Using a history of subduction to constrain timing and depth of seismic anisotropy development under the western United States

Mantle flow aligns highly anisotropic olivine crystals in the mantle, and thus anisotropy observed through seismic shear save (SKS) splitting observations are often directly associated to active mantle flow. The Western United States has a circular pattern of SKS splits, and numerous studies have been done to try and predict the cause of this phenomena. To predict the current pattern of SKS splits, we use a geodynamic numerical model to simulate anisotropy development in the mantle using both time integrated mantle velocities and instantaneous mantle velocities. In summary, our models show that anisotropy patterns that more generally match the observed SKS signal around Nevada are the direct result of complex 3D toroidal flow from the segmenting of the Farallon slab over the last 10 million years. We found that a strong circular pattern of anisotropy can be generated at depths between 200-400 km using mantle flow velocities integrated from 10 Ma to present. Our geodynamic models also suggest that geophysical studies should incorporate the full time-dependent history of mantle flow when studying SKS splits, as models that use present day velocities do not provide good agreement to current SKS splitting observations. Thus, SKS measurements represent the full history of olivine deformation due to changes in mantle flow regimes and not necessarily what is currently happening in present day mantle flow field. This work uses more simplified geodynamic models (slab only) compared to the work done by Lijun Liu and Quan Zhou (please see related presentation- "Understanding Complex Seismic Anisotropy in Western U.S. using 4D Mantle Flow Models" in session DI002).