Collaborative Research: Joint inversion of crust and upper mantle structure in central and eastern Tibetan plateau and its margins

该奖项是根据2009年《美国复苏和再投资法案》(公法111-5)资助的。(A)该项目的广泛意义喜马拉雅山脉是如何上升到今天的高度的？青藏高原(TP)是如何形成的？高原地壳是如何变形而产生大地震的，比如我们在2008年5月12日目睹的汶川大地震中国，那次地震造成7万多人死亡？我们知道，高原是由印度和欧亚大陆之间的碰撞产生的，碰撞始于大约6000万年前。然而，重要的细节被遗漏。一个主要的障碍是我们透过地表下的3D结构“看”的能力有限，这使得我们很难将地表地质结构和变形与潜在的力联系起来。该项目的目的是利用各种地震成像技术和我们将从全球数据库以及中国内部收集的史无前例的大量地震数据来成像TP中东部及其边缘的地下结构。这些图像将为检验高原形成和变形的关键假说提供关键信息。我们的研究对该地区造山、高原形成、大陆变形和地震灾害的机制和过程等基本问题具有广泛的影响。该项目将是与中国进行国际科学合作的绝佳机会。它将支持一名来自伊利诺伊州大学的研究生和一名来自圣路易斯大学的研究生。我们还将聘请本科生进行地震学培训和地震数据处理技能。(B)项目的技术描述。人们提出了各种模型来解释青藏高原的隆起、形成和变形。一个主要问题是地下三维结构的地震成像分辨率有限，使得很难将地震参数与地质结构和过程联系起来。我们建议使用多个数据集的联合反演方法来提高TP中东部岩石圈P和S构造的分辨率。我们建议从环境噪声相关和传统的基于地震的方法联合解释P走时、接收函数和面波频散测量，以得到P和S速度和各向异性的三维模型。在地震反演中，模型参数往往相互取舍。为了提高分辨率和解决模糊性，需要组合对不同参数具有敏感性的不同数据集，或者必须施加先验约束。现在可获得的丰富数据使我们的联合反演成为可能。我们特别感兴趣的是地壳通道流动模型，该模型认为中下地壳流对地形载荷的响应，上地壳的变形与下地幔的变形脱钩。我们选择西藏中部和东部是基于需要足够的数据覆盖率和我们希望研究足够大的区域以避免局部非均质性可能产生的偏差，并比较会聚体制(西藏中部)和挤压体制(西藏东部)。我们寻求解决的关键问题包括：(1)中地壳水道流动：是否有证据表明存在广泛的中地壳水道流动？它发生在高原的什么地方？(2)地壳通道流动的方向：通道流动的方向是什么？从藏中到藏东、到东南部和东北缘的方向是如何变化的？(3)壳幔变形的耦合或解耦：变形如何随深度变化？上地壳形变是否与地幔岩石圈的形变脱钩？(4)从藏中到藏东的构造和形变变化：印度岩石圈在青藏高原之下的俯冲程度如何？不同地区在构造和深部变形方面有何区别和联系？边缘的主要构造对地壳和地幔的变形有什么控制作用？

This award is funded under the American Recovery and Reinvestment Actof 2009 (Public Law 111-5).(a) Broader significance of the projectHow does the Himalaya rise up to today's height? How is the Tibetan Plateau (TP) formed? How is the crust of plateau deforming to produce great earthquakes, such as the one we witnessed on May 12, 2008 in Wenchuan, China, which killed over 70,000 people? We know that the plateau is generated by the collision between India and Eurasia, which started about 60 million years ago. However, important details are missing. A major obstacle is our limited ability to "see" through 3D structure below the surface, making it difficult to relate surface geological structures and deformation to the underlying forces. The purpose of this project is to use a variety of seismic imaging techniques and unprecedented amount of seismic data that we will collect from global databases as well as those inside China to image the subsurface structure of central and eastern TP and its margins. These images will provide critical information to test key hypotheses on plateau formation and deformation.Our research has broad implications for fundamental questions about the mechanisms and processes of mountain building, plateau formation, continental deformation, and seismic hazards in the region. The project will be an excellent opportunity for international scientific collaboration with China. It will support one graduate student from U. Illinois and one from Saint Louis U. We will also engage undergraduates for seismology training and seismic data processing skills.(b) Technical description of the project.A great variety of models have been proposed to explain the uplifting, formation, and deformation of the Tibetan Plateau. A major problem is the limited resolution of seismic imaging of the sub-surface 3D structure, making it difficult to relate seismic parameters to geological structures and processes. We propose to use joint-inversion methods involving multiple datasets to improve resolution of both P and S structures of the lithosphere in the central and eastern TP. We propose to jointly interpret P travel times, receiver functions, and surface-wave dispersion measurements from both ambient noise correlation and traditional earthquake-based method to derive 3D models of P and S velocities and anisotropies. In seismic inversion, model parameters often trade off with each other. To improve resolution and to resolve the ambiguity, a combination of different data sets that have sensitivities to different parameters is required or a priori constraints have to be imposed. The abundance of data now accessible makes our joint inversions feasible.We are particularly interested in the crustal channel flow model, which suggests that mid-lower crust flows in response to topographic loading and the deformation of the upper crust is decoupled from the underlying mantle. We select central and eastern Tibet based on the need for sufficient data coverage and on our desire to study a sufficient large area to avoid possible bias from local heterogeneity and to compare the convergence regime (central Tibet) with the extrusion regime (E. Tibet).The key questions we seek to address include:(1) mid-crust channel flow: Is there evidence for widespread mid-crust channel flow? Where in the plateau does it occur?(2) directions of crustal channel flow: What are the directions of the channel flow? How does the direction change from central Tibet to eastern Tibet and to the southeastern and northeastern margins?(3) coupling or decoupling of crustal and mantle deformation: How does deformation change with depth? Is the upper crust deformation decoupled from the deformation in the mantle lithosphere?(4) changes of structure and deformation from central Tibet to eastern Tibet: What is the extent of the India lithosphere underthrusting beneath the TP? What are the differences and connections in structures and deformation at depth among different regions? What controls do the major structures at the margins exert on the crustal and mantle deformation?