Collaborative Research: Thermochemical Models of Mantle Dynamics and Plate Motions

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The goal of the proposed work is to understand the dynamics of the Earth?s interior and the long-term evolution of our planet. The motion of tectonic plates and the attendant earthquakes are surface expressions of a large-scale flow in the interior, which is driven by a combination of thermal and compositional buoyancy as the planet cools. Most of the geological processes we observe at the surface are related in one way or another to this large-scale flow. However, our understanding of the flow from a dynamical perspective is far from complete. What is the origin of the buoyancy that drives the flow? How does this flow interact with tectonic plates at the surface? How and why does the flow reorganize and cause (relatively) abruptly changes in plate motions. We propose to address these questions developing a new theoretical model for the large-scale flow and by introducing several new observations to test and refine the model.We propose several important advances over previous studies. First, we plan to develop a more complete treatment of subduction zones in global models of flow. The dynamics of subduction is described by a new viscous sheet model that explicitly includes the effects of plate bending as well as the tensile stresses inside the plate due to the weight of the cold and dense subducted plate. Observations of deep earthquakes in subducted plates provide valuable information about the stress state inside the plates. We plan to make use of this information for the first time in global flow models. Second, we propose to develop a self-consistent description of lateral variations in viscosity. Thermal buoyancy is expected to cause large variations in viscosity due to the strong temperature dependence of important transport properties (like viscosity). We propose to use this self-consistent model to predict flow when the buoyancy forces are inferred from tomographic models of seismic heterogeneity. The conversion from seismic anomaly to density anomaly is a controversial issue, particularly in the lower part of the mantle. The relative importance of thermal and composition buoyancy is not well known. We plan to use a recently detected free oscillation of the Earth to constrain the gravity field in the interior. Different conversions from seismic anomaly to density anomaly have different consequences for the global gravity field, which can be tested using gravity measurements at the surface and our new constraint on the gravity field in the interior. We hope to gain a better understanding of the buoyancy forces that drive the flow and provide new insights into the role of plates in organizing the flow. The proposed work supports two young female investigators (Dr. Kayla Lewis and Ms. Melanie Gerault) and fosters a new collaboration between USC and UC Berkeley. The proposed work will also use, adapt and improve an existing computer code in the CIG repository (an NSF-funded initiative). We intend to contribute the new code back to CIG when the project is completed.