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Using geodynamic models to constrain the timing and depth of seismic anisotropy development under the western United States

  • Author(s): Chaparro, Thomas Robert
  • Advisor(s): Stegman, Dave R
  • et al.

Mantle flow aligns highly anisotropic olivine crystals in the mantle, and thus anisotropy observed through seismic shear save (SKS) splitting observations is often directly associated to active mantle flow. The Western United States has a circular pattern of SKS splits, and numerous studies have attempted to 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, we model 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. We found that a strong circular pattern of anisotropy can be predicted for 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 today's relatively static mantle flow field

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