THE SOUTH TIBETAN DETACHEMENT SYSTEM (STDS): ITS GEOLOGY AND SHEAR SENSE ANALYSIS OF THE HIGHER HIMALAYAN CRSYTALLINES (HHC),DHAULIGANGA VALLEY, GARHWAL HIMALAYA

Abstract

In the central laIs of Uttarakhand Himalaya, more than 20 km thick homoclinally and NE-dipping Higher Himalayan Crystalline (HI-IC) belt is thrust over the Lesser Himalaya Sequence (LHS) along the Main Central Thrust (MCT), and is almost continuously exposed between 1-lelang and Malari along the Alaknanda and Dhauli Ganga valleys. The upper contact of this belt with the Martoli Formation of the Tethyan Himalayan Sequence (THS) is demarcated by the South Tibetan Detachment System (STDS). The belt is ubiquitously marked by small-scale asymmetrical structures like S-C, S-C' and S-C" fabrics, mantled porphyroclasts, mineral fish, sigmoidal structures and boudins. Sense of ductile to brittle-ductile shearing has been determined from these structures which revealed two phases of ductile shear deformation: (a) Dl phase: an older top-to-SW upwards phase throughout the HHC having an overall thrust geometry, and (b) D2 - phase: a younger superposed top-to-NE downwards phase with normal fault sense - from the middle to upper parts. The whole section was divided into five zones on the basis of the distribution pattern of the shear sense indicators and their analysis. Then, the Channel flow model was critically assessed with respect to these observations which revealed that an earlier Couette flow was superposed with a later stage Poiseuille flow in the channel which can explain all the observed features.