High-resolution 3-D S-velocity structure in the D″ region at the western margin of the Pacific LLSVP: Evidence for small-scale plumes and paleoslabs

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Although previous tomographic studies found a large low S-velocity province (LLSVP) in the lowermost mantle beneath the Pacific, due to a lack of resolution it was unclear whether the LLSVP consists of clusters of small-scale low-velocity anomalies or large-scale anomalies. We recently deployed a seismic-array in Thailand which provides a dataset with wide azimuthal coverage of the western Pacific LLSVP. We analyze the new dataset using waveform inversion, and find high-velocity anomalies extending vertically to a height of ~400 km above the core-mantle boundary (CMB) beneath the Philippine Sea and small-scale low-velocity patches with a diameter of ~300 km at the CMB beneath New Guinea. The locations of the high-velocity anomalies are consistent with the past Izanagi-plate subduction boundary, and the low-velocity anomalies can be interpreted as a small-scale plume cluster. Hence we conclude that vertical flow (upwelling plumes and downwelling of slabs) is dominant in the lowermost mantle beneath the western Pacific region.