Depth-dependent azimuthal seismic anisotropy governed by Couette/Poiseuille flow partitioning in the asthenosphere

Azimuthal seismic anisotropy provides crucial knowledge on spatial patterns of past and present upper mantle deformation. Origins of this deformation were traditionally tied to relative shear between surface plates and mantle, and in turn a constant orientation of anisotropy azimuths with depth. However, observations of azimuthal seismic anisotropy based on surface-wave tomography often feature depth-dependent azimuths in the upper mantle. This is consistent with the existence of low-viscosity, thin asthenosphere that facilitates the channelization of both plate-driven Couette flow and pressure-driven Poiseuille flow. If the two flows are not aligned, their combination yields depth rotations of asthenospheric shear, giving rise to depth dependence of azimuthal seismic anisotropy. In this study, we utilize publicly available azimuthal seismic anisotropy together with predictions from a global mantle flow model that incorporates Couette/Poiseuille flow. We find that Poiseuille flow has significant influence on depth rotations of seismically inferred azimuthal anisotropy. Depth rotations are prominent under the Atlantic basin and the Nazca plate, where modeled asthenospheric flow regimes are dominated by Poiseuille flow. Significant Poiseuille flow may exist beneath the Indian basin, but its depth rotations are small, probably because subduction zones to the north align Couette and Poiseuille flows into the same direction. Our results indicate that interpretation of azimuthal seismic anisotropy cannot be simply associated with relative shearing between plates and mantle. Instead, the relative importance of Couette and Poiseuille flows must be considered to account its depth dependence.