Erosion and Exhumation History of the Nepalese Frontal Himalaya Since Earliest Miocene Time: Constraints on Kinematic History

尼泊尔境内的喜马拉雅山脉是在过去5500万年间由印度和亚洲大陆碰撞产生的力量形成的。自碰撞开始以来，覆盖在印度上地壳上的层状岩石已经被挤压并缩短了至少650公里，因为亚洲大陆已经覆盖了北方印度。这个过程造就了地球上最高的山脉。这些山脉在许多方面都很重要：它们控制着南亚的气候系统，包括至关重要的亚洲季风，为一个本来会更加干燥的地区提供雨水;它们以冰川和雪的形式储存水，从而减轻干旱的影响，并为南亚大部分地区提供水力发电;它们产生的沉积物构成了该地区的农业粮仓，养活了世界上大约四分之一的人口？的人口;它们体现了一种戏剧性的高山景观，在这种景观上，人类受到了冒险、探索和宗教信仰的启发。随着山脉的增长，不断的侵蚀产生了大量的沉积物，这些沉积物沿着喜马拉雅山南翼的印度-恒河前陆盆地以及阿拉伯海和孟加拉湾海底印度两侧的深海扇堆积。这些沉积物提供了喜马拉雅山脉生长历史的记录。我们的项目旨在重建尼泊尔喜马拉雅山脉的变形历史。分析是法医，因为我们试图逆变形的岩石，已折叠和逆冲断层在喜马拉雅山，根据热年代学，构造地质学，地质年代学和侵蚀沉积记录所施加的限制。热年代学测量的是，随着山脉的生长和侵蚀作用对岩石的挖掘，一种矿物在到达地形表面的过程中，相对于特定的放射性同位素系统，通过其"阻塞温度"的时间。热年代学年龄可以反演来评估侵蚀速率，在某些情况下，主要断层滑动的时间。我们还将使用铀铅地质年代学来确定山脉侵蚀沉积物和基岩源区矿物颗粒的年龄。这些数据将可用于：（a）确定喜马拉雅山南半部主要逆冲断层活动的时间;（B）计算山脉的剥露速度;（c）精确确定沉积物的具体来源及其沉积时间。我们的野外工作将包括样品收集，以及沉积侵蚀产物的详细记录，这些产物在喜马拉雅山脉的正面被抬升和暴露。这项工作有望阐明喜马拉雅山的构造和侵蚀历史在一个前所未有的详细程度，并为我们了解不断变化的海水化学，南亚季风的起源和时间，以及地形系统中构造和气候之间的动态相互作用具有深远的影响。在许多国际组织正在进行的喜马拉雅构造研究的背景下，这项工作填补了对喜马拉雅山最后2000万年变形的关键理解的空白，在此期间，据推测喜马拉雅山总缩短量的一半以上发生了，在此期间，该系统的许多标志性特征，包括主中央冲断层，藏南拆离体、大喜马拉雅浅色花岗岩和小喜马拉雅复式花岗岩。人口稠密的尼泊尔和北方印度仍然地震活跃，在尼泊尔西部的一个主要地震空白区有可能发生8级地震。由于地震概率的估计部分是基于对喜马拉雅山脉前缘长期缩短历史的理解，因此长期缩短率的微小变化将对同震断层滑动的预测产生重大影响。

The Himalayan Mountains in Nepal were formed over the last 55 million years by forces associated with the collision of the Indian and Asian continents. Since the onset of the collision, layered rocks draping the upper crust of India have been crumpled and shortened by at least 650 kilometers as the Asian landmass has overridden northern India. This process has produced the highest mountains on Earth. These mountains are important in many respects: they control the climate system of southern Asia, including the all-important Asian monsoon which provides rain in a region that would otherwise be much drier; they store water in the form of glaciers and snow, thus mitigating the effects of drought and providing hydroelectric power to much of southern Asia; they generate the sediments that form the agricultural breadbasket of the region, feeding approximately one quarter of the world?s population; and they embody a dramatic high mountain landscape upon which humans have been inspired to feats of adventure, exploration, and religious devotion. As the mountains have grown, continuous erosion has generated vast quantities of sediment that accumulates along the southern flank of the Himalaya in the Indo-Gangetic foreland basin and in deep sea fans that flank India on the floors of the Arabian Sea and the Bay of Bengal. This sediment provides a record of the history of growth of the Himalaya. Our project aims to reconstruct the history of deformation of the Himalaya in Nepal. The analysis is forensic, as we attempt to retrodeform the rocks that have been folded and thrust faulted in the Himalaya, according to constraints imposed by thermochronology, structural geology, geochronology, and the erosional sedimentary record. Thermochronology measures the time at which a mineral passes through its "blocking temperature" with respect to a particular radiogenic isotopic system en route to the topographic surface as the mountains are growing and erosion is exhuming the rocks. Thermochronological ages can be inverted to assess erosion rates and in some cases the timing of slip on major faults. We will also use Uranium-Lead geochronology to determine the ages of mineral grains in both the eroded sediments and the bedrock source areas of the mountain range. Together these data will allow use to (a) determine the timing of major thrust faulting events within the southern half of the Himalaya; (b) calculate the rate of exhumation of the mountains; and (c) determine precisely the specific sources of the sediments and when they were deposited. Our fieldwork will involve sample collection, and detailed documentation of sections of the sedimentary erosional products where they are uplifted and exposed in the frontal Himalaya. This work promises to elucidate the tectonic and erosional history of the Himalaya at an unprecedented level of detail, and has far-reaching implications for our understanding of changing seawater chemistry, the origin and timing of the South Asian monsoon, and the dynamic interplay between tectonics and climate in orographic systems. Within the context of ongoing research on Himalayan tectonics by numerous international groups, this work fills a gap in understanding of the critical last ~20 million years of deformation of the Himalaya, a timespan during which it is postulated that more than one half of the total Himalayan shortening took place, and during which many of the iconic features of the system developed, including the Main Central thrust, the South Tibetan detachment, the Greater Himalayan leucogranites, and the Lesser Himalayan duplex. Densely populated Nepal and northern India remain seismically active, with potential for a M8 earthquake in a major seismic gap in the western part of Nepal. Because estimates of earthquake probability are in part based on understanding of the long-term history of shortening in the frontal Himalaya, a small change in the long-term shortening rate would have significant implications for predictions of co-seismic fault slip.