Collaborative Research: A global examination of the subduction zone flow field from seismic anisotropy

合作研究：从地震各向异性对俯冲带流场进行全球检验

• 批准号: 0911151
• 负责人: Laurent Montesi
• 金额: $ 14.99万
• 依托单位: University of Maryland, College Park
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-10-01 至 2014-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0911151&HistoricalAwards=false
• 关键词: Collaborative; Research; global; examination; subduction

What happens when a tectonic plate plunges back into the Earth?s interior at a subduction zone? Surprisingly, this simple question remains one of the fundamental unsolved problems in earth science. The flow pattern around the downgoing plate (often called the slab) is typically studied using seismic anisotropy, a property of the mantle that can be detected by recording seismic waves. In particular, measurements of shear wave splitting can be used to characterize seismic anisotropy in subduction zones. It is increasingly clear from such measurements that the simplest model of two-dimensional corner flow above the slab and entrained flow beneath the slab is likely incorrect. However, consensus on an alternative model has not been forthcoming. In this project, we will undertake a global survey of shear wave splitting observations with the goal of understanding what controls the mantle flow field in subduction zone regions. To do this, we will identify parameters that describe subduction zone dynamics that appear to exert a first-order control on shear wave splitting. Preliminary work has identified systematic variations in anisotropy both above and below the slab linked with the magnitude of trench migration velocity. This has led to the hypothesis that 3-D flow dominates beneath the slab and interacts with 2-D corner flow in the mantle wedge. In addition to a systematic evaluation of seismic anisotropy in subduction zones, we will construct laboratory and numerical models of mantle flow above and below the slab to identify diagnostic features of the flow field in shear wave splitting measurements and to explore the implications of our model for mantle dynamics. This project constitutes an interdisciplinary effort to understand and characterize the character of the mantle flow field that accompanies subduction using observations of seismic anisotropy and geodynamical modeling. With the increasing popularity of shear wave splitting as a tool for mapping mantle flow, a copious amount of data from subduction zones is now available. It is timely, therefore, to undertake a global survey of splitting observations with the goal of understanding which subduction parameters (such as convergence velocity, trench migration and curvature, age and spreading history of the downgoing plate, slab dip and morphology, seismicity, arc length, overriding plate thickness and stress, and volcanic production) appear to control the subduction zone flow field. From a preliminary survey, we hypothesize that 3-D flow dominates beneath the slab and that this flow field interacts with 2-D corner flow in the mantle wedge. We will complement our primary observational seismology goals with laboratory and numerical modeling studies. This forward modeling work will be used to validate the predictions of our working model, formulate alternative hypotheses, identify any second-order effects on the flow field, and explore the implications of our working model for larger-scale mantle dynamics. The availability of constraints on anisotropy from many regions around the globe and the combination of seismological observations and laboratory and numerical modeling suggest that a solution to the fundamental problem of interaction between downgoing slabs and the surrounding mantle is within reach.

当构造板块在俯冲带跌回地球内部时会发生什么？令人惊讶的是，这个简单的问题仍然是地球科学中未解决的基本问题之一。通常使用地震各向异性来研究下行板块（通常称为板片）周围的流动模式，地震各向异性是地幔的一种特性，可以通过记录地震波来检测。特别是，剪切波分裂的测量可用于表征俯冲带中的地震各向异性。从这些测量中越来越清楚地表明，板上方的二维角流和板下方的夹带流的最简单模型可能是不正确的。然而，尚未就替代模型达成共识。在这个项目中，我们将进行剪切波分裂观测的全球调查，目的是了解控制俯冲带区域地幔流场的因素。为此，我们将确定描述俯冲带动力学的参数，这些参数似乎对剪切波分裂施加一阶控制。初步工作已经确定了板块上方和下方各向异性的系统变化，这些变化与沟槽迁移速度的大小有关。这导致了这样的假设：3-D 流在板块下方占主导地位，并与地幔楔中的 2-D 角流相互作用。除了对俯冲带地震各向异性进行系统评估外，我们还将构建板块上方和下方地幔流的实验室和数值模型，以确定剪切波分裂测量中流场的诊断特征，并探索我们的模型对地幔动力学。该项目是一项跨学科的工作，旨在利用地震各向异性和地球动力学模型的观测来理解和表征伴随俯冲的地幔流场的特征。随着剪切波分裂作为绘制地幔流图的工具越来越受欢迎，现在可以获得来自俯冲带的大量数据。因此，现在是时候对分裂观测进行全球调查，以了解哪些俯冲参数（例如收敛速度、海沟迁移和曲率、下行板块的年龄和扩张历史、板片倾角和形态、地震活动性、弧度）。长度、上覆板块厚度和应力以及火山喷发）似乎控制着俯冲带流场。根据初步调查，我们假设 3-D 流在板片下方占主导地位，并且该流场与地幔楔中的 2-D 角流相互作用。我们将通过实验室和数值模型研究来补充我们的主要观测地震学目标。这项正演建模工作将用于验证我们的工作模型的预测，制定替代假设，识别对流场的任何二阶效应，并探索我们的工作模型对更大规模地幔动力学的影响。全球许多地区对各向异性的约束的可用性以及地震观测与实验室和数值模拟的结合表明，解决下行板块与周围地幔之间相互作用的基本问题是可以实现的。