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

尼泊尔前缘喜马拉雅山自最早中新世以来的侵蚀和剥露历史：对运动学历史的限制

• 批准号: 1140068
• 负责人: Peter DeCelles
• 金额: $ 51.31万
• 依托单位: University of Arizona
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-04-01 至 2016-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1140068&HistoricalAwards=false
• 关键词: Erosion; Exhumation; History; Nepalese; Frontal

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.

尼泊尔的喜马拉雅山脉是在过去的5500万年里由印度和亚洲大陆碰撞的力量形成的。自碰撞开始以来，覆盖在印度上地壳上的层状岩石被揉皱并缩短了至少650公里，因为亚洲大陆覆盖了印度北部。这个过程产生了地球上最高的山脉。这些山脉在很多方面都很重要：它们控制着南亚的气候系统，包括至关重要的亚洲季风，它为本应更加干燥的地区提供降雨；它们以冰川和雪的形式储存水分，从而减轻了干旱的影响，并为南亚大部分地区提供水力发电；它们产生的沉积物形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。人口;它们体现了引人注目的高山景观，激发了人类冒险、探索和宗教信仰的壮举。随着山脉的增长，持续的侵蚀产生了大量的沉积物，这些沉积物沿着喜马拉雅山脉南侧的印度-恒河前陆盆地和印度侧面的阿拉伯海和孟加拉湾海底的深海扇聚集在一起。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。根据热年代学、构造地质学、地质年代学和侵蚀沉积记录的限制，我们试图对喜马拉雅地区褶皱和逆冲断层的岩石进行逆变形，因此分析是法医鉴定的。热年代学测量的是一种矿物在到达地形表面的过程中，随着山脉的生长和侵蚀作用使岩石被挖掘出来，相对于特定的放射性同位素系统，通过其“阻塞温度”的时间。热年代学年龄可以倒转来评估侵蚀速率，在某些情况下还可以评估主要断层的滑动时间。我们还将使用铀铅地质年代学来确定侵蚀沉积物和山脉基岩源区矿物颗粒的年龄。这些数据将用于(a)确定喜马拉雅南半部主要逆冲断层事件的时间；(b)计算山区的挖掘率；(c)精确地确定沉积物的具体来源和沉积时间。我们的野外工作将包括样品收集，以及沉积侵蚀产物剖面的详细记录，这些产物是在喜马拉雅山前缘隆起和暴露的。这项工作有望以前所未有的详细程度阐明喜马拉雅的构造和侵蚀历史，并对我们理解海水化学变化，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意义。人口稠密的尼泊尔和印度北部仍然地震活跃，在尼泊尔西部的一个主要地震间隙有可能发生8级地震。由于对地震概率的估计部分是基于对喜马拉雅锋面的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。