Low Peak Metamorphic Temperatures Across the NW Indian Himalaya from the Indus Suture to the Range Front; Implications for Tectonic Models.
Master of Science (MS)
Geology and Geophysics
The mechanisms responsible for a major phase of the Himalayan tectonic evolution, namely the development and emplacement of the Himalayan crystalline core, have long been debated. For the past decade, the debate has largely focused on three competing emplacement models: extrusion via critical taper wedge processes, extrusion via channel flow processes, and emplacement at depth via tectonic wedging which may involve duplex development. The extrusion models predict exposed rocks experienced peak temperature of >600-650°C and should occupy a large swath of territory across the region. In contrast, the duplexing model allows that lower grade THS rocks (exclusively<600-650°C, commonly<400°C) could cover the whole north-south swath. To test those predictions, we investigate the temperature distributions along a transect exists from the MCT north to the Indus Suture Zone. We sampled along a NE-SW transect across the east Ladakh / Chamba region in the NW Himalaya. We use Illite/Muscovite Crystallinity (IC) to quantify metamorphic temperature and quartz microstructural analysis to determine deformation temperature. In structurally high samples, IC values range from 0.21-0.82 with corresponding temperatures of ~100-~300°C. In structural low samples, quartz boundaries analysis shows minor bulging and statically crystallized fabrics with corresponding deformation temperatures of <~350°C. Quartz c-axis analysis results show a sample that yields a single girdle pattern has deformation temperature of 280°C and other samples do not have c-axes orientations that are close to a pattern which suggests they are either not recrystallized or are fully recrystallized. Basic thin section observations preclude the second option. Overall, IC data indicates metamorphic temperature from ~100-~300°C, quartz grain boundary analysis yields a deformation temperature of an upper end of 300-350°C, and quartz c-axis pole figures show grains that are not recrystallized. In conclusion, our results and IC values from Girard et al. (2001) to the north together show a continuous exposure of low temperature rocks from the Indus Suture Zone to the foreland. These results are consistent with duplexing models which propose that lower temperature rocks of the THS (commonly <400°C) cover the whole north-south swath for the emplacement of the GHC in Ladahk - Chamba region.
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Zhang, Kexin, "Low Peak Metamorphic Temperatures Across the NW Indian Himalaya from the Indus Suture to the Range Front; Implications for Tectonic Models." (2015). LSU Master's Theses. 3065.
Webb, Andrew Alexander Gordon