This project will investigate the thermal and compositional state, melting processes, and deformation of the upper mantle beneath the Atlantic basin. These properties will be inferred from new, high-resolution, three-dimensional tomographic models of seismic velocity, attenuation, and anisotropy of the Atlantic upper mantle. Much of what is currently known about the oceanic upper mantle comes from regional seismic models of the Pacific basin. However, differences in the spreading rates and plate velocities of the Atlantic and Pacific oceans suggest that different dynamical processes are occurring in the mantle beneath the two basins. The new models will be constrained by large data sets of fundamental and higher-mode surface-wave phase delay and amplitude from events and stations located within or on the margins of the Atlantic basin. This research seeks to address three key issues: (1) What controls the seismic structure as a function of depth and seafloor age? Can these properties be attributed solely to temperature, or must composition and melt also be considered? (2) What controls along-axis and off-axis seismic anomalies? Do they reflect a variable mantle source, and what roles do temperature and composition play? (3) Is the mantle fabric associated with slower plate velocities weaker and more variable than that produced by faster spreading rates? Each of these questions invites a comparison with the faster-spreading Pacific upper mantle, and together they present an opportunity to investigate possible spreading-rate dependence of seismic structure as well as the mechanisms that might produce it. The final seismic models will ultimately be used to infer temperature, composition, partial melt content, and deformation state, aided by constraints from mineral-physics experiments and other data sets such as basalt chemistry, bathymetry, and geoid height.
Non-Technical Description of Research According to the theory of plate tectonics, the Earth?s rigid outer shell is divided into tectonic plates that slowly move relative to one another, driven by convection currents within hot, weak rocks in the mantle beneath the plates (the asthenosphere). We have a poor understanding of the processes that control this abrupt transition from rigid rocks within the plate to weak deforming rocks beneath the plate. Is it simply that the asthenospheric rocks are hotter? Are they partially molten, or do they contain compositional components (water or other volatiles) that weaken them? Because seismic waves generated by earthquakes are sensitive to temperature and other weakening processes, we can use seismic imaging of mantle structure to address these questions. Distinguishing between these processes will allow us to better understand how the Earth?s tectonic system developed and evolved over time, and also illuminates the weakening and melting processes that produce geologic hazards such as fault zones and volcanic systems.
