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Effects of crystal preferred orientation on upper-mantle flow near plate boundaries: rheologic feedbacks and seismic anisotropy

Olivine polycrystals that deform via dislocation glide develop CPO that results in their viscosity being anisotropic. The associated directional dependence in strength impacts the pattern of upper-mantle flow near a plate boundary. For background asthenosphere viscosity of ∼1020 Pa s and a rigid lithosphere, the anisotropic viscosity modification of the corner flow pattern is not drastic, but the sense of the change could affect partial melting rates and distribution. Feedback is also predicted in the development of CPO, particularly near and below the base of the lithosphere. Notably stronger fabric is predicted below the flanks of a spreading centre for fully coupled, power-law polycrystals than was determined using prior intermediate coupling polycrystal approaches. Alignment in the lithosphere is stronger and rotates into the horizontal. CPO near the corner inflection point of the flow field is reduced. These are important local behaviours from a structural viewpoint. The associated SKS splitting is modestly different (∼0.5 s) between the intermediate and fully coupled cases for oceanic plates less than 20 Myr old. Surface waves, on the other hand, show a more notable difference between the cases, with the amount of Rayleigh wave azimuthal anisotropy being twice as large (up to 5 per cent) for fully coupled power-law flow/polycrystals compared to predictions for linear, intermediate coupled flow/polycrystals.

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