Constraints on Lowermost Mantle Mineralogy and Fabric beneath Siberia from Seismic Anisotropy

Raypaths and bouncepoints for data used in the study. Panel A shows the great-circle paths for the phases studied, from Hindu-Kush and Kuril Arc events (circles) to POLARIS and GRSN stations (triangles). Stars mark the reflection point of ScS: for both paths these are very close. Panel B shows the variation of shear-wave speed at the CMB from Masters et al (2000) (blue colours are faster than average) and the predicted location of paleosubducted slab material from Lithgow-Bertelloni and Richards (1998) (blue circles). The grey region shows the approximate Fresnel zone of the ScS phases in the lowermost mantle. Also shown are the S and ScS raypaths; S turns above D$''$ whilst ScS samples it.
Seismic anisotropy is an important tool for studying the nature, origin and dynamics of the lowermost mantle (D"). Here we present analysis of the seismic anisotropy beneath Siberia from shear-wave splitting measured in ScS phases. Data come from two near-perpendicular raypaths (Hindu-Kush to Northern Canada and the Kuril Arc to Germany; both at ~80 degrees epicentral distance) with close ScS reflection points on the Core-Mantle boundary (CMB). We apply differential S-ScS splitting to minimise contamination from the source and receiver side upper mantle. The two raypaths show different ScS splitting times and fast shear-wave orientations, incompatible with the VTI style of anisotropy inferred for much of the lowermost mantle. The availability of data at two azimuths give us an opportunity to better understand D" anisotropy than in previous studies. For example, the results provide the first accurate measurement of the dip of the symmetry plane. Several mechanisms have been suggested to explain lowermost mantle anisotropy, including the lattice-preferred orientation of lower mantle minerals such as perovskite or post-perovskite, or the shape-preferred orientation of inclusions of melt. In order to infer the flow regime implied by these mechanisms we use elasticities from published deformation experiments to forward model shear-wave splitting. Tomography of the region suggests a north-south trend in the geodynamics, and a model incorporating post-perovskite with a [100](010) slip system or aligned melt inclusions are most naturally compatible with such a trend. This may suggest a connection with remnant slab material from past subduction in the north Pacific.
</p><p>Wookey, J., Kendall, J-M. (2008) Constraints on Lowermost Mantle Mineralogy and Fabric beneath Siberia from Seismic Anisotropy, Earth Planet. Sci. Lett., 275, 32-42
</p><p>Acknowledgements: Data came from the CNSN and SZGRF datacentres. JW was supported by a NERC fellowship grant.</p>


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