Do changes in monsoon intensity reposition Himalayan erosion? A combined cyclostratigraphic-detrital thermochronology approach

从活跃的山脉中侵蚀出来的沉积物包含了丰富的记录，记录了这些山脉是如何生长的，以及固体地球内部的各种过程如何与侵蚀过程相结合来塑造地貌。一个重要的问题涉及这个系统的侵蚀部分的重要性，以及记录在多大程度上受固体地球过程、侵蚀变化或两者兼而有之的支配。在尼泊尔，岩石记录了1500万年来喜马拉雅山脉的地形变化。在此期间，强烈的南亚季风气候发展起来，这一现象现在集中在喜马拉雅山脉南侧夏季的强烈侵蚀上。有强有力的证据表明，这种季风的强度在2万到10万年的时间跨度内发生变化。这些变化可以用来准确地确定沉积岩的年代，这些沉积岩是由不断增长的山脉中的沉积物脱落形成的。通过测定沉积物中发现的矿物的年代，可以确定特定的侵蚀位置，因为季风的强度和位置的变化不同。该项目将开发一种结合不同但互补的测年工具的新方法，这种方法将提供一种更详细地读取沉积岩记录的手段。这个项目还将帮助回答这个问题：气候和岩石变形在塑造山体景观方面起到了什么相对作用？该项目将支持一名博士生的培训，还将制作有关山脉地形演化和地质学家用于研究沉积岩序列的工具的公共教育视频。该项目有两个技术目标。第一个目标涉及整合岩石-磁性旋回地层学和碎屑热年代学的方法，以追踪尼泊尔Siwalik群的物源随时间的变化。沉积物中的岩石磁性提供了一种探测持续数万年的轨道周期的方法，从而以1000年的分辨率限制了不同地点沉积物的相对年龄。以这一高分辨率测年为框架，将使用Ar-Ar分析确定Siwalik群沉积物中包含的单个白云母颗粒的年龄，以确定这些颗粒从高温中冷却的时间，从而推断它们来自沉积物源区的哪里。第二个项目目标是开发喜马拉雅活跃地区众所周知的沉积物来源区。由岩石磁性旋回地层学校准的千年尺度的白云母测年，可以用来检查南亚季风强度与喜马拉雅地区侵蚀中心之间的可能相关性。在这些短时间尺度上的相关性应该不受任何固体地球构造信号的影响，从而便于单独审查气候变化，而气候变化可归因于南亚季风的变化。该项目将开发和评估旋回地层学/碎屑热年代学相结合的方法，作为物源分析的一种方法，然后使用结果来评估构造、气候和侵蚀如何共同作用形成地貌。结果还将为解释同造山沉积岩中其他矿物学和地球化学记录的周期性变化提供更一般的指导。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Sediments eroded from active mountains contain a rich record of how those mountains grew and how the combination of processes within the solid Earth combine with the process of erosion to shape landscapes. An important question concerns the significance of the erosional part of this system, and how much the record is dominated by either solid-Earth processes, erosional changes, or both. In Nepal, rocks provide a record of 15 million years of Himalayan topographic change. During this time period the strong South Asian monsoon climate developed, a phenomenon that now focuses intense erosion on the south flanks of the Himalayan mountains in the summer months. There is strong evidence that the strength of this monsoon changes over timespans of 20,000 to 100,000 years. These changes can be used to precisely date sedimentary rocks formed from sediment shed off of growing mountain ranges. By dating minerals found in the sediment, specific locations of erosion can be determined as changes in the strength and location of the monsoon varied. This project will develop a new approach that combines different but complementary dating tools, an approach that will provide a means of reading the sedimentary rock record in far greater detail. This project will also help answer the question: What are the relative roles of climate and rock deformation in shaping mountain landscapes? The project will support training of a Ph.D. student and will also develop public-education videos about the topographic evolution of mountains and the tools geologists use to study sedimentary rock sequences.The project has two technical goals. The first goal involves integrating the methods of rock-magnetic cyclostratigraphy and detrital thermochronology to track changes in the provenance of the Siwalik Group in Nepal through time. Rock magnetics in sediments provide a way of detecting orbital cycles lasting tens of thousands of years, thereby constraining the relative ages of sediments in different locations with 1000-year resolution. Using this high-resolution dating as a framework, single white-mica grains contained in the Siwalik Group sediments will be dated using Ar-Ar analysis to determine when these grains cooled from high temperatures and thus infer where they came from in the sediment source area. The second project goal exploits a well-understood sediment source area in the active Himalayan region. White-mica dating, calibrated at the thousand-year scale by rock magnetic cyclostratigraphy, can be used to examine possible correlations between the intensity of the South Asian monsoon and foci of erosion within the Himalaya. Correlations at these short time scales should be free of any solid-Earth tectonic signal, thus facilitating the examination of climate variations alone, which are attributable to variations in the South Asian monsoon. This project will develop and evaluate combined cyclostratigraphy/detrital thermochronology as an approach to provenance analysis, and then use the results to assess how tectonics, climate, and erosion work together to shape landscapes. Results will also provide more general guidance for interpreting cyclical changes in other mineralogical and geochemical records from synorogenic sedimentary rocks.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.