Lesser Himalayan thrusting followed by out-of-sequence channel flow in the Greater Himalayan Sequence, north-western Garhwal Himalaya, India: evidence from integrated structural and microtectonic studies

Abstract

In continent-continent collision zones, the mechanically weak, hot middle crust plays a significant role in the accommodation of convergence between the colliding plates. Thus, multiple tectonothermal episodes are preserved in the middle crust represented in general, by the metamorphic core of collisional orogens like the Himalaya. The present study demonstrates that the Greater Himalayan Sequence (GHS) evolved by mid-crustal channel flow that is out-of sequence with respect to the penetrative fabric forming event in the Paleoproterozoic Lesser Himalayan Sequence (LH) occurring SSW of the GHS, northwestern Garhwal Himalaya, India. Structural and microtectonic studies reveal three phases of deformation at varying thermal regimes across the Himalayan Metamorphic Core (HMC) that constitutes the GHS, the Main Central Thrust (MCT) and the Paleoproterozoic LH, south of the MCT. Out-of-sequence deformation in the LH-GHS package is revealed by the prevalence of the earlier D1 related structures in the Paleoproterozoic LH, and their gradual transposition and reorientation parallel to the late D2 ductile thrusts preserved in the hinterland part (GHS). Crystallographic preferred orientation studies suggest that the GHS extruded as a mid-crustal channel evinced by reversal in the asymmetry of mid-crustal flow across the GHS. SSW-ward extrusion of the GHS as a mid-crustal channel involved a significant pure-shear component (41–66 %) of deformation in transport-parallel section. Fluid infiltration along the MCT was pre-to syn-kinematic with respect to its localized thrust-sense reactivation.