Contraining the large-scale dynamics and structure of the lower mantle using observations of the geoid, dynamic topography and plate tectonics

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Analyzing seismic waves traveling through the inside of the Earth has revealed that the Earth's mantle has a predominately long-wavelength (5000 km) structure. Particularly, the lower mantle beneath the circum-Pacific regions has seismic wave speeds that are relatively fast, while seismic wave speeds in the lower mantle beneath Africa and Pacific are relatively slow. These two seismically slow areas beneath Africa and the Pacific are are considered anomalies, and are most likely hot regions within the mantle because seismic waves travel slower in hot materials. These anomalies are particularly strong near the boundary between the core and mantle (Core-Mantle Boundary or CMB) regions, and are often termed as large low shear-velocity provinces or LLSVPs. The LLSVPs encompass most volcanism and large igneous provinces for the last 200 million years. This pattern in mantle seismic structure also highly correlates with the gravity anomalies at the Earth's surface. Together with seismic studies, geochemical studies also suggest that the LLSVPs may be compositionally different from the bulk of the mantle, being enriched in heavy, incompatible elements such as radioactive elements U and Th. Considering the significant influence of volcanic degassing on Earth's climate and the important effect of mantle viscosity on post-glacial rebound and sea-level change, it is therefore important to understand the dynamics of the long-wavelength mantle convection, the generation of the LLSVPs, and the relationship between the LLSVPs and their surface expression including volcanism and gravity anomalies. The goal of this 3-year project is to seek understanding of these fundamental questions, by analyzing and modeling fluid dynamical processes of mantle convection. This project will also lead to significant improvement in computational techniques in modeling mantle convection via international collaboration with an Australian geodynamicist. The project includes training of a graduate student. The project seeks to address the following three specific questions: 1) Are the LLSVPs purely thermal or thermochemical features? 2) Can the observations of the geoid and dynamic topography be made consistent with stable, thermochemical LLSVPs? 3) Can the long-wavelength mantle structure and convection (e.g., degree-2) be generated dynamically self-consistently with dynamic plates and realistic lithospheric rheology including the low-temperature plasticity? Four tasks for the proposed three-year project include: 1) to seek buoyancy and viscosity structures that explain the geoid, CMB excess ellipticity, and stability of the chemically distinct LLSVPs, by formulating mantle flow models with buoyancy derived from seismic models with compositional effects from the LLSVPs and post-perovskite phase change; 2) to test the hypothesis that the LLSVPs are purely thermal or thermochemical anomalies, using time-dependent convection models with imposed plate motion history; 3) to understand the origin of long-wavelength mantle structures including the LLSVPs, by formulating fully dynamic models of mantle convection with realistic lithospheric rheology; 4) to improve solvers in CitcomS for efficiency and robustness. The project should significantly improve understanding on the origins of the LLSVPs, long-wavelength mantle convection and plate tectonics. The results should have direct implications for studies in other areas beyond geodynamics, including mantle geochemistry, seismology, supercontinent cycles, gravity anomalies, and volcanism.