The tectonometamorphic evolution of the greater Himalayan sequence along the Zanskar Shear Zone, NW India

Abstract

The Greater Himalayan Sequence (GHS) includes high-grade rocks and granites that have been exhumed along the Zanskar Shear Zone (ZSZ) in the Zanskar region of the western Himalaya. GHS rocks in the Suru Valley record metamorphism and deformation during the Himalayan collision and Miocene exhumation. The prograde regional metamorphic event (Mi) is associated with burial of GHS rocks during the Himalayan orogeny, and a retrograde metamorphic event (M2) that is associated with reactivation of the ZSZ as a detachment and exhumation of high-grade rocks. Mineral assemblages consist of Qz + PI + Bt + Ms ± Grt ± St ± Ky ± Sil ± And ± Chi ± Crd ± Zo ± Tur ± Rt ± Zm ± Opq. Field and petrographic observations indicate metamorphic grade in the GHS increases from the chlorite-biotite to the kyanite zone from E to W along the Suru River, and comprising a complete succession of prograde Barrovian-type metamorphism. Rotated garnets, recrystallized quartz grains, irregular grain boundaries, kink bands, microfolds, deformation bands, changes in grain size, and variable crenulation cleavage development suggest different deformation mechanisms and temperatures corresponding to decreasing strain with distance from the ZSZ. Shear sense indicators show that a topto- the-SW thrusting was overprinted by a top-to-the-NE normal-sense shearing along the ZSZ. Strain is greatest within the ZSZ and seen as reduced grain size, interlobate grain boundaries, anhedral quartz grains, and inequigranular size distribution. Metamorphic temperatures are highest in the structurally-deepest rocks near the Suru Dome in which quartz grains show dynamic recrystallization and late fibrolitic sillimanite growth. Modeled peak P-T conditions for schists from the study area indicate ~1.18-1.23 GPa at ~550-590 °C for Mi metapelites from the garnet zone, and -1.10 GPa at ~625 °C for M2 rocks from the kyanite zone in the Suru Dome. While temperature increases from the ZSZ toward the Suru Dome, the Suru Dome records lower pressures related to the late M2 doming event. U-Pb zircon analyses give ages of 21.6 ± 2.7 to 17.2 ±5.3 Ma for the period at the end of the Mi event and the beginning of the M2 retrogression, and that coincide with the reactivation of the ZSZ as an extensional structure. The late-stage doming event in the early part of the M2 occurred at 19.9±0.9 Ma.