Probing the Rheology of Tibetan Lithosphere: Surface Deformation in Response to Climatically-Induced Changes in Lake Loads

该奖项是根据2009年《美国复苏和再投资法》(公法111-5)资助的。直接测量活跃变形岩石圈的变形性质仍然难以实现，导致在理解流变学的演变如何控制造山系统的演化方面存在根本差距。尽管印亚碰撞造山带提供了丰富的地球物理和地质资料，但西藏中部下地壳的活动性、物理状态以及局部渠道流在高原和喜马拉雅演化中的作用仍然是一个首要问题。该项目的重点是表征西藏岩石圈对时变地表载荷的变形响应，以对地壳流变性施加定量约束。该项目利用青藏高原内部排水的内陆地区几个大型湖泊周围残留的海岸线飞行。这些海岸线表明了以前更大范围的湖泊水位的位置，而湖泊因气候驱动的水文变化而撤退代表着从岩石圈中移除的负荷。由于海岸线代表的是古水平基准面，这些标志物在弯曲均衡回弹过程中的偏转可能会对岩石圈的弹性强度和下伏基质的粘度施加限制。这项研究将把详细的野外观测和地质年代学与严格的变形模拟(弹性弯曲和三维粘弹性变形)结合起来，将湖泊卸荷历史与下地壳和地幔的物理性质联系起来。在此过程中，关于西藏-喜马拉雅造山带的两个基本问题将被解决：(1)西藏地壳在地质时间尺度上是否具有侧向流动的能力，以及(2)西藏岩石圈的流变性在构成高原的各个地体之间是否存在差异？这些问题的答案远远超出了西藏-喜马拉雅造山带的具体情况，并将为活动和古山带造山过程中有利于或阻碍下地壳流动的条件提供额外的约束。地壳深部的变形是将板块构造的驱动力--地幔流动--连接到脆性地壳上部的构造断层上的一个基本过程。由于直接测量深部地壳的物理性质在技术上是不可行的，因此深部地壳在造山过程中的行为受到了激烈的辩论。特别是，深部地壳是否能够广泛传播，侧向流动对板块边界断层系统的应变积累(美国西部卡斯卡迪亚边缘的瞬时变形被认为与地壳深层变形有关)以及亚洲高地形的发展速度和模式都有影响。后一个问题是人们非常感兴趣的问题之一，因为青藏高原的高地形从根本上影响着大气环流。了解这种高地势发展的过程和速度，将使人们能够更严格地测试青藏高原的增长在地质时间尺度上对气候变化发挥核心作用的可能性。这项研究将对西藏深部地壳内变形的性质产生新的见解，而这些信息在今天的S造山模型中是非常缺乏的。通过这样做，该项目还将对气候驱动的湖泊水位变化提供新的限制，这些信息对于更全面地了解世界上这个资源有限地区的水平衡至关重要。最后，这些结果将指导我们更全面地理解地壳深部的流动如何影响地球的整体变形--S地表，包括构造断层上发生地震滑动的可能性。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Direct measurements of the deformational properties of actively deforming lithosphere remain elusive and lead to fundamental gaps in the understanding of how the evolution of rheology controls the evolution of orogenic systems. Despite abundant geophysical and geological data for the Indo-Asian collisional orogen, the mobility of the lower crust beneath central Tibet, its physical state, and the role that localized channel flow has played in the evolution of the Plateau and the Himalaya remain first-order questions. This project focuses on characterizing the deformational response of Tibetan lithosphere to time-varying surface loads to place quantitative constraints on the rheology of the crust. The project utilizes flights of relict shorelines around several large lakes within the internally-drained interior of the Tibetan Plateau. These shorelines indicate former positions of more extensive lake levels, and the withdrawal of the lakes in response to climatically-driven hydrologic changes represents a load removed from the lithosphere. Because the shorelines represent a paleo-horizontal datum, deflection of these markers during flexural isostatic rebound can place constraints on the elastic strength of the lithosphere and the viscosity of the underlying substrate. The study will couple detailed field observations and geochronology with rigorous deformational modeling (elastic flexure and 3-D visco-elastic deformation) to link the history of lake unloading to physical properties of the lower crust and mantle. In doing so, two fundamental questions about the Tibet-Himalayan orogen will be addressed: (1) Is Tibetan crust capable of lateral flow on geologic timescales, and (2) Do the rheologic properties of Tibetan lithosphere vary among the individual terranes comprising the plateau? The answers to these questions extend far beyond the specifics of the Tibet-Himalayan orogen and will provide additional constraints on the conditions that favor or impede lower-crustal flow during orogenesis in both active and ancient mountain belts.Deformation within the deep crust is a fundamental process that links the driving forces for plate tectonics - flow in the earth's mantle - to slip on tectonic faults in the upper, brittle crust. Because direct measurements of the physical properties of the deep crust are technically infeasible, the behavior of the deep crust during mountain building is hotly debated. In particular, whether the deep crust is capable of widespread, lateral flow has implications for both strain accumulation on plate boundary fault systems - transient deformation along the Cascadia margin in the Western US is thought to be related to deformation in the deep crust - as well as the rates and patterns of development of high topography in Asia. The latter problem is one of great interest in that high topography of the Tibetan Plateau fundamentally influences atmospheric circulation. Understanding the processes and rates by which this high topography developed will enable more rigorous tests of the possibility that growth of the Tibetan Plateau played a central role in climate change over geologic timescales. This study will yield new insight into the nature of deformation within the deep crust beneath Tibet, information that is sorely lacking in today?s models of mountain building. In doing so, the project will also provide new constraints on climatically-driven changes in lake levels, information that is central to a more comprehensive understanding of the water budget in this resource-limited part of the world. Finally, the results will guide a more general understanding of how flow in the deep crust influences the overall deformation of the earth?s surface, including the potential for earthquake-generating slip on tectonic faults.