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.

核心掩体边界（CMB）是地球内部最戏剧性的物理边界。 CMB是岩石，对流地幔和液态铁外芯之间的界面，它们的运动产生了地球的磁场。跨CMB区域的组成，密度，粘度和温度的主要对比意味着该界面在控制地球内部的动力学和演变中起着至关重要的作用。具体而言，CMB表示地幔对流的底部边界层，随着相对热的材料上升和相对较酷的材料下沉，地球在地质时间上冷却的过程。一个未解决的主要问题是，地幔对流的模式看起来像CMB上方，以及该模式如何与地幔其余部分中的对流运动相互作用及其在板构造特征（例如俯冲带）中的表面表达式。该项目的目的是利用已经通过最低地幔的地震波的观察来限制CMB上方地幔流的模式。该项目涉及三年的努力，以研究地震偏见和通过观测和建模的地幔基础的地震障碍和流动模式。由于变形与地震各向异性之间的因果关系，各向异性的表征和解释可以在地幔中的流动模式上提供至关重要的约束。虽然通常在地幔中研究地震各向异性，但将信号从最低地幔各向异性分离出来要困难得多。此外，在最低地幔矿物质中菌株和各向异性之间的关系仍然存在主要不确定性。 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? 2）最低地幔中的流动模式是什么，什么物理过程驱动了这一流量？为了解决这些科学问题，研究人员建议进行五项活动。首先，该团队将对S-SC和SKS-SKKS相进行微分剪切波分裂的观测，以限制由于射线传播方向在选定区域的壁炉底部的各向异性引起的分裂。其次，他们将对已经反映在d“不连续性（PDP和SDS）的相位的阶段进行阵列分析；这些相的极性受D“各向异性的影响，并与剪切波分裂测量相结合可以更加紧密地约束各向异性几何形状。第三，他们将采用基于矿物质物理学的前向建模框架，该框架使用单晶弹性来识别与地震观测一致的合理各向异性几何形状。第四，他们将使用各向异性的这些观察结果来测试全球模型在地幔底部的流动和弹性的预测。最后，他们将整合项目的各个阶段的结果，以测试一组关于地幔底部流动驱动力的假设做出的预测。这项工作的更广泛影响包括培训研究生，国际合作的培养，针对公众的深层过程的网站创建网站，以及在科学出版物，公共教育和外展活动中传播结果。