IMBRICATION AND EROSIONAL TECTONICS RECORDED BY GARNETS IN THE SIKKIM HIMALAYAS

The Sikkim region of NE India plays a central role in developing ideas regarding the development of the Himalayan range. The region may form a microplate between Nepal and Bhutan. Here, the convergence vector is almost perpendicular to the Himalayan deformation front. Convergence is estimated to be ~450 km and is accommodated by the Main Central Thrust (MCT) and its imbrications. We present high-resolution P-T paths and monazite dates from garnet-bearing rocks collected across the Sikkim MCT shear zone. We aimed to understand its timing and tectonic development. Using closely spaced analytical data points across garnets and the rock bulk composition, with the assumption that the rocks collected experienced closed system behavior and equilibrium conditions, we generated high-resolution P-T paths from footwall (Lesser Himalaya) and hanging wall (Greater Himalaya) rocks. The high-resolution P-T paths approximate how a garnet with a specific type of compositional zoning would behave as reactions proceeded following equilibrium in a closed system of the known bulk composition. Rocks are open systems, but our approach treats them as ideal closed systems that follow the constraints outlined by the thermodynamic datasets and programs used. Lesser Himalaya P-T path conditions and those obtained using conventional thermobarometry are best in agreement. Tests for evaluating results included whether garnet compositional isopleths intersect and whether the results are geologically reasonable and consistent with mineral assemblages. Modeled P-T paths also predict garnet zoning. Late Miocene (10-14 Ma) monazite dates are found in Lesser Himalaya garnets, whereas those from the Greater Himalaya are Miocene (18–20 Ma). Some P-T paths reveal a hairpin shape consistent with imbrication models for the MCT shear zone. We also found that decreases in P recorded by garnet can be modeled as exhumation due to overburden removal instead of tectonically controlled extension. We propose using garnet as a recorder of erosion as a new tool to decipher how mountain ranges evolve. A complicating factor is the presence of numerous lineaments throughout the Sikkim. These lineaments highlight the potential for isograd disruption through later-stage deformation. Speculation exists that these are vertical faults with a right-lateral sense of motion.