Anisotropic Mantle: Advancing Models of Regional Flow and Associated Seismic Signature

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Flow within the mantle is the key process by which heat and material are transferred between the deep Earth and the surface we live on. One of the few ways of determining the pattern of this flow is to measure seismic anisotropy- an indication that propagation of waves occurs faster in one direction than another. Anisotropy can be linked to mantle flow because minerals that make up the mantle tend to slowly align when subjected to shear forces and the direction they align typically has faster seismic wave speeds compared to the cross direction. In this project, prior laboratory rock deformation results will be combined with computer simulations of regional flow, where motion of tectonic plates generates the forces to deform mantle rocks. The new question to be addressed is whether alignment of minerals can significantly stiffen, or weaken, the mantle and whether this may influence the overall pattern of flow, thus potentially altering exchange mechanisms such as production of magma. Seismic waves that travel along Earths surface are well suited for documenting the variation in anisotropy with depth. A secondary aspect of the project is to determine optimum surface wave data analysis methods for detecting complex patterns of anisotropy such as are expected to develop near tectonic plate boundaries.A suite of numerical models will elucidate the effects that flow-induced texturing can have on the viscosity structure of the upper mantle and the extent to which it influences the pattern of flow that develops, as well as the associated seismic signatures. The main advance that the geodynamical part of this study will provide is explicit inclusion of anisotropic rheology in models of upper mantle flow. A multi-scale (nanometer-100's km), linked numerical procedure is required and updates to existing programs are an integral part of the effort. The flow modeling will emphasize evolution away from an oceanic spreading center, but insights on the relationships between flow structure and the patterns/magnitudes of seismic anisotropy will be relevant to other settings such as the Hawaiian plume and subduction zones. The new seismological modeling for this study will document how surface wave dispersion depends on the distribution of (elastic) anisotropy. This has not been studied previously for linked models of mantle flow and texturing but it is a key aspect, especially in light of seismic array data that are now becoming available. The complexity of expected anisotropic structure within ~1000 km of plate boundaries and intraplate hotspots require that the full suite of body and surface wave analyses be performed in order to discern structure with the minimum possible non-uniqueness. At the same time, a quantitative assessment of the potential bias and limits of resolution is needed for seismic waves in settings where both vertical and lateral structure vary continuously. By providing a strong suite of forward models, we will set the stage for inverse approaches that can be applied during seismic data analysis.