A three-dimensional radially anisotropic model of shear velocity in the whole mantle

Abstract

SUMMARY We present a 3-D radially anisotropic S velocity model of the whole mantle (SAW642AN), obtained using a large three component surface and body waveform data set and an iterative inversion for structure and source parameters based on Non-linear Asymptotic Coupling Theory (NACT). The model is parametrized in level 4 spherical splines, which have a spacing of ! 8 . The model shows a link between mantle flow and anisotropy in a variety of depth ranges. In the uppermost mantle, we confirm observations of regions with VSH > VSV starting at ! 80 km under oceanic regions and ! 200 km under stable continental lithosphere, suggesting horizontal flow beneath the lithosphere. We also observe a VSV > VSH signature at ! 150‐300 km depth beneath major ridge systems with amplitude correlated with spreading rate for fast-spreading segments. In the transition zone (400‐700 km depth), regions of subducted slab material are associated with VSV > VSH, while the ridge signal decreases. Whilethemid-mantlehasloweramplitudeanisotropy( VSV in the lowermost 300 km, which appears to be a robust conclusion, despite an error in our previous paper which has been corrected here. The 3-D deviations from this signature are associated with the large-scale low-velocity superplumes underthecentralPacificandAfrica,suggestingthatVSH >VSV isgeneratedinthepredominant horizontal flow of a mechanical boundary layer, with a change in signature related to transition to upwelling at the superplumes.