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

研究山带的演化可以让我们了解大陆板块是如何变形的，它们的隆起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中，进而影响全球气候和海洋化学。喜马拉雅是研究这些相互作用的理想的自然实验室，因为人们认为它的演化引发了气候变化：西藏的隆升导致了季风的加剧，喜马拉雅的隆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。喜马拉雅山的例子支持了快速侵蚀可以通过反馈机制“触发”进一步地壳变形的概念。拟议的研究旨在进一步了解固体地球和地表侵蚀过程之间的相互作用。河流流域演化是理解构造和侵蚀过程的关键。喜马拉雅东部角落的排水模式是非典型的，主要河流异常紧密平行。这种结构被解释为印度-亚洲碰撞的结果。一方面，这种模式被解释为由于地壳物质被侧向挤压离开西藏而导致流域形态扭曲的先前河流。在这种情况下，河流充当了“被动的菌株标记”。也有人认为，该流域形态是由西藏变形和隆升过程中构造诱导的河流捕获所致。这些河流捕获事件的一个被提出的例子是雅鲁藏布江的雅鲁藏布江，这可能导致了河流流经的东部构造区非常迅速和最近的挖掘。“构造性动脉瘤”的概念被用来解释这一过程；河流的快速切割降低了下伏岩石的强度，使较弱的较热的中部地壳岩石迅速上升，导致进一步的隆起。通过新近纪对雅鲁藏布江沉积物组成演化的多技术物源研究，将使我们能够检验有关流域演化和侵蚀-构造耦合的相互竞争的假设。我们将确定：1)雅鲁藏布江是否和/或何时捕获了雅鲁藏布江；2)河流捕获是否先于syntaxis的快速挖掘开始，如果是，则是在什么时间滞后。雅鲁藏布江流域(a)、雅鲁藏布江流域(b)和雅鲁藏布江流域(c)合成区(c)的岩石年龄、同位素地球化学和矿物学差异较大，将有助于确定它们对古雅鲁藏布江沉积的相对变化贡献。因此，雅鲁藏布江捕获和合成发掘的时间可以根据其特征碎屑的首次输入到沉积物记录中来确定。我们将从连续的新近纪沉积记录中对单一矿物颗粒进行综合分析。如果需要，古近系样品也可获得。我们将使用复杂的同位素技术，能够确定每个颗粒的年代并确定其同位素地球化学指纹，以独特地表征每个颗粒的来源区域。我们还将改进我们的分析方法，使它们更加灵活和有效，以便使它们适应手头的问题。我们将对各自的“来源”进行进一步分析，以更好地了解其特征的可变性。有了完整的新近系侵蚀记录中沉积物来源的连续和突变记录，我们将能够测试河流捕获和侵蚀-构造耦合模型。这些数据随后可用于约束包括地壳变形和气候驱动因素在内的亚洲演化的数值模式。此外，我们在同位素分析方法改进方面的投资将有利于国际社会在未来几年使用这些工具来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.