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High-resolution P-T paths from garnet-bearing rocks across the Himalayan Main Central Thrust: Implications for understanding the crustal response to orogenic processes
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  • Elizabeth Catlos,
  • Thomas Etzel,
  • C. Dubey,
  • Oscar Lovera
Elizabeth Catlos
University of Texas at Austin

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Thomas Etzel
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C. Dubey
Centre for Advanced Studies
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Oscar Lovera
University of California Los Angeles
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Barrovian-grade pelites in the Greater Himalayan Crystallines and Lesser Himalayan Formations exposed in the Himalayan core are separated by the Main Central Thrust (MCT). This fault system accommodated a significant amount of India-Asia convergence and is the focus of several models that explore ideas about the development of the range and collisional belts in general. Units separated by the MCT provide critical information regarding the mechanisms of heat transfer within collisional belts. Garnets collected across the MCT record their growth history through changes in chemistry. These chemical changes can be extracted and modeled using a variety of thermodynamic approaches. Here we describe and apply particular thermobarometric techniques to decipher the metamorphic history of several garnet-bearing rocks collected across the MCT in central Nepal, the Sikkim region, and NW India. Comparisons are made between the results of previously-reported conventional rim P-T conditions and P-T paths extracted using the Gibb’s method to isopleth thermobarometry and high-resolution P-T path modeling using the same data and assemblages. Regardless of calibrations used, the P-T conditions and paths, along with previously-reported timing constraints, are consistent with an imbrication model that suggest the MCT shear zone developed as rock packages within the LHF were progressively transferred. In this model, samples within the LHF travel along the MCT at a 5 km/Ma speed rate from 25 to 18 Ma. The hanging wall speed rate is 10 km/Ma, and topography progressively accumulates until a maximum height of 3.5 km. Once the topography is achieved at 18 Ma, a period of cessation is applied to the MCT between 18 and 15 Ma, and topography is reduced at a rate of 1.5 km/Ma. The model returns to activity within the MCT shear zone with the activation of the MCT footwall slivers from 8 to 2 Ma. P‐T changes recorded by the footwall garnets result from thermal advection combined with alterations in topography. For most MCT footwall samples, the P-T paths match the model predictions remarkably well. The P-T paths for some samples in central Nepal are also consistent high exhumation rates (>12mm/year) within the MCT shear zone since the Pliocene, a scenario predicted by this imbrication model.