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

探讨青藏高原岩石圈的流变性：气候引起的湖泊荷载变化引起的地表变形

• 批准号: 0911587
• 负责人: Eric Kirby
• 金额: $ 45.99万
• 依托单位: Pennsylvania State Univ University Park
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-01-01 至 2013-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0911587&HistoricalAwards=false
• 关键词: Probing; Rheology; Tibetan; Lithosphere; Surface

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.

该奖项由 2009 年美国复苏和再投资法案（公法 111-5）资助。对主动变形岩石圈变形特性的直接测量仍然难以捉摸，导致在理解流变学演化如何控制演化方面存在根本性差距造山系统。尽管印度-亚洲碰撞造山带有丰富的地球物理和地质数据，但青藏高原中部下方地壳的活动性、其物理状态以及局域河道流在高原和喜马拉雅山演化中所发挥的作用仍然是一阶的问题。该项目的重点是表征青藏高原岩石圈对时变表面载荷的变形响应，以对地壳流变学进行定量限制。该项目利用了青藏高原内陆地区几个大型湖泊周围的废弃海岸线。这些海岸线表明了更广泛的湖泊水位以前的位置，而因气候驱动的水文变化而导致的湖泊退缩代表了岩石圈上的负载被移除。由于海岸线代表古水平基准，因此弯曲均衡回弹期间这些标记的偏转可能会对岩石圈的弹性强度和下方基质的粘度产生限制。该研究将详细的现场观测和地质年代学与严格的变形模型（弹性弯曲和三维粘弹性变形）结合起来，将湖泊卸荷的历史与下地壳和地幔的物理性质联系起来。在此过程中，有关青藏高原-喜马拉雅造山带的两个基本问题将得到解决：（1）青藏高原地壳是否能够在地质时间尺度上进行横向流动，以及（2）青藏高原岩石圈的流变特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，并将为活动山带和古代山带造山作用期间有利于或阻碍下地壳流动的条件提供额外的约束。深部地壳内的变形是一个基本因素。这一过程将板块构造的驱动力（地幔中的流动）与上层脆性地壳的构造断层上的滑动联系起来。由于直接测量深部地壳的物理特性在技术上是不可行的，因此造山过程中深部地壳的行为引起了激烈的争论。特别是，深部地壳是否具有广泛的侧向流动能力，对于板块边界断层系统上的应变积累（美国西部卡斯卡迪亚边缘的瞬态变形被认为与深部地壳的变形有关）以及亚洲高地形的发展速度和模式。后一个问题备受关注，因为青藏高原的高地形从根本上影响大气环流。了解这种高地貌发展的过程和速度，将能够更严格地检验青藏高原的生长在地质时间尺度上的气候变化中发挥核心作用的可能性。这项研究将对西藏地下深部地壳变形的本质产生新的认识，而这些信息是当今造山模型中非常缺乏的。在此过程中，该项目还将为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预算至关重要。最后，这些结果将指导人们更全面地了解地壳深处的流动如何影响地球表面的整体变形，包括构造断层上产生地震的滑动的可能性。