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

Cottaar, S. and Li, M. and McNamara, A. K. and Romanowicz, B. and Wenk, H.-R. (2014) Synthetic seismic anisotropy models within a slab impinging on the core-mantle boundary. Geophysical Journal International, 199 (1). pp. 164-177. ISSN 0956-540X DOI https://doi.org/10.1093/gji/ggu244

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Abstract

The 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 temperature-dependent 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.

Item Type:	Article
Additional Information:	This article has been accepted for publication in Geophysical Journal International © The Authors 2015. Published by Oxford University Press on behalf of The Royal Astronomical Society.All rights reserved.
Uncontrolled Keywords:	2014AREP, IA68,
Subjects:	02 - Geodynamics, Geophysics and Tectonics
Divisions:	02 - Geodynamics, Geophysics and Tectonics 08 - Green Open Access
Journal or Publication Title:	Geophysical Journal International
Volume:	199
Page Range:	pp. 164-177
Identification Number:	https://doi.org/10.1093/gji/ggu244
Depositing User:	Sarah Humbert
Date Deposited:	02 Nov 2014 01:32
Last Modified:	30 Aug 2015 02:28
URI:	http://eprints.esc.cam.ac.uk/id/eprint/3146

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