The Tsergo Ri landslide: An uncommon area of high morphological activity in the Langthang valley, Nepal

The large-scale mass movement of Tsergo Ri (Langthang valley, central-north Nepal) was investigated from the viewpoint of morphological activity for the first time. The Quaternary mass-movement (fission track age: about 4 x 10(4) yr) only affected crystalline hard rocks (gneisses, migmatites and leucogranites) of a dimension of 10(10) m(3). During the mass displacement an extreme shattering of the rocks happened, which is mainly responsible for the high actual morphological activity in the Tsergo Ri landslide deposit area. Three different zones of highest morphological activity (= the sum of the presently occurring geomorphic processes) can be distinguished: (A) High, very steep slopes (mainly in bedrock) show usual morphological activity, except when they were affected by the landslide event - e.g., the barrier where the sliding mass crashed against: E-slope of Pangshungtramo Peak. (B) Slopes and erosional channels underneath glaciated or perennial firn areas, or areas which are snow covered most of the year show a permanent or seasonal morphological activity - e.g., the area in front of the Yala Tsang. (C) The zone with the biggest extension of actual morphological activity is the area of the Tsergo Ri landslide deposit with intense rock shattering of the sliding mass - e.g., the Dranglung valley or the S- and SW-slopes of Tsergo Ri. Zones A and B come up to the normal expectations of morphological activity for high mountain regions. Zone C makes the whole Tsergo Ri landslide deposit area an outstanding terrain of the upper Langthang valley with extremely high morphological activity.