Climatic and Tectonic Insights from Low-Temperature Thermochronometry Across the Himalayan Rain Shadow, Everest Region, Nepal and Tibet

从恒河平原到青藏高原的地理过渡在空间上与我们地球上最深刻的气候过渡之一相对应。几种流行的喜马拉雅晚新生代演化地球动力学模型具有降水、剥蚀岩石折返和变形之间的强烈反馈关系。如果这些模型是正确的，这些模型预测，喜马拉雅地区的极端降水梯度应该伴随着剥蚀速度和变形模式的变化。之前在喜马拉雅山脉南侧进行的验证这些预测的研究结果喜忧参半，部分原因是这些研究是在横穿喜马拉雅山脉的主要河流的山谷中进行的。这些“跨希玛拉雅河”河谷是季风风暴向北渗透的通道，因此山脉峰顶两侧的侵蚀速率可能无法真正反映出雨影的潜在影响。此外，喜马拉雅山脉抬升期间，特兰希玛拉雅河的极端切割可能会集中在出土作用上，并掩盖与气候和区域构造有关的更广泛的模式。我们试图通过将(铀-钍)/氦温度计时仪应用于沿着不顺着特兰希玛拉雅河的横断面收集的岩石，来更多地了解跨越雨影的挖掘模式。几个研究小组在尼泊尔中部珠穆朗玛峰地区和西藏南部进行了近三十年的研究，对该地区的基本构造地质有了非常好的了解，从而能够在适当的构造背景下解释获得的数据。我们的目标是建立一个适用于热运动学模拟的综合数据集，旨在更好地了解降水和喜马拉雅造山楔动力学的耦合影响。我们还利用产生的部分数据来探索珠穆朗玛峰以北荣布谷洛兹和珠穆朗玛峰支队的时间和滑移历史。这是所有藏南拆离系统中研究得最好的两条，这是喜马拉雅山脉的一个主要构造特征。虽然已知藏南支队系统在中新世早至中期活跃在整个喜马拉雅地区，但在包括荣布谷地在内的大多数地区，最年轻活动的实际年龄限制很少。我们正在通过(铀-钍)/氦和裂变径迹工作对这里的两个支队底部的低温热结构提供新的限制。荣布谷支队的暴露是不寻常的：浅的断层倾角和高起伏共同提供了在藏南支队系统紧邻下盘的运输方向上超过35公里的距离上的采样机会。虽然许多研究的重点是气候在空间和时间上的变化对全球的影响，但一些最大的研究前沿是较小的区域规模。在这些尺度上，我们正在解决气候变化如何与其他自然地球系统过程相关的问题。这项研究特别感兴趣的是山脉如何影响当地气候，反过来，当地气候如何影响山脉的侵蚀历史。喜马拉雅山脉的高山延缓了南亚季风系统向北的轨迹，形成了两种截然不同的景观：喜马拉雅南侧茂密的森林斜坡和青藏高原南部的高沙漠。喜马拉雅山脉南侧的森林覆盖着地球上有记录以来最高的年降雨量。我们的工作旨在了解这种气候模式可能延续了多长时间，以及它如何导致喜马拉雅峰顶北部和南部侵蚀速率的变化。从这里学到的经验教训将使人们认识到区域气候变化在美国和国外的侵蚀过程中所起的作用。该项目正在支持一位才华横溢的女性博士候选人，从而有助于继续努力缩小男性和女性博士级科学家之间的性别差距，这一问题在自然科学中尤为严重。此外，正在为公共博物馆展览创造特殊的多媒体展示，以探索山区景观中气候变化的科学。

The physiographic transition from the Gangetic Plains to the Tibetan Plateau corresponds in space to one of our planet's most profound climate transitions. Several popular geodynamic models for the late Cenozoic evolution of the Himalaya feature a strong feedback relationship between precipitation, erosional rock exhumation, and deformation. If valid, these models predict that the extreme precipitation gradient across the Himalaya should be accompanied by variations in denudation rate and deformational patterns. Previous studies along the southern Himalayan flank conducted to verify these predictions have had mixed results, partly because they were done along the valleys of major rivers that cut across the Himalaya. These 'Transhimalayan' river valleys serve as pathways for northern penetration of monsoon storms, such that erosion rates on either side of the range crest may not truly reflect potential effects of the rain shadow. In addition, extreme incision of the Transhimalayan rivers during uplift of the Himalaya may focus exhumation and obscure broader patterns related to climate and regional tectonics. We endeavor to learn more about exhumation patterns across the rain shadow by applying the (Uranium-Thorium)/Helium thermochronometer to rocks collected along a transect that does not follow a Transhimalayan river. Nearly three decades of research in in the Everest region of central Nepal and southern Tibet by several researcher groups has resulted in an unusually good understanding of the basic structural geology of the region, permitting interpretation of the data obtained in appropriate structural context. Our goal is a comprehensive dataset suitable for thermal-kinematic modeling aimed at providing an improved understanding of the coupled influence of precipitation and dynamics of Himalayan orogenic wedge. We are also taking advantage of a portion of the data produced to explore the timing and slip history of the Lhotse and Qomolangma detachments in the Rongbuk Valley north of Mount Everest. These are two of the best studied of all strands of the South Tibetan detachment system, a major tectonic feature in the Himalaya. While the South Tibetan detachment system is known to have been active throughout the Himalaya in Early to Middle Miocene time, the actual age of youngest movement is poorly constrained in most areas, including the Rongbuk Valley. We are providing new constraints on the low-temperature thermal structure of the footwalls of the two detachments here through (Uranium-Thorium)/Helium and fission track work on accessory minerals. Exposures of the detachments in the Rongbuk Valley are unusual: shallow fault dip angles and high relief conspire to provide sampling opportunities over as distance of greater than 35 kilometers in the direction of transport in the immediate footwall of the South Tibetan detachment system. While much research focuses on the global impacts of climate variations in space and time, some of the greatest frontiers for research are at smaller, regional scales. On those scales, we are addressing questions about how climate variations relate to other natural earth system processes. Of particular interest for this study is how mountain ranges affect local climate and, conversely, how local climate can influence the erosional history of a mountain range. The high mountain ranges of the Himalaya retard the northward track of the South Asian monsoon system, creating two very different landscapes: the heavily forested slopes of the southern Himalayan flank, which receive some of the highest recorded annual rainfall totals on earth, and the high deserts of the southern Tibetan Plateau. Our work is aimed at understanding how far back in time this climatic pattern may have extended and how it might cause variations in erosion rate north and south of the Himalayan crest. The lessons learned here will inform perceptions of the role of regional climate variation in the erosional process both in the United States and abroad. The project is supporting a talented female Ph.D. candidate, thereby contributing to on-going efforts to reduce the gender disparity between male and female doctoral level scientists, a problem that is particularly acute in the physical sciences. In addition, special multimedia displays are being created for public museum exhibitions exploring the science of climate variations in mountainous landscapes.