On preparatory causal factors, initiating the prehistoric Tsergo Ri landslide (Langthang Himal, Nepal)

The most probable series of causal factors and their complexity make analysis and synthesis of the Tsergo Ri landslide (Langthang Himal, central-north Nepal) restricted, especially in this case of a prehistoric event with a spread of age data. Interpreting morphologic, lithologic, structural and engineering-geologic aspects in relation to preparatory causal factors and subsequent processes will therefore be only an approach. The solid basement and the broken crest of the landslide, as well as the surrounding areas of the upper section of Langthang valley, are to be included in the High Himalayan Crystalline: leucogranitic dikes (dipping towards the southwest) intruded gneisses and migmatites (primary foliation towards the northeast). Formation of these dikes caused one of many factors or preexisting structures, leading to the development of the landslide. In addition, subsequent tectonic movements and related seismicity forced specific deformation of metamorphic rocks: ultramylonites, two generations of faults with striated slickensides, and pseudotachylites degraded to substantial horizons of weakness. The spatial orientation enabled predetermination of subsequent morphologic processes (e.g., slope destabilising forces, trend and dip of sliding). In association with leucogranitic intrusions, a disseminated mineralized, extensive sulphidic ore structure outcrops at the broken crest of the landslide. Due to brittle theologic behaviour and poor weathering resistance this ore-bearing horizon (parallel to the main sliding plane) also made the slopes susceptible to sliding. Material displaced from the scarp by landsliding, was mixed up with rocks from undisturbed ground and subsequently cemented by secondary ore mineralization, thus forming ore-breccias at the top of the surface of rupture. Neotectonic structures, caused by stress release associated with erosional processes and similarly oriented as the existing zones of weakness, may have acted as a preparatory and/or triggering causal factor for slope destabilisation too. Hyalomylonite from primary and secondary sliding surfaces, as well as gneiss from the solid basement of the landslide, analysed by means of scanning electron microscopy and energy-dispersive X-ray analysis, indicate deformational patterns attributed to tectonics and landslide mechanisms. The deformation feature is distinguished clearly from those created by shock-wave events (i.e. impact-triggered Kofels landslide, Tyrolean Alps, Austria). Thus, seismic activity might have been the triggering causal factor for the Tsergo Ri landslide event.