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

从活山上侵蚀下来的沉积物包含了丰富的记录，记录了这些山脉是如何形成的，以及固体地球内部的各种过程是如何与侵蚀过程结合起来形成地貌的。一个重要的问题涉及到这个系统的侵蚀部分的意义，以及这些记录在多大程度上是由固体地球过程、侵蚀变化或两者共同主导的。在尼泊尔，岩石提供了喜马拉雅地区1500万年地形变化的记录。在这段时间里，南亚季风气候形成，这种现象现在集中在夏季喜马拉雅山脉南侧的强烈侵蚀。有强有力的证据表明，季风的强度会在2万到10万年的时间跨度内发生变化。这些变化可以用来精确地确定沉积岩的年代，这些沉积岩是由生长中的山脉脱落的沉积物形成的。通过测定沉积物中发现的矿物质的年代，可以根据季风强度和位置的变化确定侵蚀的具体位置。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方法。这个项目也将有助于回答这样一个问题：气候和岩石变形在塑造山地景观中的相对作用是什么？该项目将支持一名博士生的培训，并将制作有关山脉地形演变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。第一个目标是整合岩石-磁旋回地层学和碎屑热年代学方法，追踪尼泊尔Siwalik群物源的时间变化。沉积物中的岩石磁学提供了一种检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。以这种高分辨率测年为框架，将使用Ar-Ar分析来确定Siwalik群沉积物中含有的单个白色云母颗粒的年代，以确定这些颗粒在高温下冷却的时间，从而推断它们来自沉积物源区的位置。第二个项目目标是在活跃的喜马拉雅地区开发一个很好理解的沉积物源区。白云母测年，通过岩石磁旋回地层学在千年尺度上进行校准，可以用来检验南亚季风强度与喜马拉雅内部侵蚀焦点之间可能存在的相关性。在这些短时间尺度上的相关性应该没有任何固体地球构造信号，从而有利于单独研究气候变化，这可归因于南亚季风的变化。该项目将开发和评估综合旋回地层学/碎屑热年代学作为物源分析的方法，然后使用结果来评估构造、气候和侵蚀如何共同塑造景观。研究结果也将为解释同造面沉积岩的其他矿物学和地球化学记录的周期变化提供更普遍的指导。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.