Conclusions
Various small-scale ductile to brittle shear structures such as S-C and S-Cʹ shear fabric, porphyroclasts and porphyroblasts, mineral fish, asymmetric boudins, duplex and shear zones have been used for deciphering the shear sense within the Higher Himalayan Crystallines along the Alaknanda and Dhauli Ganga Valleys of the Uttarakhand Himalaya. This analysis revealed crucial information regarding two phases of ductile shear deformation, the DS1 and DS2, where the DS1 is the older phase of SW-verging shearing associated with the compressional tectonics and the DS2 is the younger phase of late orogenic NE-verging normal faulting/shearing. On close examination of these structures, five distinct zones have been mapped in this section: (i) pure compressional zone (PCZ) with only the DS1 shear fabric, (ii) dominant compressional zone (DCZ) with DS1 fabric dominating over the DS2 phase, (iii) transition zone (TZ) with extensive flowage having no shear sense indicators, (iv) dominant extensional zone (DEZ) with more frequent DS2 phase structures over the DS1 phase, and (v) pure extensional zone (EZ) having only the DS2 phase within the THS. The NE-directed extensional ductile shearing is distributed for nearly 20 km wide zone in the Dhauli Ganga Valley. The STDS shear fabric appears to be highly localized in character. These fabric/kinematic indicators become useful tools in critically assessing various tectonic models like ductile shear, critical wedge and channel flow models for the evolution of this part of the Himalaya, though quantitative work on strain, metamorphic gradients and geochronology is required to precisely select the model for evolution of the HHC. A possible model for the evolution of this part of the Himalaya is therefore either a ductile shear zone in which an older SW-verging ductile shearing associated with the compressional tectonics (the Couette flow) is superposed by the younger DS2 late orogenic NE-verging normal faulting/shearing phase as a consequence of the Poiseuille flow. Another possibility to explain the described kinematics is the critical taper model proposed by Kohn (2008) in which contractional and extensional shearing act in response of the chaning dynamic of the orogenic wedge. Only after a precise geochronology of the shear zone it is possible to fully discriminate among the proposed models.