Structural and sedimentary characteristics of the Tsergo Ri rockslide breccia, Langtang, Nepal, Himalaya

The high magnitude Tsergo Ri landslide (c.100ka); Langtang Himal, with a volume in excess of 15km3 and a vertical displacement of more than 1500m remains the largest crystalline rockslide in Nepal Himalaya. The Quaternary landslide affected rocks in gneissic facies of the High Himalayan Crystalline Sequence adjacent to the Tibetan Sedimentary Series. Depositional characteristics from exhumed transport slope sites were analysed at both the macroscopic and the microscopic scale to investigate (i) localised fracture in terms of orientation and density, and (ii) petrography of slide induced host rock deformation.; Fracture orientation analysis indicate the spatial orientation of preexisting discontinuities to NE and SW played a principal causal role in destabilisation, establishment of the primary slide plane and movement to the SW. Density data show fractures increase non-uniformly and do not show a positive linear relationship with distance. Most instances show fracture densities are greater in the slide mass than in the basement and surrounding rockwalls, excluding impact sites with obstacles to movement.; In contrast to the almost exclusive interpretation of brittle processes from previous descriptions of brecciated rocks from similar landslides, evidence indicates the operation of tectonic style deformation. (Abstract shortened by UMI.)...