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

由研究高品位岩石的首席研究员发展的一种新的岩石学和年代学方法表明，大约2000万年前以来的逆冲断层运动速率与印度次大陆以大约2厘米/年(相当于现代速率)的会聚速率基本一致，但逆冲速率是不同的，一个逆冲板块的侵位速率是平均速率的2-3倍。这些数据还突显了过去300-400万年来快速侵蚀(5-10毫米/年)在变质岩折返过程中的重要性。这项拟议研究的目的是测试这些模式是否也出现在另外两个地区--西边的安纳普尔纳地区和尼泊尔远东的阿伦地区--并测试大喜马拉雅序列是否经历了高压或超高压变质作用。采用了以下方法：(1)独居石颗粒的化学测绘和定量显微分析，以及通过离子探针测定化学结构域的年龄。这些年代记录了冷却的时间，对许多结构层面来说，这是逆冲运动的直接结果。(2)通过已发表的喜马拉雅热模型确定压力-温度条件和古位置转换。每个薄板的位移、冷却和推力移动的时间被转换为推进率的估计。(3)测定阿润地区榴辉岩的年龄和P-T条件，以确定它们是否与喜马拉雅早期变形有关。这项工作对构造学研究及其与侵蚀和地球化学演化的相互关系具有重大而广泛的意义。这项工作还将支持一名地球化学/构造学研究生的教育。

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.