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

研究山脉带的演变使我们能够了解大陆板块如何变形，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，进而影响全球气候和海洋化学。喜马拉雅山是研究这些相互作用的理想天然实验室，因为它被认为是其演变引发了气候变化：西藏的隆起导致季风的加剧，喜马拉雅山的隆起和侵蚀可能导致新生代全球变冷和海洋87 Sr/86 Sr值的变化。喜马拉雅山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。拟议的研究旨在进一步了解这些知之甚少的固体地球和表面侵蚀过程之间的相互作用。河流水系演化是理解构造和侵蚀过程的关键。喜马拉雅山东角的排水模式是非典型的，主要河流异常密集和平行。这种构造被解释为印度-亚洲碰撞的结果。一方面，这种模式被解释为先前的河流，其排水形态随着地壳物质从西藏侧向挤出而扭曲。在这种情况下，河流充当“被动应变标记”。也有人认为，这种水系形态是由西藏变形隆升过程中的构造运动引起的河流截留作用造成的。雅鲁藏布江被雅鲁藏布江夺去的事件是这些河流夺去事件的一个例子，雅鲁藏布江的夺去可能导致了雅鲁藏布江流经的东部结合带地区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底层岩石的强度，使较弱的较热的中地壳岩石快速上升，导致进一步的隆起。通过新近纪时间的雅鲁藏布江沉积物的演变组成的多技术物源研究将使我们能够测试排水演变的竞争假说，从而侵蚀构造耦合。我们将确定：1）雅鲁藏布江是否和/或何时被雅鲁藏布江占据; 2）河流被占据是否先于构造结中快速折返的开始，如果是的话，时间间隔是多少。a）雅鲁藏布江流域，B）雅鲁藏布江流域前和c）同轴区域的岩石在年龄、同位素地球化学和矿物学方面的巨大差异，将使它们对古雅鲁藏布江沉积物的相对贡献发生变化。因此，雅鲁藏布江的捕获和同向折返的时间可以从它们的特征碎屑首次输入到沉积物记录中来确定。我们将从一个连续的新近纪沉积记录中对单个矿物颗粒进行综合分析。如果需要，可获得古近纪样品。我们将使用先进的同位素技术，能够确定每一粒谷物的年代，并确定其同位素地球化学指纹，以独特地确定每一粒谷物的来源地区。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，以更好地了解其特征的可变性。通过对整个新近纪侵蚀记录中沉积物来源的连续和突变的记录，我们将能够检验河流捕获和侵蚀-构造耦合模型。这些数据随后可用于约束亚洲演变的数值模型，包括地壳变形和气候驱动因素。此外，我们在同位素分析方法改进方面的投资将使全球社会受益，在未来几年中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.