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

由尼泊尔中东部朗唐地区高级岩石的首席研究员开发的一种新的岩石学和年代学方法表明，自大约2000万年前以来，逆冲断层的运动速度与印度次大陆的每年约2厘米（相当于现代速度）的融合速度大致一致，但逆冲速度是可变的，一个逆冲片的侵位速度是平均速度的2-3倍。这些数据还强调了在过去的3-4百万年间，在挖掘出的变质岩中快速侵蚀（5-10毫米/年）的重要性。这项研究的目的是测试这些模式是否也存在于其他两个地区——西部的安纳普尔纳地区和尼泊尔远东的阿伦地区——并测试大喜马拉雅层序是否经历了高压或超高压变质作用。主要采用以下方法：(1)对独居石颗粒进行化学作图和定量微量分析，并通过离子探针对化学结构域进行定年。这些年代记录了冷却的时间，对于许多构造层来说，这是推力运动的直接结果。(2)利用已发表的喜马拉雅地区热模式确定压力-温度条件并转换为古位置。每个薄片的位移量、冷却时间和推力运动被转换为推力速率的估计。(3)确定Arun地区榴辉岩的年龄和P-T条件，确定其是否与喜马拉雅早期变形有关。这项工作对构造研究及其与侵蚀和地球化学演化的相互关系具有重要的广泛意义。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.