Remnants of shifting early Cenozoic Pacific lower mantle flow imaged beneath the Philippine Sea Plate

Seismic anisotropy could provide vital information about the evolution and internal convection of the deep Earth interior. Although previous seismological studies have revealed a wide distribution of seismic anisotropy in the upper portion of the lower mantle beneath many subduction zones, the existence of anisotropy at these depths away from subducted slabs remains debated. Here we use P-wave azimuthal anisotropy tomography to image the crust and mantle down to 1,600-km depth. We find prominent anisotropic patterns in the upper portion of the lower mantle beneath the Philippine Sea Plate. Substantial azimuthal anisotropy with N–S fast-velocity directions occurs at 700–900-km depths. We interpret this azimuthal anisotropy as a remnant of the Pacific lower mantle flow field about 50 million years ago. Two isolated high-velocity anomalies at 700–1,600-km depths may be vestigial pieces of the subducted Izanagi slab with seismic velocity features suggesting a shift in the Pacific lower mantle flow field by about 40 million years ago. Our findings provide seismic evidence for the existence of complex lower mantle flows and deformation mechanisms away from subduction zones.