Timing, Conditions, and Rates of Thrust Transport in the Nepalese Himalaya

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A new petrologic and chronologic approach developed by the principal investigator for high-grade rocks in the Langtang region (east-central Nepal) demonstrates that thrust fault movement rates since about 20 million years ago are generally consistent with convergence of the Indian subcontinent at about 2 cm/yr (equivalent to modern rates), but that thrust rates were variable, with emplacement of one thrust sheet at a rate 2-3 times the average rate. These data also highlight the importance of rapid erosion (5-10 mm/yr) in exhuming metamorphic rocks over the last 3-4 million years. The purpose of the proposed research is to test whether these patterns are also found in two other areas - the Annapurna region to the west, and the Arun region in far eastern Nepal - and to test whether the Greater Himalayan Sequence experienced high-pressure or ultra-high-pressure metamorphism. The following methods are being used: (1) Chemical mapping and quantitative microanalysis of monazite grains, and dating of chemically distinct domains via ion microprobe. These ages are documenting the timing of cooling, which for many structural levels is a direct result of thrust movement. (2) Determination of pressure-temperature conditions and conversion to paleoposition via published thermal models for the Himalaya. The amount of displacement, and timing of cooling and thrust movement for each sheet is being converted to estimates of thrust rate. (3) Determination of ages and P-T conditions of eclogites in the Arun region to determine whether these are related to early Himalayan deformation. This work has major broad implications for tectonics research and its interrelationship with erosion and Earth's chemical evolution. This work will also support the education of one graduate student in geochemistry/tectonics.