Frictionite as evidence for a large Late Quaternary rockslide near Kanchenjunga, Sikkim Himalayas, India - Implications for extreme events in mountain relief destruction

We present sedimentologic and petrographic field evidence of an extremely large (~ 2.5 × 109 m3) Late Quaternary rockslide around Dzongri, 24 km S of Kanchenjunga (8585 m a.s.l.). Widespread occurrence of shattered leucogranites and migmatites deposited discordantly on top of undisturbed augen gneisses, and separated by micro-brecciated sliding surfaces indicate rapid movement during a large rockslide event. Occurrence of frictionite (= hyalomylonite) in mineral samples indicates short-term temperatures of ~ 600 °C in the moving rockslide mass along a 12° sliding plane. From a SW direction of movement we infer a source 8 km NE of the deposit, ~ 5 km S of Pandim (6691 m a.s.l.), where today a thick leucogranite intrusion marks a zone of weakness and failure of a postulated former mountain crest. The mechanism of movement, the dip of the sliding surface, and the deposit preservation along ridge tops require that the Dzongri rockslide ran out over an ice-filled Prek Chu valley. Hence, we prefer earthquake shaking over undercutting and glacial debuttressing as a possible trigger mechanism. A regional review of other similarly large and catastrophic Himalayan rockslides indicates that leucogranite intrusions are highly conducive to large-scale failure in the Higher Himalayan Crystalline (HHC). Importantly, high erosion rates have rendered this large rockslide deposit undetectable by remote sensing or digital elevation data, which are commonly used for identifying evidence of such extreme events. Quantifying the contribution of such giant landslides in destroying some of the highest relief on Earth will thus need further research attention.