Shear sense analysis of the Higher Himalayan Crystalline Belt and tectonics of the South Tibetan Detachment System, Alaknanda-Dhauli Ganga valleys, Uttarakhand Himalaya

摘要

In the central parts of Uttarakhand Himalaya, more than 20 km thick homoclinally NE-dipping Higher Himalayan Crystalline (HHC) Belt is thrust over the Lesser Himalaya Sedimentary Belt along the Main Central Thrust (MCT), and is almost continuously exposed between Helang and Malari along the Alaknanda and Dhauli Ganga valleys. The upper contact of this belt with the Martoli Formation of the Tethyan Himalayan Sequence is demarcated by the South Tibetan Detachment System (STDS). The belt is ubiquitously marked by small-scale asymmetrical structures like S-C and S-C' foliation, porphyroclasts and porphyroblasts, mineral fishes, intrafolial folds, duplex structures, ductile-brittle shear zones, and asymmetric shear boudins. Sense of ductile to brittleductile shearing has been determined from these structures across the whole belt, the MCT and the STDS, and reveals two phases of shear deformation: (a) an older top-to-SW upwards phase throughout the HHC, having an overall thrust geometry (DS1), and (b) a younger superposed top-to-NE downwards phase with normal fault sense from the middle to upper parts (DS2). These shear senses provide invaluable constraints on various tectonic models currently in use for the evolution of the Himalayan metamorphics.