Constraining lowermost mantle flow through observations and models of seismic anisotropy

通过地震各向异性观测和模型约束最低地幔流

• 批准号: 1547499
• 负责人: Maureen Long
• 金额: $ 26.99万
• 依托单位: Yale University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-06-01 至 2020-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1547499&HistoricalAwards=false
• 关键词: Constraining; lowermost; mantle; flow; through

The core-mantle boundary (CMB) is the most dramatic physical boundary within the Earth's interior. The CMB is the interface between the rocky, convecting mantle and the liquid iron outer core, whose motions give rise to the Earth's magnetic field. The major contrasts in composition, density, viscosity, and temperature across the CMB region mean that this interface plays a critical role in controlling the dynamics and evolution of the Earth's interior. Specifically, the CMB represents the bottom boundary layer for mantle convection, the process through which the Earth cools off over geologic time as relatively hot material rises and relatively cool material sinks. A major unsolved problem is what the pattern of mantle convection looks like just above the CMB, and how that pattern interacts with convective motions in the rest of the mantle and their surface expressions in plate tectonic features such as subduction zones. The goal of this project is to use observations of seismic waves that have passed through the lowermost mantle to constrain the pattern of mantle flow just above the CMB.This project involves a three-year effort to study seismic anisotropy and flow patterns at the base of the mantle via observations and modeling. Because of the causative link between deformation and seismic anisotropy, the characterization and interpretation of anisotropy can provide crucial constraints on flow patterns in the mantle. While seismic anisotropy is commonly studied in the upper mantle, it is much more difficult to isolate the signal from lowermost mantle anisotropy; furthermore, major uncertainties remain about the relationships between strain and anisotropy in lowermost mantle minerals. Despite the challenges inherent in studying D" anisotropy, however, it holds exceptional promise as a tool for deciphering patterns of flow at the base of the mantle and understanding the processes that drive these patterns. This project addresses two fundamental unsolved problems related to the structure and dynamics of the lowermost mantle: 1) What is the geometry of seismic anisotropy in the D" layer? and 2) What is the pattern of flow in the lowermost mantle, and what physical processes drive this flow? In order to address these science questions, the investigator proposes to carry out five activities. First, the team will carry out differential shear wave splitting observations of S-ScS and SKS-SKKS phases to constrain splitting due to anisotropy at the base of the mantle in selected regions over a range of ray propagation directions. Second, they will carry out array analysis of phases that have been reflected off the D" discontinuity (PdP and SdS); the polarities of these phases are affected by D" anisotropy and in combination with shear wave splitting measurements can more tightly constrain the anisotropic geometry. Third, they will apply a mineral physics-based forward modeling framework that uses single-crystal elasticity to identify plausible anisotropic geometries that are consistent with seismic observations. Fourth, they will use these observations of anisotropy to test the predictions of global models for flow and elasticity at the base of the mantle. Finally, they will integrate results from all phases of the project to test the predictions made by a set of hypotheses about the driving forces for flow at the base of the mantle. Broader impacts of this work include the training of a graduate student, the cultivation of international collaborations, the creation of a website on deep Earth processes aimed at the general public, and the dissemination of the results in both scientific publications and public education and outreach presentations.

核心-地幔边界（CMB）是地球内部最引人注目的物理边界。宇宙微波背景是岩石对流地幔和液态铁外核之间的界面，其运动产生了地球磁场。 CMB 区域在成分、密度、粘度和温度方面的主要差异意味着该界面在控制地球内部的动力学和演化方面发挥着关键作用。具体来说，CMB 代表地幔对流的底部边界层，这是地球在地质时期随着相对热的物质上升和相对冷的物质下沉而冷却的过程。一个尚未解决的主要问题是宇宙微波背景上方的地幔对流模式是什么样的，以及该模式如何与地幔其余部分的对流运动及其在板块构造特征（例如俯冲带）中的表面表现相互作用。该项目的目标是利用对穿过最下地幔的地震波的观测来限制 CMB 上方的地幔流模式。该项目为期三年，旨在研究地幔底部的地震各向异性和流动模式。通过观察和建模来了解地幔。由于变形和地震各向异性之间存在因果关系，各向异性的表征和解释可以为地幔中的流动模式提供关键的约束。虽然地震各向异性通常在上地幔中进行研究，但将信号与下地幔各向异性分离要困难得多。此外，关于最低地幔矿物的应变和各向异性之间的关系仍然存在重大不确定性。尽管研究 D" 各向异性存在固有的挑战，但它作为破译地幔底部流动模式和理解驱动这些模式的过程的工具具有非凡的前景。该项目解决了与结构相关的两个基本未解决问题和最低地幔动力学： 1）D"层地震各向异性的几何形状是什么？ 2）最下地幔的流动模式是什么，什么物理过程驱动这种流动？为了解决这些科学问题，研究者建议开展五项活动。首先，研究小组将对S-ScS和SKS-SKKS相进行差分剪切波分裂观测，以限制在一系列射线传播方向上选定区域中地幔底部各向异性造成的分裂。其次，他们将对 D" 不连续性反射的相位（PdP 和 SdS）进行阵列分析；这些相位的极性受到 D" 各向异性的影响，结合剪切波分裂测量可以更严格地约束各向异性几何学。第三，他们将应用基于矿物物理学的正演建模框架，该框架使用单晶弹性来识别与地震观测一致的合理的各向异性几何形状。第四，他们将利用这些各向异性的观测结果来测试全球模型对地幔底部流动和弹性的预测。最后，他们将整合该项目各个阶段的结果，以测试一系列关于地幔底部流动驱动力的假设所做出的预测。这项工作的更广泛影响包括培养研究生、培养国际合作、创建面向公众的地球深部过程网站，以及在科学出版物和公共教育和外展演示中传播研究结果。