3D effects of sharp boundaries at the borders of the African and Pacific Superplumes: Observation and modeling

We report that a sharp lateral boundary exists at the southern edge of the Pacific superplume. The set of SHdiff waveforms, which graze the South Pacific, have similar features to those observed previously at the southeastern edge of the African superplume. Both waveform sets show a rapid shift of the arrival time and the broadening of the waveforms with respect to the azimuth as previously reported in the case of the African plume. We also document here that they both show a secondary pulse that follows the direct Sdiff phase. The coupled mode/spectral element method, which can handle strong lateral variations of shear velocity in Dʺ, is used to construct synthetic waveforms. The postcursors can be explained by simple effects of 3D structure in the Dʺ region with a sharp quasi vertical boundary aligned almost parallel to the ray path. The existence of these pulses suggests that modeling of heterogeneity outside of the great circle path can help constrain the 3D structure at the base of the mantle. When including 3D effects in the modeling, we find that the velocity contrast across the sharp boundary is of the order of 4-5%, averaged over the last 300 km of the mantle, which is smaller than has been proposed in some studies, but larger than in existing tomographic models, implying that the “superplume” features at the base of the mantle cannot be purely thermal. The similarity of the two observed SHdiff waveform sets at relatively high frequencies indicates that the low velocity regions in the lower mantle under Pacific and Africa, corresponding to the strong degree-2 pattern in shear velocity tomographic models, have a similar nature also at finer scales.