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

这项工作旨在通过模拟地震各向异性来提高对西藏形变的认识。这种方法并不是全新的，但目前的观察要么高度本地化，要么解决得不好，因此洞察力仍然不完整。这项拟议的研究将基于PASSCAL实验数据和通过中国合作者的中国地震台阵，产生关于西藏地壳和最上地幔的径向和方位各向异性的新的高分辨率信息。将进行三种特定类型的地震观测。(1)利用全西藏的环境噪声和地震资料获得的高分辨率Love波相速度，将用于推断地壳中的VSH。这些观测将有助于确定所观测到的地壳低空区是完全还是仅部分地由矿物排列所致。(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的方位各向异性，限制了应变的垂直连续性。这些观测将有助于判断河道水流和垂直相干变形之间的关系。(3)接收函数与面波频散的联合反演将有助于标定地壳中的模型参数，提供有关地壳低速带顶部和底部的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，这对于确定矿物取向是否足以产生观测到的低速带或是否需要部分熔融至关重要。所有的反演都利用面波层析成像方法的最新进展(例如，eikonal和Helmholtz层析成像)来提高估计波速的保真度。这项工作的一个关键组成部分是使用蒙特卡洛和相关的贝叶斯统计估计从初始测量到最终地震模型的不确定性。西藏提供了一个天然实验室来研究大陆-大陆碰撞，这一过程在其历史上塑造了地球，并留下了伤痕，但目前是西藏独有的。这项拟议的研究旨在阐明西藏地壳形变的性质，并特别受到以下问题的推动。中地壳的变形在多大程度上局限于河道(河道流)，因此与河道上方或下方的变形不同？大范围的部分熔融是否有利于渠道流动？尤其是，西藏中地壳的低速带(LVZ)是由部分熔体的广泛存在引起的，还是它们主要反映了由先前或正在进行的变形所导致的各向异性矿物的排列？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地震波速度方向相关模型(各向异性)来解决，这些模型将在西藏下面的地壳和最上地幔中进行。由中国和美国与中国合作安装的新地震数据资源将构成研究的基础，新的层析成像方法将产生比以前可能的更高分辨率和更可靠的图像。

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.