Evaluating post-perovskite as a cause of D anisotropy in regions of palaeosubduction

Seismic anisotropy in the Earth's lowermost mantle (D-'') is often attributed to the alignment of MgSiO3 post-perovskite (ppv) by the movement of dislocations in response to mantle flow. However, ppv's plastic yield surface is not known; nor do we know if this is the main deformation mechanism. We make use of a heterogeneous, generally anisotropic model of elasticity in D-'' derived from a 3-D model of mantle flow, which is obtained by inversion of geophysical observables. Unlike previous approaches, completely general, 3-D flow and full anisotropy are permitted, yielding more information to compare with observations than has been possible before. We model observations of anisotropy in D-'' by calculating the shear wave splitting predicted in ScS waves for a series of models of ppv plasticity. We find that observations in regions of the lowermost mantle beneath subduction zones are best fit by a model which accommodates slip on (010). Our results show that, within one standard deviation, slip on (010)-or a mechanism giving the same style of anisotropy-explains D-'' anisotropy beneath these regions.