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)项目更广泛的意义喜马拉雅山脉是如何上升到今天的高度的？青藏高原是如何形成的？2008年5月12日发生在中国汶川、造成7万多人死亡的大地震，高原地壳是如何变形产生的？我们知道，高原是由印度和欧亚大陆之间的碰撞产生的，这种碰撞始于大约6000万年前。然而，遗漏了一些重要的细节。一个主要的障碍是我们通过地表以下的3D结构“看到”的能力有限，这使得很难将地表地质结构和变形与潜在的力量联系起来。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境内数据库收集的前所未有的地震数据，对青藏高原中东部及其边缘的地下结构进行成像。这些图像将为检验关于高原形成和变形的关键假设提供关键信息。我们的研究对该地区的造山、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意义。该项目将是与中国开展国际科学合作的绝佳机会。它将支持一名来自伊利诺伊大学的研究生和一名来自圣路易斯大学的研究生。我们还将吸引本科生进行地震学培训和地震数据处理技能。(b)项目的技术说明。人们提出了各种各样的模式来解释青藏高原的隆升、形成和变形。一个主要问题是地下三维构造的地震成像分辨率有限，这使得地震参数与地质构造和过程之间的联系变得困难。我们建议采用多数据集联合反演的方法来提高青藏高原中东部岩石圈P、S结构的分辨率。我们建议联合利用环境噪声相关性和传统的基于地震的方法来解释P传播时间、接收函数和表面波色散测量，以获得P和S速度和各向异性的3D模型。在地震反演中，模型参数往往相互权衡。为了提高分辨率和解决歧义，需要对不同参数具有敏感性的不同数据集的组合，或者必须施加先验约束。现在可访问的大量数据使我们的联合反演成为可能。我们对地壳通道流动模型特别感兴趣，该模型表明，中下地壳流动是对地形负荷的响应，而上地壳的变形与下地幔是分离的。我们选择西藏中部和东部是基于对足够数据覆盖的需求，以及我们希望研究一个足够大的区域，以避免可能的局部异质性偏差，并比较辐合区（西藏中部）和挤压区（西藏东部）。我们寻求解决的关键问题包括：(1)中地壳通道流动：是否有证据表明中地壳通道流动广泛存在？它发生在高原的什么地方？(2)地壳河道流动方向：河道流动的方向是什么？从西藏中部到西藏东部，再到西藏东南和东北边缘，方向是如何变化的？(3)地壳与地幔变形的耦合或解耦：变形如何随深度变化？上地壳的变形是否与地幔岩石圈的变形解耦？(4)从藏中到藏东的构造和变形变化：青藏高原下逆冲构造的印度岩石圈范围有多大？不同地区在构造和深部变形方面有什么区别和联系？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?