Mechanisms leading to the 2016 giant twin glacier collapses, Aru range, Tibet

Abstract. In northwestern Tibet (34.0° N, 82.2° E) near lake Aru Co, the entire ablation area of two glaciers (Aru-1 and Aru-2) suddenly collapsed on 17 July 2016 and 21 September 2016, respectively, and transformed into 68 and 83 106 m3 mass flows that ran out up to 7 km, killing nine people. The only similar event currently documented is the 2002 Kolka Glacier mass flow (Caucasus Mountains). Using climatic reanalysis, remote sensing and 3D thermo-mechanical modeling, we reconstructed in detail the glaciers' thermal regimes, thicknesses, velocities, basal shear stresses and ice damage prior to the collapse. We show that frictional change leading to the collapses occurred in the temperate areas of polythermal glacier structures and are not linked to thaw of cold based ice. The two glaciers experienced a similar stress transfer from predominant basal drag towards predominant lateral shearing in the later detachment areas and during the 5–6 years before the collapses, though with a high friction patch on Aru-2 tongue which is inexistent on Aru-1. The latter led to distinctly disparate behaviour making the development of the instability more visible for the Aru-1 glacier compared to Aru-2 through enhanced crevassing over a longer period and terminus advance. Field investigations reveal that those two glaciers are flowing on a soft, highly erodible, and fine-grained sedimentary lithology. We propose that specific bedrock lithology played a key role in the two Tibet, and also in the Caucasus gigantic glacier collapses documented to date by producing low bed roughness and large amount of till rich in clay/silt with low friction angle. The twin Aru collapses would have been driven by a failing substrate linked to increasing water pore pressure in the subglacial drainage system in response to recent increases of surface melting and rain.