Collaborative Research: The role of viscosity heterogeneity in plate-mantle coupling

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The earth's surface is broken into more than a dozen mobile "plates" that move relative to one another at rates of a few cm per year. Over millions of years, these plates deform at their boundaries as well as internally, producing plate-bounding shear zones such as the San Andreas Fault, basins, and mountain ranges. Much of this deformation occurs seismically, which generates an earthquake hazard in regions that are experiencing deformation. Ultimately, the forces that drive these dynamic surface processes originate within the Earth's deep interior, which flows as it convects. While Earth's surface plates are the surface expression of this convection, the details of how their motion and deformation is related to the motion of the convecting mantle is not well constrained. Yet, it is essential to understand this plate-mantle interaction, which ultimately controls plate tectonics and the surface deformation that we observe. This project is designed to constrain the interaction between mantle flow, tectonic stresses, and plate motions. PIs Conrad and Lithgow-Bertelloni are using spherical finite element models of viscous mantle flow to predict the forces that the mantle exerts on the base of Earth's tectonic plates. These forces are then used to predict two fields that can be observed from the Earth's surface. First, plate velocities are predicted by balancing the forces on each of Earth's tectonic plates. The comparison of predicted motions to observations lead to further improvements in the numerical model. Second, the PIs compute the stresses experienced by an elastic lithosphere that is subjected to basal shear tractions associated with mantle flow and plate motions. These stresses are statistically analyzed against observations of lithospheric stress made in boreholes, from moment tensor solutions for earthquakes, and from geological observations of lithospheric deformation. To quantitatively assess the degree and variability of plate-mantle coupling, the PIs focus on the influence of viscosity heterogeneity beneath the elastic lithosphere. The material strength of the rocks beneath the lithosphere is thought to vary between geological provinces. For example, oceanic and thin continental lithosphere may be underlain by low-viscosity asthenosphere that is weaker than the upper mantle while old continental shields may reside over thick, strong cratons that protrude deeply into the upper mantle. These extreme lateral variations in viscosity greatly influence the coupling between the mantle and the lithospheric plates. In doing so they affect plate motions, patterns of lithospheric stresses at the surface, and the long-term geological deformation of continents. The PIs are developing rigorous models of lithosphere-mantle interaction, calibrated against a variety of observations. Specifically, the PIs hope to further illuminate the range of material properties that characterize the sub-lithospheric mantle, and to gain a better understanding of how the mantle controls surface deformation and the background stresses associated with seismic hazards. This funded project involves the close collaboration between the two PIs with varying scientific and technical expertise ranging from numerical modeling to geology. Hence, the results of these experiments are likely to have a broad impact and to be of interest to diverse segments of the Earth Sciences community. Beyond the scientific collaboration, the project will involve the education of two PhD students (one female), the establishment of a new geodynamic research laboratory at Johns Hopkins University, and the continuing support of a Hispanic female PI.