Structural and mineralogical studies of the Tso Morari Dome: Insight into the deformation kinematics of the eclogitic gneiss, Ladakh Himalaya, India

Coesite-bearing eclogites from the Tso Morari Dome (TMD) and the Kaghan valley (Pakistan) are two examples from the Himalayan orogen that attained UHP conditions within 5 Ma, by subducting the frontal part of the advancing Indian plate through a subduction channel, and subsequently extruded rapidly ( 17 mm yr-1). This study focuses on the deformation of the gneissic rock that hosts the UHP eclogites. 25 rock samples were collected from two transects viz. (A) Sumdo-Karzok and (B) Sumdo-Debring. Preliminary thin-section studies reveal differences in microstructural characters between the rocks of A and B. Although dynamically recrystallised quartz grains are present in all these samples, grain boundary migration recyrstallisation ( 530-650 °C) are better preserved in the rocks of A. Similarly, intra-granular fractures in both quartz and feldspars, the latter being dominant, are more prominent in the samples along A. Chessboard extinction patterns (> 700 °C) in quartz, micro-faults in plagioclase grains and undulatory extinction in micas are also present. Samples close to the Zildat shear zone (ZSZ; N margin of the TMD) exhibit medium-sized, lenticular quartzo-feldspathic grains. Their abundance wanes away from the fault possibly due to decreasing deformation intensity. XRD studies reveal a decline in the ratio of modal percentage K-feldspar to that of muscovite towards the N margin: the fall being more gradual along B. Biotites are less abundant (< 1%) in the samples near the ZSZ, but the total content of phyllosilicates (Ms+Bt+Chl) show a rise of > 14 % towards the ZSZ. Previous workers reported similar increase in micaceous minerals in ductile- and brittle shear zones from other terrains, and suggested higher fluid activity as the key factor. Subduction of the Indian continental crust and subsequent exhumation, along the subduction channel, followed Coutte- and Poiseuille flows, respectively. Hence, rocks near the ZSZ should exhibit opposing shear senses, which we encounter both at micro- and meso-scales. Besides, Google Earth images show geomorphologic features viz. displaced NW trending ridges, linear lake margins etc., which probably indicate regional scale (neotectonic?) NNW trending strike-slip. This can also explain the origin of the nearby major lakes viz. Tso Moriri, Tso Kar and Kiagar Tso.