Deformational and strain patterns of an intracontinental collision ductile shear zone—an example from the Higher Garhwal Himalaya

Abstract

The Higher Himalayan metamorphic belt of Garhwal appears to have evolved in a major 15–20 km wide, NE-dipping ductile shear zone of the overthrust-type due to intracontinental crustal shortening during Cenozoic continental collision within the Indian Plate. Out of the four major distinct deformation phases, the most widespread D 2 deformation phase is marked by a prominent S 2 foliation axial-planar to the reclined and recumbent F 2 folds, a coaxial NE-plunging L 2 stretching lineation and syntectonic growth of index metamorphic minerals. These deformational structures are developed during the SW-directed D 2 ductile shear deformation, irrespective of the orientation of the S 2 foliation and thrust zones. Regional strain patterns in quartzite of the Lesser Himalayan Garhwal Group beneath the Main Central Thrust (MCT) and the Central Crystalline Zone using R f/ φ data on quartz clasts and aggregates reveal that the maximum values of finite strain ε s are attained within narrow mylonitic shear zones within the broad ductile shear zone. During its progressive translation towards higher structural levels in late D 2 and D 3 deformation phases, the shortening was accommodated by development of thrust sheets with the MCT and Jutogh Thrust surfaces coinciding with zones of mylonite and maximum strain. At higher levels, ductile shear zones became compressional brittle-ductile zones which resulted in the development of SW-verging F 3a and F 3b folds during the southward migration of the thrust sheets in the Lesser Himalaya.