Synthetic seismic anisotropy models within a slab impinging on the core–mantle boundary

Abstract

S U M M A R YThe lowermost few hundreds of kilometres of the Earth's mantle are elastically anisotropic; seismic velocities vary with direction of propagation and polarization. Observations of strong seismic anisotropy correlate with regions where subducted slab material is expected. In this study, we evaluate the hypothesis that crystal preferred orientation (CPO) in a slab, as it impinges on the core-mantle boundary, is the cause of the observed anisotropy. Next, we determine if fast polarization directions seen by shear waves can be mapped to directions of geodynamic flow. This approach is similar to our previous study performed for a 2-D geodynamic model. In this study, we employ a 3-D geodynamic model with temperaturedependent viscosity and kinematic velocity boundary conditions defined at the surface of the Earth to create a broad downwelling slab. Tracers track the deformation that we assume to be accommodated by dislocation creep. We evaluate the models for the presence of perovskite or post-perovskite and for different main slip systems along which dislocation creep may occur in post-perovskite [(100),(010) and (001)]-resulting in four different mineralogical models of CPO. Combining the crystal pole orientations with single crystal elastic constants results in seismically distinguishable models of seismic anisotropy. The models are evaluated against published seismic observations by analysing different anisotropic components: the radial anisotropy, the splitting for (sub-)vertical phases (i.e. azimuthal anisotropy), and the splitting for subhorizontal phases. The patterns in radial anisotropy confirm our earlier results in 2-D. Observations of radial anisotropy and splitting in subhorizontal phases are mostly consistent with our models of post-perovskite with (010)-slip and (001)-slip. Our model of (001)-slip predicts stronger splitting than for (010)-slip for horizontally propagating phases in all directions. The strongest seismic anisotropy in this model occurs where the slab impinges on the core-mantle boundary. The azimuthal anisotropy pattern for (001)-slip shows fast axis directions at the edges of the slab (sub-)parallel to flow directions, suggesting horizontal flows may be mapped out in the lowermost mantle using seismic observations.

Seismic anisotropy of the D″ layer induced by (001) deformation of post-perovskite

“…Previous investigations of the source of anisotropy in the D″ have shown that the expected texture of bmg in D″ layer does not produce a seismic signature consistent with seismic observations and thus is not expected to contribute significantly to the observed seismic anisotropy in the region27. (Mg,Fe)O fp with high V s anisotropy of ∼40% and abundance of ∼25% volume in the D″ layer, has been suggested as a source for the observed anisotropy in D″ (ref.…”

“…Seismic velocities resulting from slip on (001)[100] and (001)[010] or on (001)<110> alone in a slab descending to the bottom of the mantle were predicted with a three-dimensional geophysical model27, which assumes deformation is accommodated by dislocation slip. Tracers in the model record temperature and strain history as the slab descends.…”

Two-phase deformation of lower mantle mineral analogs

“…Ferropericlase may not only influence the development of crystallographic preferred orientation and seismic anisotropy more than previously thought (e.g. Cottaar et al, 2014), but mantle mixing (Girard et al, 2016), viscosity (Yamazaki and Karato, 2001) and slab stagnation (Marquardt and Miyagi, 2015) as well.…”

The limits of ray theory when measuring shear wave splitting in the lowermost mantle withScSwaves

“…However, in general φ ′ is not the same for the two methods, and δt is smaller for the finite-frequency methodthough this δt relationship is not universally true, notably for the S2 path. This immediately implies that previous interpretations of flow direction or likely mechanisms for anisotropy in D ′′ using ray methods (e.g., Wookey et al 2005;Wookey & Kendall 2008;Long 2009;Nowacki et al 2010;He & Long 2011;Nowacki et al 2013;Cottaar et al 2014;) may need to be revisited, even in areas of apparently 'weak' and simple anisotropy.…”