Crustal Anisotropy Across Tibet: Implications for the Existence of Partial Melt and the Vertical Coherence of Deformation

拟议的工作旨在通过模拟地震各向异性来提高对西藏变形的理解。这种方法并不完全是新的，但目前的观测要么高度局部化，要么分辨率很差，因此洞察力仍然不完整。拟议的研究将根据PASSCAL实验的数据，并通过中国合作者，中国地震台阵，产生关于西藏地壳和上地幔径向和方位各向异性的新的高分辨率信息。将进行三种具体类型的地震观测。(1)通过对西藏全境的环境噪声和地震资料的高分辨率Love波相速度测量，推断地壳中的Vsh。这些观测结果将有助于确定所观测到的地壳LVZ是完全还是部分来自矿物排列。(2)从环境噪声中观测到的地壳方位各向异性以及从地震记录中观测到的下地壳和上地幔方位各向异性限制了应变的垂直连续性。这些观测结果将有助于判断河道水流和垂直相干变形之间的关系。(3)联合反演接收器函数与面波频散将有助于校准地壳中的模型参数化，提供有关地壳LVZ顶部和底部潜在速度跳跃以及地壳底部潜在榴辉岩层的信息。这将改善对中地壳径向各向异性强度的估计，这对于确定矿物排列是否足以产生所观察到的LVZ或是否需要部分熔融至关重要。所有反演都利用了表面波层析成像方法的最新进展（例如，程函和亥姆霍兹层析成像）以提高估计的波速的保真度。这项工作的一个关键组成部分是使用蒙特卡罗和相关的贝叶斯统计来估计从初步测量到最终地震模型的不确定性。西藏是研究大陆-大陆碰撞的天然实验室，这一过程在整个历史上塑造了地球并留下了疤痕，但目前是西藏独有的。本研究的目的是阐明西藏地壳变形的性质，特别是以下问题的动机。中地壳的变形在多大程度上局限于水道（水道流）中，因而与水道上方或下方的变形不同？大量的部分熔化是否有利于通道流动？特别是，低速带（LVZ）观察到的西藏中地壳造成的广泛存在的部分熔融，或者他们主要反映了各向异性矿物的排列所造成的先前或正在进行的变形？在另一个极端，整个岩石圈的变形在多大程度上是垂直一致的？这些问题将通过开发新的高分辨率模型来解决，这些模型可以反映西藏地壳和上地幔中地震波速度（各向异性）的方向性。中国安装的新地震数据资源以及中美合作将成为研究的基础，新的层析成像方法将产生比以前更高分辨率和可靠的图像。

The proposed work aims to improve understanding of the deformation of Tibet by modeling seismic anisotropy. This approach is not entirely new, but current observations are either highly localized or poorly resolved so that insight remains incomplete. The proposed research will produce new, high-resolution information about radial and azimuthal anisotropy in the Tibetan crust and uppermost mantle based on data from PASSCAL experiments and, through Chinese collaborators, the Chinese Earthquake Array. Three specific types of seismic observations will be made. (1) High resolution Love wave phase velocity measurements obtained from ambient noise and earthquakes across all of Tibet will be obtained to infer Vsh in the crust. These observations will help to determine whether the observed crustal LVZs result entirely or only partially from mineral alignment. (2) Azimuthal anisotropy observed for the crust from ambient noise and for the lower crust and uppermost mantle from earthquake records constrain vertical continuity of strain. These observations will help to adjudicate between channel flow and vertically coherent deformation. (3) Joint inversion of receiver functions with surface wave dispersion will help to calibrate the model parameterization in the crust, providing information about potential velocity jumps at the top and bottom of the crustal LVZ and about a potential eclogitized layer at the base of the crust. This will improve the estimate of the strength of mid-crustal radial anisotropy, which is crucial to determine whether mineral alignment suffices to produce the observed LVZs or if partial melt may be required. All inversions leverage recent advances in surface wave tomography methods (e.g., eikonal and Helmholtz tomography) to improve the fidelity of estimated wave speeds. A key component of this work is the estimation of uncertainties using Monte Carlo and related Bayesian statistics from primary measurements to the final seismological models.Tibet presents a natural laboratory to study continent- continent collision, a process that has shaped and scarred the Earth throughout its history but is at present unique to Tibet. The proposed research aims to illuminate the nature of deformation in the Tibetan crust and is motivated particularly by the following questions. To what extent is deformation of the middle crust localized in a channel (channel flow) and hence different from that above and perhaps below it? Does extensive partial melt facilitate channel flow? In particular, are the low velocity zones (LVZs) observed across the Tibetan middle crust caused by the extensive presence of partial melt, or do they dominantly reflect the alignment of anisotropic minerals caused by previous or on-going deformation? At the opposite extreme, to what extent is deformation of the entire lithosphere vertically coherent? These question will be addressed through the development of new high resolution models of directionally-dependent seismic wave speeds (anisotropy) in the crust and uppermost mantle beneath Tibet. New seismic data resources installed by the Chinese and collaboratively between the US and Chinese will form the basis for the research and new methods of tomography will be to produce much higher resolution and reliable images than were possible previously.