Global upper-mantle structure from finite-frequency surface-wave tomography

We report global shear-wave velocity structure and radial anisotropy in the upper mantle obtained using finite-frequency surface-wave tomography, based upon complete three-dimensional Born sensitivity kernels. Because wavefront healing effects are properly taken into account, finite-frequency surface-wave tomography improves the resolution of small-scale mantle heterogeneities, especially for deep anomalies that are constrained by the longest-period surface waves. In our finite-frequency model FFSW1, the globally averaged radial anisotropy shows a transition from positive (SH>SV) to negative anisotropy (SV>SH) at about 220 km, consistent with a change in the dominant mantle circulation pattern from predominantly horizontal flow at shallow depths to vertical flow at greater depths. The radial anisotropy beneath cratons and the old Pacific plate agrees well with previous studies. However, our model exhibits a strong negative radial anisotropy at depths greater than 120 km beneath mid-ocean ridges, a feature that is not present in previous upper-mantle models. More interestingly, the depth extent of the ridge anomalies is distinctly different beneath fast- and slow-spreading centers; anomalies beneath fast- spreading centers are stronger, but the strength decreases rapidly below 250 km. In contrast, beneath slow-spreading centers such as the northern Mid-Atlantic Ridge and the Red Sea, anomalies extend down at least to the top of the transition zone. The different depth extent of the ridge anomalies suggests that the primary driving force of slow-spreading seafloor may be different from that of fast- spreading seafloor and that active upwelling beneath slow-spreading ridges may play a major role in the opening of the seafloor.