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 15km^3 and a vertical displacement of more than 1500m remains the largest crystalline rockslide in Nepal Himalaya. The Quaternary landslide affected rocks in gneissic fancies 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. This is complex polyphase deformation episodes producing both brittle and ductile processes in response to slide-induced stress. In the bottom 20 metres of the Basal Zone (BZ) cataclasis occurred predominantly by brittle fracture and rigid body rotation. Localised strength anisotropies due to juxtaposition of leucogranite sheet intrusions (\u3c50cm \u3ethick) and biotite gneiss result in a range of microscopic deformational features. Using texture as a basis a transition is distinguished from (1) simple fracture dominated crackle breccia; through (2) foliated cataclasites; (3) matrix-poor jigsaw breccia to matrix rich jigsaw breccia with clast rotation. The character of crackle breccia suggests that sealing of fractures takes place by recrystallization of chlorite grains. In addition, dykes of fine grained cataclasite (\u3c0.1mm) generated and injected from the slide plane into the overlying block contain a range of microfabrics including: colour anisotropies, shape preferred orientation in biotite, and localised ultra cataclasite domains occurring as linear and non-linear foliations. The observed microstructures are interpreted for each sub-process during transport slope deformation in Basal Zone (primary slide plane) and impact generated Transform Fault (secondary slide plane) settings. Observed microstructures indicate that deformation of crystalline rock close to the slide planes (primary and secondary) are not simply the result of particle size comminution due to shear stress accommodation. Instead, it seems more likely that microbreccia develops during complex polyphase deformation episodes with cyclic strain hardening and softening episodes. Preexisting lithological textures are disrupted without disturbing the original stratigraphy of the block. Because the significance of high magnitude landslides in the Nepal Himalaya is not well known, this set of features, and their interpretation, may provide additional criteria for distinguishing other ancient rockslide deposits that remain, hitherto, unrecognised