Giant collapses of high Himalayan peaks and their impact on the erosion of Himalayan landscapes

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 headwalls 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 denudation requires a distinct erosional process. Based on the example of the giant collapse of the paleo-Annapurna IV, Lavé et al. (2023) proposed that this erosion occurs episodically and catastrophically during such giant rockslides.

To test this conceptual model and evaluate the modes of sediment export associated with these extreme events, we explored the entire Annapurna massif more systematically. In addition to dating (³⁶Cl exposure age and ¹⁴C burial age) several rockslide deposits already described in the literature, we identified for the first time a giant rockslide in the upper Marsyandi Valley (central Nepal). This rockslide, which corresponds to the collapse of the Khangsar Khang paleo-peak during the early-Holocene, formed a huge breccia deposit of ~20 km³ damming the valley and creating the Tilicho Lake (~5000 m a.s.l.).

The cumulative contribution of giant Holocene rockslides in the Annapurna massif represents an average erosion rate of approximately 3 mm/a, equivalent to long-term denudation rate. This equivalence confirms that the main mode of high-altitude erosion could be mega-rockslides that lead to the catastrophic reduction of the high peaks elevation by several hundred meters. This erosion mode of the High Himalaya, associated to steep slopes and high relief, might arise from a higher mechanical strength of the substratum, probably due to the presence of permafrost at high altitude and the absence of bedrock weathering (Lavé et al., 2023).

This major contribution from giant rockslides primarily concerns the denudation of peaks and hillslopes in the High Himalaya. At the scale of the mountain range, the question then arises of the export of breccia deposits produced by these rockslides. Using VolcFlow, a numerical code for granular avalanche flow (Kelfoun & Druitt 2005), we first explore the dynamics of avalanche deposit formation and their final geometry in the upper valleys. Secondly, by comparing with their present residual volumes, we estimate the erosion and export rates of the brecciated deposits. We thus highlight a significant contrast (> one order of magnitude in rates) between the southern flank of the massif, which is directly exposed to the monsoon precipitations and has steeper valleys, and the northern flank, which receives little rainfall. Depending on their location within the range, the giant rockslides can therefore have very different impacts on Himalayan landscape and downstream river evolution.