Syn-orogenic lateral extrusion across a Pre-Detachment Shear Zone, Garhwal Tethyan Himalayan Sequence, India: tectonothermal insights into the transition between frontal and lateral, extrusion processes

In the context of collisional orogenic belts the importance of lateral extrusion lies in the dissipation of energy accumulated by tectonically driven crustal thickening. The present study characterizes lateral extrusion across the newly reported (in this study), Pre-Detachment Shear Zone (PDSZ), in the Garhwal Tethyan Himalayan Sequence (THS), India, and highlights it’s significance in the Middle–Late Oligocene tectonothermal evolution of the Himalaya-Tibetan orogen. Integrated structural, microstructural, and kinematic vorticity analyses suggests crustal over-thickening in the THS, due to thrusting in the ductile regime. Increased overburden (~24 km) on the lower THS (deformed and metamorphosed at ~550–620 °) facilitated extrusion. However, greater uplift rate of the Greater Himalayan Sequence (GHS, south of the THS) than that of the THS, obstructed frontal extrusion and triggered orogen-parallel lateral tectonic escape of the THS. Lateral tectonic escape was dominated by dextral shearing in a general shear regime. Thrusting-dominated deformation in a general shear regime characterizes the southern boundary of the PDSZ. The study suggests that in the Garhwal Himalaya, the southern boundary of the PDSZ represents the tectonic front separating the domain of lateral extrusion (northwards into the Tibetan crust), from the domain of frontal extrusion and thrusting prevalent in the GHS during Oligocene–Miocene.