River capture in the Easternmost Himalaya:Testing erosion-tectonic feedback models using palaeo-Brahmaputra deposits of the Bengal Basin, Bangladesh

研究山脉带的演化使我们能够了解大陆板块是如何变形的，它们的抬升如何影响天气和气候，以及气候引起的侵蚀如何反馈到构造过程，进而影响全球气候和海洋化学。喜马拉雅山脉被认为是研究这些相互作用的理想天然实验室，因为它的演化引发了气候变化：西藏的抬升导致季风的加强，喜马拉雅的抬升和侵蚀可能导致了新生代全球变冷和海洋87Sr/86Sr值的变化。喜马拉雅山脉的例子支持这样的概念，即快速侵蚀可以通过反馈机制‘触发’进一步的地壳变形。这项拟议的研究旨在加深我们对固体地球和地表侵蚀过程之间这些鲜为人知的相互作用的了解。河流水系演化为理解构造和侵蚀过程提供了一把钥匙。喜马拉雅山东角的排水格局是非典型的，主要河流异常紧密和平行。这种格局被解读为印度-亚洲碰撞的结果。一方面，这种模式被解释为先行河流，由于地壳物质向远离西藏的方向挤压，导致流域地貌扭曲。在这种情况下，河流扮演着“被动应变标记”的角色。其他人认为，这种排水形态是由于西藏变形和隆升过程中构造引起的河流俘获造成的。这些河流俘获事件的一个拟议例子是雅鲁藏布江的雅鲁藏布江事件，这可能导致了河流流经的东部语法区域非常迅速和最近的折返。“构造动脉瘤”的概念被用来解释这一过程；河流的快速下切降低了下伏岩石的强度，使较弱、较热的中地壳岩石迅速上升，导致进一步抬升。对新近纪以来雅鲁藏布江沉积物组成演化的多技术来源研究将使我们能够检验关于排水演化和侵蚀-构造耦合的相互竞争的假说。我们将确定：1)雅鲁藏布江是否和/或何时占领雅鲁藏布江；2)河流捕获是否先于语法中快速挖掘身体的开始，如果是，滞后时间是什么。A)雅鲁藏布江流域、b)前捕获雅鲁藏布江流域和c)联结区的岩石在年龄、同位素地球化学和矿物学方面的巨大反差，将使我们能够确定它们对古雅鲁藏布江沉积的变化的相对贡献。因此，雅鲁江捕获和同构折返的时间可以从其特征碎屑首次输入到沉积物记录中确定出来。我们将从连续的新近系沉积物记录中进行单矿物颗粒的综合分析。如果需要，可以获取古近纪样品。我们将使用先进的同位素技术，既能够测定每一颗谷物的年龄，也能够确定其同位素地球化学指纹，以唯一地描述每一颗谷物的来源区域。我们还将改进我们的分析方法，使其更加灵活和有效，以便使其适应手头的问题。我们将对各自的“来源”进行额外的分析，以更好地了解它们特征的可变性。通过完整的新近纪侵蚀记录中沉积物来源的连续和突然变化的记录，我们将能够检验河流捕获和侵蚀-构造耦合的模型。这些数据随后可用于约束包括地壳形变和气候驱动因素在内的亚洲演化的数值模型。此外，我们在改进同位素分析方法方面的投资将使全球社会受益，在未来几年使用这种工具解决地球系统科学问题。

Studying the evolution of mountain belts allows us to understand how continental plates deform, how their uplift impacts on weather and climate, and how climatically-induced erosion may feed back to tectonic processes that in turn affect global climate and ocean chemistry. The Himalaya is an ideal natural laboratory to study these interactions since it is thought that its evolution triggered climate change: uplift of Tibet caused intensification of the monsoon, and Himalayan uplift and erosion may have resulted in Cenozoic global cooling and changes in ocean 87Sr/86Sr values. Examples in the Himalaya support the concept that rapid erosion can 'trigger' further crustal deformation by feedback mechanisms. The proposed research aims to further our knowledge of these poorly understood interactions between the solid Earth and surface erosional processes. River drainage evolution provides a key to understanding both tectonic and erosion processes. The drainage pattern of the eastern Himalayan corner is atypical, with the major rivers being unusually closely spaced and parallel. This configuration has been interpreted as resulting from India-Asia collision. On the one hand the pattern has been interpreted as antecedent rivers with drainage morphology distorted as crustal material is extruded sideways away from Tibet. In this scenario the rivers act as 'passive strain markers'. Others consider that the drainage configuration was caused by tectonically-induced river capture during deformation and uplift of Tibet. One proposed example of these river capture events is that of the Yarlung Tsangpo by the Brahmaputra, which may have caused the very rapid and recent exhumation of the eastern syntaxis region, through which the river flows. The concept of a 'tectonic aneurysm' has been used to explain this process; rapid downcutting by the river reduces underlying rock strength, allowing the rapid rise of weaker hotter middle crustal rocks, leading to further uplift. A multi-technique provenance study of the evolving composition of Brahmaputra River sediment through Neogene time will allow us to test competing hypotheses of drainage evolution and thus erosion-tectonic coupling. We will determine: 1) if and/or when the Brahmaputra captured the Yarlung Tsangpo and 2) whether river capture preceded the beginning of rapid exhumation in the syntaxis and if so by what time lag. The large contrasts in age, isotope geochemistry, and mineralogy of the rocks of a) the Yarlung-Tsangpo drainage basin, b) the pre-capture Brahmaputra drainage basin and c) the syntaxial region, will allow their changing relative contributions to the dated paleo-Brahmaputra sediments to be identified. Thus the time of Yarlung river capture and syntaxial exhumation can be determined from the first input of their characteristic detritus to the sediment record. We will conduct comprehensive analysis of single mineral grains from a continuous Neogene sediment record. Paleogene samples are accessible if required. We will use sophisticated isotope techniques, capable of both dating each grain and determining its isotope geochemistry fingerprint, to uniquely characterise each grains's source region. We will also refine our methods of analysis to make them more flexible and efficient in order to adapt them to the problem at hand. We will undertake additional analyses of the respective 'sources' to better understand the variability of their characteristics. With a record of both continuous and abrupt changes to sediment provenance from the full Neogene erosional record, we will be able to test the models of river capture and erosion-tectonic coupling. These data can be subsequently used to constrain numerical models of Asian evolution that include crustal deformation and climatic drivers. Additionally our investment in method improvement in isotope analysis will benefit the worldwide community using such tools to address Earth System Science problems in the years ahead.