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 a ~700 m thick, primarily top-to-the-north shear zone within the klippe; we term this 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 rightway-up thermal gradient of ~77-189 °C/km. U-Pb zircon dating of post-kinematic leucogranite dikes suggests ductile shearing along the Tila shear zone occurred prior to ~17-14 Ma. 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 ~3 km of structural section in the northern portion of our study area, and become close to within ~1 km of separation, in the southern portion. Interpolation suggests that the STD (Tila shear zone) and MCT merge 15±10 km 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.

Preservation of the Early Evolution of the Himalayan Middle Crust in Foreland Klippen: Insights from the Karnali Klippe, West Nepal

“…Sample JD‐06D is from the middle of the GHS on the northeast flank of the Karnali klippe. It is located within the Tila shear zone (Figure ; He et al, ). The CPO fabric forms a weakly asymmetric type I crossed girdle suggesting a top‐to‐the‐SSE sense of shear, inconsistent with the top‐to‐the‐north sense of shear interpretation of He et al (), but consistent with top‐to‐the‐SE shear bands observed in a nearby sample (JD‐06B).…”

“…Top‐to‐the‐SW shear‐sense indicators are abundant in the MCT high‐strain zone and sparse above it. A top‐to‐the‐north structure, the Tila shear zone, has been described in the middle of the GHS on the north flank of the Karnali klippe (Figure a; He et al, ). However, we have observed ambiguous evidence for top‐to‐the‐north shear at that location (see also Soucy La Roche et al, ).…”

“…The Karnali klippe is the eastern termination of the Almora‐Dadeldhura composite klippe in west Nepal (Figures and ) and represents an erosional outlier of GHS and TSS rocks (Arita et al, ; Frank & Fuchs, ; Fuchs, , ; Hayashi et al, ; He et al, ; Soucy La Roche et al, ; Upreti & Le Fort, ).…”

“…The structurally lower biotite‐rich quartzo‐feldspathic schist is likely a highly strained equivalent of the augen gneiss (e.g., Godin, Gleeson, et al, ). All GHS units above the MCT are commonly intruded by undated deformed leucogranitic sills and dikes, and middle Miocene undeformed dikes (e.g., He et al, ).…”

Gangdese culmination model: Oligocene–Miocene duplexing along the India-Asia suture zone, Lazi region, southern Tibet

“…8), spatial convergence ( Figs. 1 and 10), and consistent top-to-the-north kinematics suggest that linkage was possible, if not probable (Yin et al, 1994(Yin et al, , 1999(Yin et al, , 2010Yin, 2006;Webb et al, 2007Webb et al, , 2017He et al, 2016). The South Tibetan detachment system forms the roof fault of the North Himalayan domes (e.g., Larson et al, 2010), exposed just to the south of the Yarlung suture zone (Fig.…”