Structure of the Tibetan Crust and Upper Mantle and Its Geodynamic Implications

本研究将研究西藏地壳和上地幔的大尺度结构，并探讨其地球动力学意义。初步三维地震模型的两个特征激发了这项工作，并定义了这项工作提出的主要问题。首先，青藏高原下地幔中高速和低速异常的连贯三维模式。什么样的地球动力学过程可以解释这种模式？第二，西藏地壳具有强烈的中地壳径向各向异性，初步分析表明，中地壳的方位各向异性是相干的、与周期相关的。西藏地壳有限应变发展的哪些模型与这些观测相一致？这项工作将改进目前的地震模型，并研究其地球动力学意义。通过增加更多的表面波频散测量和在反演中引入新的测量方法，将对模型进行改进；例如，地壳和地幔泛音测量和接收函数。反演产生一系列可接受的模型，而不是单一的最佳拟合模型，从而产生检验假设所需的有意义的不确定性信息。本文将研究几种可能解释青藏高原上地幔速度异常模式的地球动力学情景，包括：印度岩石圈的逆冲作用，亚洲岩石圈的西南逆冲作用，以及西藏岩石圈的分离或对流移动作用。此外，还研究了推断出的地壳各向异性与三种有限应变发展模式之间的关系。研究生将接受地震层析成像和地球动力学界面的培训。地震模型将以在课堂上有用的形式放置。这项工作的合作者包括地球系统教育数字图书馆（DLESE, www.dlese.org）和联安办事处的研究人员，他们也是地球信息系统大规模信息技术研究项目（www.geongrid.org）的一部分。

This research will study the large-scale structure of the crust and upper mantle beneath Tibet andinvestigate its geodynamic implications. Two features of a preliminary 3-D seismic model motivate this work and define the primary questions that the work proposes to address.First, a coherent 3-D pattern of high and low speed anomalies in the mantle underlies the Tibetan plateau. What geodynamic processes can account for this pattern? Second, strong mid-crustal radial anisotropy characterizes the Tibetan crust, and preliminary analysis implies coherent, period-dependent azimuthal anisotropy. Which models of the development of finite strain in the Tibetan crust are consistent with these observations?The work will improve the current seismic model and investigate its geodynamical implications.The model will be improved by adding more surface wave dispersion measurements and by introducing new kinds of measurements into the inversion; e.g., crustal and mantle overtone measurements and receiverfunctions. The inversion generates an ensemble of acceptable models, rather than a single best-fitting model, thereby producing meaningful uncertainty information needed to test hypotheses. Several geodynamic scenarios that may explain aspects of the pattern of velocity anomalies in the upper mantlebeneath Tibet will be investigated, including; the underthrusting of Indian lithosphere, southwestwardthrusting of Asian lithosphere, and the detachment or convective removal of Tibetan lithosphere.In addition, the relation between inferred crustal anisotropy and three models of the development of finitestrain in the Tibetan crust will be investigated.A graduate student will receive training at the interface between seismic tomography andgeodynamics. The seismic models will be placed in a form that is useful in the classroom. Collaborators in this effort include researchers at the Digital Library for Earth Systems Education (DLESE, www.dlese.org) and UNAVCO, who are also part of the GEON large scale information technology research project (www.geongrid.org).