Collaborative Research: Quantifying Plume-Lithosphere Interactions with GNSS Geodesy, Seismology, and Geodynamic Modeling

Part 1: Continental rifting is a physical process that shapes the Earth's surface and causes a range of geohazards for local populations. The East African Rift System (EARS) is the Earth's archetype continental rift system in its early stages of development and one of the focus sites of the NSF GeoPRISMS program. Understanding the physics of deeper mantle interactions with continental rifting at the shallow parts of the Earth remains an outstanding challenge in the geosciences. In this project the physics of mantle-lithosphere interactions will be addressed, with a focus on testing long-standing mantle plume hypotheses using computational modeling and new seismic and Global Navigational Satellite System / Global Positioning System (GNSS/GPS) precision positioning data to constrain movements in the mantle and at the surface.

Part 2: It has long been recognized that the mantle beneath the East African Rift System (EARS) is thermally perturbed. A host of geophysical and geochemical data are used to argue for a plume origin of the thermal perturbations. Whether the EARS is underlain by a single, superplume or multiple plumes is hotly debated. Fundamental to understanding the physical processes influencing the East African GeoPRISMS focus site (Eastern Branch), and more broadly of large-scale mantle dynamics, is testing the superplume model. New seismic and GNSS data are needed from the GeoPRISMS focus-site along with thermomechanical modeling of lithosphere-plume interactions to fully evaluate the superplume and multiple plume models and advance our understanding of continental rifting processes. The proposed work will allow two GeoPRISMS questions to be addressed: (1) How does the presence or absence of an upper-mantle plume influence extension? (2) How is strain accommodated and partitioned throughout the lithosphere, and what are the controls on strain localization and migration? Seismic data from 9 new stations in northeastern Uganda will be installed to collect data for generating mantle flow indicators (seismic shear wave splitting, SKS) and improved sub-surface images (seismic tomography) of the Eastern Branch. If a consistent northerly fast polarization direction in the SKS measurements is discovered for most or all of the seismic stations the superplume model would be corroborated. If a more complex pattern of fast polarization directions is observed, then alternative interpretations will be examined, including the multiple plume hypothesis. The tomography models will constrain initial conditions of the proposed geodynamic modeling. A transect of new GNSS stations across the Eastern Branch spanning Kenya (7) and Uganda (3 co-located) will also be installed. If the GNSS data have along-rift motions that align with SKS observations, then the superplume model with strong plume-lithosphere interactions is favored. If SKS observations are consistent with the superplume model, but GNSS are not, then surface-mantle decoupling is indicated. These and alternative outcomes of the GNSS-SKS comparisons will be explored with 3D computational modeling of the lithosphere-mantle system that is constrained by the new seismic tomography. To characterize lithospheric coupling mechanics to upper mantle flow a suite of material models will be developed to determine coupling properties at the lithosphere-asthenosphere boundary (LAB). Modeled flow will be scored with new GNSS and SKS-splitting observations at the surface and LAB, respectively. In addition to the above scientific objectives this project will (1) enhance diversity by providing graduate research opportunities in seismology for underrepresented minority students supported through AfricaArray; (2) provide training to African scientists through GNSS short courses and the AfricaArray education program, (3) geodynamic modeling extensions will become publically available as part of the ASPECT software package, (4) the GNSS stations will become part of the permanent AfricaArray network and provide open data to the community long after the project is over, and (5) seismic data will become publically available. This project has been supported in part by the Office of International Science and Engineering at NSF.