Collaborative Research: The Asthenosphere and Mantle Dynamics

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The theory of plate tectonics is one of the great intellectual revolutions of the past 50 years. The theory remains a kinematic one in that it does not self-consistently address the connection between plate motions and the Earth's internal energy sources which must, in some way, be the drivers of plate motion. Extending plate tectonics to a fully dynamic theory is a fundamental current challenge in Earth science. The motivations for doing so go beyond studies of the Earth's interior as the operation of plate tectonics has implications for long term climate history and for the evolution of life on our planet. Although it is agreed that the convective cooling of the Earth's interior is associated with plate motions, the specifics of the connection are not fully understood. Our work will make a vital step in closing the loop, so to speak, by elucidating the dynamics and feedback mechanisms associated with plate tectonics and convection in the Earth's mantle. It has long been suspected that a region of low relative strength beneath tectonic plates, i.e., the asthenosphere, plays a fundamental role in connecting plate motions to convection in the Earth's interior. We will quantify this idea through a combination of data analysis and state of the art numerical modeling. By comparing our modeling predictions to observations we will be able to test the hypothesis that plate tectonics depends on the presence of a low viscosity asthenosphere. If correct, this will have implications beyond a better understanding of our own planet (e.g., Venus lacks evidence for an asthenosphere and for currently active plate tectonics). We will perform 3D spherical mantle convection simulations to explore the combined effects of a low-viscosity asthenosphere and plate boundary failure, at parameter values appropriate to Earth's mantle. We will augment our numerical work with theoretical scaling analysis to keep interpretations of the numerical work on a solid footing, and to predict model behavior in previously unexplored regimes. Our analysis will provide insight into model dynamics in terms of energetic balances between the resistance to plate motion associated with dissipation at plate margins, within the asthenosphere, and in the bulk mantle. This will allow us to link our dynamic modeling to instantaneous flow studies that isolate the balance of dissipation between plate processes and the bulk mantle. This linkage will bring added observational constraints to bear on our numerical simulations. Our modeling will also make predictions regarding plate size distribution, plate velocities, heat flow, dynamic topography, gravity signals, the power spectrum of mantle thermal anomalies, and seismic anisotropy to be compared (statistically) with observations. The combination of simulations, physical analysis, and data constraints will provide insight into the geologically recent dynamics of the solid Earth. With this in hand, our analysis and modeling will be extended to address the role of the asthenosphere for the thermal history of our planet.