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Pressure-temperature-time paths from across the exhumed Himalayan Metamorphic Core and their implication for orogenic processes Shrestha, Sudip


The recent identification of multiple strike-parallel discontinuities within the exhumed Himalayan metamorphic core, which was long thought to be a continuous, undisrupted unit, has demonstrated the potential complexity in the geologic evolution of the former mid-crust exposed in the Himalaya. While the significance of these discontinuities to the overall development of the mountain belt is still being investigated, their widespread occurrence indicates that our previous understanding of convergence accommodation processes in the orogen is incomplete. This project investigates previously identified discontinuities in two different portions of the Nepalese Himalaya, using advanced petrographic techniques including phase equilibria modelling, quartz-in-garnet barometry, trace element analysis, in-situ monazite petrochronology, and Lu-Hf garnet geochronology to elucidate the role of these structures in the overall development of the Himalaya. The results from the Likhu Khola and Modi Khola regions of east-central and central Nepal, respectively, confirms the presence of cryptic thrust-sense tectonometamorphic discontinuities and provides new information on the pressure-temperature (P-T) development of those regions. The discontinuities are marked by abrupt changes in the P-T-time (t) paths of the rocks across them indicating metamorphism in the footwall during exhumation of the hanging wall. The general down-structural section migration of prograde metamorphism, anatexis, cooling and exhumation in these regions are consistent with models of progressive underplating and in-sequence thrusting during the development of the Himalayan mid-crust. Furthermore, a break in cooling ages between two adjacent rock packages in the Modi Khola region is consistent with late stage, normal-sense fault reactivation, perhaps tied kinematically to the foreland propagation of strain during mid-late Miocene. This work also extends the capabilities of existing thermodynamic datasets used for phase equilibria modelling to include phosphate phases. The addition of accessory phosphate phases like monazite, xenotime, and apatite, which are commonly used as geochronometers, not only yields a better understanding on the relationships between co-existing silicate and phosphate mineral phases, but, with available trace element data, also allows significantly more precise P-T-t paths especially for rocks in which major elements in phases such as garnet may have been diffusionally homogenized.

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