Constraining lowermost mantle flow through observations and models of seismic anisotropy

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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.