Extrusion vs. duplexing models of Himalayan mountain building 2: The South Tibet detachment at the Dadeldhura klippe

Abstract

Himalayan mountain building has been dominantly explained by two types of models: extrusion and duplexing. To elucidate possible roles of these mechanisms during emplacement of the Himalayan crystalline core, we investigate an area speculated to contain the southern leading edge of the crystalline core: the northeastern margin of the Dadeldhura klippe, western Nepal. We found an ~700m thick, primarily top-to-the-north shear zone within the klippe; we term this as the Tila shear zone. The shear zone occurs within a right-way-up metamorphic field gradient, and separates footwall gneiss from hanging wall schist. Similarly, deformation temperatures estimated from quartz and feldspar microstructures and quartz c-axis fabrics indicate a right-way-up thermal gradient of ~77–189°C/km. U–Pb zircon dating of post-kinematic leucogranite dikes suggests that ductile shearing along the Tila shear zone occurred prior to ~17–14Ma. We correlate the Tila shear zone to the South Tibet detachment (STD) on the basis of consistent structural fabrics (shear sense), lithologies, metamorphism, and deformation timing. This interpretation, in combination with regional constraints, indicates southwards-increasing proximity of the STD (Tila shear zone) and the Main Central thrust (MCT). These two shear zones are separated by ~3km of structural section in the northern portion of our study area, and become close to within ~1km of separation, in the southern portion. Interpolation suggests that the STD (Tila shear zone) and MCT merge 15±10km southwest of our study area. The increasing-to-south proximity and potential merger of the two shear zones suggest that the STD formed as a backthrust from the MCT. This interpretation contrasts with the long-standing normal fault interpretation of the STD. Because the STD and MCT bound the Himalayan crystalline core, these findings document crystalline core emplacement at depth via tectonic wedging. This kinematic evolution is consistent with duplexing, but not extrusion to the surface.