Extrusion kinematics of UHP terrane in a collisional orogen: EBSD and microstructure-based approach from the Tso Morari Crystallines (Ladakh Himalaya)

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Several extrusion mechanisms have been proposed to explain the occurrences of ultra-high pressure (UHP) rocks at collisional margins. Here we assess the suitability of the channel flow model of extrusion for the Tso Morari Crystallines (TMC) by investigating the syn-extrusion deformation kinematics of the UHP eclogite-bearing gneiss that dominates the TMC. Samples collected from the two transects viz. Sumdo-Karzok and Sumdo-Debring, have been examined. The channel flow model predicts high simple shear component at the channel margins during extrusion. Gneisses near the upper margin of the subduction channel – also the northern margin of the TMC on map – support this claim and exhibit closely spaced shear planes, and elevated fluid activity as revealed by X-ray diffraction results. Quartz grain parameters e.g. Feret diameter, aspect ratio, and vector mean strength measured from oriented thin-sections of the gneiss do not exhibit distinct pattern of variation of deformation intensity across the TMC. EBSD-based low-angle intragranular 'neighbor to neighbor' misorientation analysis and micro-textural evidences of incipient partial melting in the gneiss reflect temperatures >600 °C. We propose that return flow and buoyancy of the subducted continental crust triggered the extrusion, which was then facilitated by the Indo-Eurasia collision. The mechanism could thus be a combination of both channel flow and ductile wedge extrusion. Continued convergence possibly promoted non-planar triclinic transpression – also evident from the crystallographic vorticity axis (CVA) analysis – such that the extrusion direction varied spatially, producing the diverse shear senses observed at both meso- and micro-scales.