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

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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?