Developing a Seismic Model for Investigating Layering in Cratonic Lithosphere beneath Africa

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This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).The lithosphere is the stiff outer shell of our planet - it is the rigid plate of plate tectonics. In the oldest and most stable regions of continents (cratons) the lithosphere is expected to be cold and thick, yet we observe a wide-spread, near-constant layer internal to the lithosphere, almost akin to the crust-mantle boundary, but expressed as a velocity reduction. This puzzling and ubiquitous observation of ‘mid-lithospheric layering’ has resulted in different, sometimes contradictory, explanations (i.e., partial melt, anisotropy, chemical stratification, or short-term rheological weakening). In this study, the investigators will develop a new seismic model of the lithosphere useful for testing proposed models of lithospheric layering beneath Africa. The multi-decadal investment in the innovative AfricaArray initiative, the diversity of cratons, and a growing number of permanent and temporary seismic networks on the continent, with publicly available data, has opened a new vista on our ability to resolve fine-scale lithospheric layering on the continent. Similarly, new passive-source imaging using spectral coherence of ambient noise, machine learning, and probabilistic analysis of converted body waves is poised to ensure maximum utility from the archived seismic datasets. This project will engage early career scientists, train minority Ph.D. students, and promote education and outreach goals for recruiting under-represented undergraduate students by designing new courses that expose students to the optimal use and management of high-performance computing resources. The project will address gaps in current efforts to study lithospheric layering on the continent. The investigators will conduct analysis of all publicly available seismic data on the continent, with sensitivity to the crust and upper mantle lithosphere, and prepare and publishing measurements of: (1) group and phase velocity of broad-band ambient noise and earthquake records for Love and Rayleigh surface waves, (2) depth and sharpness of compressional and shear body wave reflectivity of lithospheric discontinuities, (3) quantify uncertainty in phase velocity maps and, (4) provide an updated continent-wide reference model of the African crust and upper mantle (LithoAFR+), using a probabilistic joint inverse approach to reporting uncertainty in each stage of measurement and model-construction (i.e., 1-4). The expected peer-reviewed studies, when interpreted with complementary experimental and geophysical constraints (i.e, conductivity and mineral physics), will advance our understanding of cratonic lithospheric layering and the geological evolution of continents. Integrated modeling, using other geophysical constraints, will also advance the understanding of (1) the origin of the elevated African topography, (2) the origin of orphan tremors, intra-plate earthquakes, and volcanism, and their connection to the well-known African superplume, (3) the broad-scale evolution of continental rifting and formation of new ocean basins, and (4) multi-stage assembly and break up of Gondwanaland.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.