Crustal structure, restoration and evolution of the Greater Himalaya in Nepal-South Tibet: implications for channel flow and ductile extrusion of the middle crust

Abstract

Abstract Recent suggestions that the Greater Himalayan Sequence (GHS) represents a mid-crustal channel of low viscosity, partially molten Indian plate crust extruding southward between two major ductile shear zones, the Main Central thrust (MCT) below, and the South Tibetan detachment (STD) normal fault above, are examined, with particular reference to the Everest transect across Nepal-south Tibet. The catalyst for the early kyanite ± sillimanite metamorphism (650–680°C, 7–8 kbar, 32–30 Ma) was crustal thickening and regional Barrovian metamorphism. Later sillimanite ± cordierite metamorphism (600–680°C, 4–5 kbar, 23–17 Ma) is attributed to increased heat input and partial melting of the crust. Crustal melting occurred at relatively shallow depths (15–19 km, 4–5 kbar) in the crust. The presence of highly radiogenic Proterozoic black shales (Haimanta-Cheka Groups) at this unique stratigraphic horizon promoted melting due to the high concentration of heat-producing elements, particularly U-bearing minerals. It is suggested that crustal melting triggered channel flow and ductile extrusion of the GHS, and that when the leucogranites cooled rapidly at 17–16 Ma the flow ended, as deformation propagated southward into the Lesser Himalaya. Kinematic indicators record a dominant south-vergent simple shear component across the Greater Himalaya. An important component of pure shear is also recorded in flattening and boudinage fabrics within the STD zone, and compressed metamorphic isograds along both the STD and MCT shear zones. These kinematic factors suggest that the ductile GHS channel was subjected to subvertical thinning during southward extrusion. However, dating of the shear zones along the top and base of the channel shows that the deformation propagated outward with time over the period 20–16 Ma, expanding the extruding channel. The last brittle faulting episode occurred along the southern (structurally lower) limits of the MCT shear zone and the northern (structurally higher) limits of the STD normal fault zone. Late-stage breakback thrusting occurred along the MCT and at the back of the orogenic wedge in the Tethyan zone. Our model shows that the Himalayan-south Tibetan crust is rheologically layered, and has several major low-angle detachments that separate layers of crust and upper mantle, each deforming in different ways, at different times.