Petrologic and structural controls on geomorphology of prehistoric Tsergo Ri slope failure, Langtang Himal, Nepal

Abstract

High relief (Δ h up to 2600 m) and spatial orientation of discontinuities, such as leucogranitic intrusions, mylonitic and pseudotachylitic horizons and related ore structures, were the preparatory causal factors for the giant prehistoric Tsergo Ri landslide (High Himalayan Crystalline, Langtang Himal, Nepal). Other controlling factors include overthrusting, detachment faulting, and paleoseismics related to the Main Central Thrust (MCT) and Southern Tibet Detachment System (STDS), neotectonically generated structures, and predesigned form of the scarp (broken crest) and the direction of movement towards the WSW. Coherently displacement of several 10 9 m 3 materials caused frictional fusion. Landslide dynamics and specific morphologic conditions created four different types of sliding surfaces (in chronological sequence): (i) primary—at the basement, (ii) secondary—parallel or subparallel to (i), (iii) tertiary—vertical to (i) and (ii), and (iv) quarternary sliding surfaces—local variety of (iii). (Note: ordinal numeric terms are not in time-stratigraphic sense.) These planes represented the main discontinuities conducting different syn-event stresses in the sliding mass and for further landslide dynamics. Spatial difference and widespread distribution of sliding planes and sets of discontinuities all over the landslide area caused different classes of rock quality, which subsequently determined specific erosional processes within the landslide accumulation area. The subsequent glacial and fluvial erosional processes were controlled by the correlating preexisting lithotectonic patterns, the hyalomylonitic and/or breccious sliding planes and the recent morphologic features or shapes of Tsergo Ri. (i) The SE part is gradationally brecciated to the top and includes Tsergo Ri itself. The residual mass of the bulk has been eroded almost isometrically as far as the primary sliding plane crops out along the intact gneissic basement. Differences in rock quality along a horizon of secondary sliding planes indicate a steplike break in the relief after the last main glaciation. (ii) The earlier-halted NW part still preserved micromountainous shape of compact sliding blocks (Phushung I and II, Kyimoshung), separated by landslide-dynamic-triggered faults (tertiary and quarternary sliding planes). Primary and secondary sliding planes, almost covered by Holocene sediments, produced an insignificant morphology. (iii) The highest brecciated strike–slip fault, generated during collision with the obstacle of Pangshungtramo Peak in the southwest, is the deeply eroded Dranglung Chu valley, which has been kept morphologically active in a major way until recent times. (iv) Except for the collision obstacle of Pangshungtramo Peak with recent semi-active erosion and a small secondary rockslide (block of Tsangbu) north of it, all surrounding parts show typical features of high altitude erosion.