Collaborative Research: Mantle Dynamics and Plate Tectonics Constrained by Converted and Reflected Seismic Wave Imaging Beneath Hotspots

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Since its formation billions of years ago, Earth has been slowly cooling via convection, where warmer material rises to the surface and cold tectonic plates plunge deep into the interior. Understanding this process is important for a wide range of problems, such as determining the factors that drive plate tectonics to sustaining deep water and carbon cycles that stabilize the atmosphere/hydrosphere and climate throughout Earth history. This is particularly relevant for society, both because plate tectonic processes are the driving forces behind hazards such as earthquakes and volcanoes and also because our climate and atmosphere make Earth habitable. This project studies large scale convection in the solid Earth by using earthquake data recorded at distant seismic stations in regions of upwelling. Upwellings are notoriously difficult to constrain. The focus will be on three end-member cases: 1) Hawaii, the classic example where deep upwelling occurs beneath ocean lithosphere, 2) Yellowstone, the classic example of deep upwelling that occurs beneath a continent, and 3) the equatorial mid-Atlantic Ridge, which is typically assumed to be a location without deep upwellings. The project will use classic techniques and also some newly developed approaches to better constrain the scale of the upwellings and their pathways. An outreach program will increase diversity in the Earth Sciences via public engagement and education and training of students and early career researchers. The outreach portion includes visits to core discipline classrooms at Morgan State University, a historically black university, with an established connection with two faculty members there. The goal of the visits and the materials is to increase awareness of career possibilities and also the societal relevance of the Earth Sciences. The approach is modeled on the success of the "Google in Residence" program that successfully placed Google engineers on HBCU campuses. Eventually a wider range of Earth scientists will be included by developing an archive of materials that can be scaled and adapted according to needs and also best-practice advice, both of which will be made publicly available to the broader scientific community. The project also provides training for undergraduate students, a graduate student, and a post doc in cutting-edge methodologies and use of seafloor seismic data. Earth’s convective system is important for understanding the evolution of the planet, including everything from the factors that drive and enable plate tectonics to sustaining deep water and carbon cycles that stabilize the atmosphere/hydrosphere and climate over billions of years. It is generally accepted that cool mantle sinks back into the Earth, e.g., at subduction zones, and hot mantle rises, e.g., beneath hotspots and mid ocean ridges. Although mantle tomography has resolved many seismically fast anomalies associated with subduction, imaging upwellings has proven more challenging, potentially because seismic waveforms have difficulty resolving thin slow conduits. Thus, the exact dimensions, locations, characteristics, origin depths and magnitudes of these thermal anomalies are poorly known, as well as their chemical and physical interaction with the surrounding mantle, such as in the transition zone and uppermost mantle. Similarly, the degree to which upwellings change in size and/or are deflected during their ascent and how they vary among tectonic environments is uncertain. Converted and reflected wave imaging of the transition zone discontinuities and the lithosphere-asthenosphere boundary should provide tighter constraints. However, there are some discrepancies in studies using these methods regarding hotspot character and location, perhaps because they have used different methodologies and approaches with different sensitivities in different locations. This study will examine this issue systematically at a varied suite of tectonic environments. The planned approach will use the complementary sensitives of converted and reflected seismic phases to image the lithosphere-asthenosphere boundary and transition zone discontinuities beneath three key regions that are representative of the range of tectonic environments where variability in upwelling characteristics might be expected. These include the iconic hotspot of Hawaii near the center of an old oceanic plate, the classic example of Yellowstone hotspot beneath a continental interior, and finally the mid-Atlantic Ridge where deep upwellings are not predicted by classic models. Analyses will include P-to-S imaging of the transition zone discontinuities and S-to-P imaging of the lithosphere-asthenosphere boundary, both sensitive to vertical changes in shear wave velocity, and also use S-reflections, which have the added advantage of high depth resolution. A systematic approach will allow comparisons among the regions and anisotropic testing and F-K full-waveform modelling will be performed to determine the influence of anisotropy and/or focussing/defocussing for any apparent discrepancies. Once a full range of possibilities is defined from the seismic waveforms, inversions for Earth properties based on experimental and ab initio constraints will determine the properties that can explain the observations.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.