditions were analyzed. The results show that particle shape has more influence than particle density on the resting time between particle displacement and the mean traveling distance. For all densities investigated, the particle shape systematically induced differences in travel distance that were strongly correlated (R2 = 0.94) with the Sneed and Folks shape index. Such shape influences, although often mentioned, are here quantified for the first time, demonstrating why and how they can be included in bedload transport models.

Lake Tauca highstand (Heinrich Stadial 1a) driven by a southward shift of the Bolivian High.

Heinrich events are characterized by worldwide climate modifications. Over the Altiplano endorheic basin (high tropical Andes), the second half of Heinrich Stadial 1 (HS1a) was coeval with the highstand of the giant paleolake Tauca. However, the atmospheric mechanisms underlying this wet event are still unknown at the regional to global scale. We use cosmic-ray exposure ages of glacial landforms to reconstruct the spatial variability in the equilibrium line altitude of the HS1a Altiplano glaciers. By combining glacier and lake modeling, we reconstruct a precipitation map for the HS1a period. Our results show that paleoprecipitation mainly increased along the Eastern Cordillera, whereas the southwestern region of the basin remained relatively dry. This pattern indicates a southward expansion of the easterlies, which is interpreted as being a consequence of a southward shift of the Bolivian High. The results provide a new understanding of atmospheric teleconnections during HS1 and of rainfall redistribution in a changing climate.