Vorticity patterns along the Main Central Thrust Zone, Alaknanda–Dhauli Ganga Valleys (Garhwal), Uttarakhand Himalaya

The Greater Himalayan Sequence (GHS), constituting the anatectic core of the Himalaya, is generally modelled as a mid-crustal southward extruding channel or wedge. Movements along the Main Central Thrust (MCT) in the south and the South Tibetan Detachment System (STDS) in the north and exhumation along the Himalayan front played an important role in the extrusion of the GHS from beneath the Tibetan plateau during the Miocene. To understand the kinematics of these orogen-scale shear zones, it is important to constrain the percentage of pure shear associated with them. In this paper, we present the kinematic vorticity data from the Main Central Thrust Zone (MCTZ), Alaknanda and Dhauli Ganga Valleys (Garhwal), Uttarakhand Himalaya. The mean kinematic vorticity number (Wm), which can be used to calculate the percentage of pure shear, has been estimated by analysing the rotational behaviour of rigid grains in a ductile matrix. The analysis reveals that pure shear provides significant contribution (30–52%) to the deformation associated with southward ductile shearing along the MCT, with the highest mean kinematic vorticity number (Wm) values close to the MCT. The results provide important quantitative constraints for the boundary conditions in the extrusion models. The Wm values from within the anatectic core have not been reported as most of the vorticity gauges fail due to increased deformation temperatures in this region.