The Role of Post-Perovskite in Explaining Observations of Seismic Anisotropy

Abstract:

Increasing evidence is emerging for the presence of strong seismic anisotropy in D'' based on the observation of various seismic phases. To explain these observations in terms of crystal preferred orientation, and test for the presence and deformation mechanisms of post-perovskite, we apply a multi-disciplinary forward model approach. Our setup in this study is quite similar to our earlier study in 2D (Wenk et al., 2011). We employ a 3D geodynamical model with temperature-dependent viscosity and kinematic velocity boundary conditions defined at the surface of the Earth to form a broad downwelling slab. Tracers in the geodynamical model track the velocity gradient tensor in the slab at the surface down to the core-mantle boundary. The deformational information in the lower mantle is fed into a viscoplastic polycrystal plascticity model, in which we assume all deformation is accommodated by dislocation creep. We test models of either perovskite or post-perovskite mixed with periclase. For the post-perovskite phase we vary which slip system is most active. We average single crystal elastic constants over the crystal pole orientations to obtain seismically distinguishable models of anisotropy.

The four resulting synthetic anisotropy models are evaluated against published seismic observations by comparing different anisotropic components: the radial anisotropy, the splitting for (sub-)vertical phases (i.e. azimuthal anisotropy), and the splitting for sub-horizontal phases. The patterns in shear radial anisotropy and splitting in sub-horizontal phases are consistent with post-perovskite with dominant slip on the (010)- and (001)-planes, confirming our earlier results in 2D. Overall, the (001)-model shows stronger anisotropy than the (010)-model, and its strongest patch localizes where the slab impinges on the core-mantle boundary. Radial anisotropy for P-waves could further constrain whether a slip system on the (010)-plane or the (001)-plane is most active. The azimuthal anisotropy patterns for all post-perovskite models shows fast axis directions at the edges of the slab (sub-)parallel to flow directions, suggesting horizontal flow directions may be mapped out in the lowermost mantle using seismic observations.