Scientists from China and Japan discovered unique features of the flow field in the lower mantle. By examining the seismic anisotropy in the upper part of the lower mantle beneath the Philippine Sea Plate, they found that the old lower mantle flow field still persists there. The study was published on: Natural Geology.

The lower mantle is an important layer of the Earth and can play an important role in the evolution and circulation of materials in the Earth’s interior. It is generally accepted that this is not only the final destination of subduction plates, but also the birthplace of mantle plumes, two major styles of evolution and circulation of material at the surface and within the Earth. However, our knowledge about the characteristics of the flow field and the geodynamics of the lower mantle is still insufficient.

Research methodology and results

In this study, researchers performed P-wave azimuthal anisotropy tomography to image the 3D anisotropic structure of the crust and mantle below the PSP to a depth of 1,600 km. Tomography results show that NS fast velocity directions (FVDs) are located at depths 700–900 km below the middle of the PSP. They also observed two isolated velocity anomalies with NW-SE FVD beneath the PSP at depths of 700–1600 km.

They found that NS FVDs at depths of 700–900 km were not associated with plate subduction because they occurred far from present-day subduction zones. They are also independent of the mantle plume, as there has been no active mantle plume beneath the PSP since the early Cenozoic.

Based on previous geodynamic modeling and seismological results, the researchers concluded that NS FVDs at depths of 700–900 km reflect an approximately 50 Ma residual flow field in the Pacific lower mantle.

In addition, two isolated velocity anomalies are consistent with seismic scatterers at depths of 1000–1800 km identified by previous seismological studies, and their locations are generally consistent with the spreading center between the Izanagi and Pacific Plates, where this spreading center is about to subduct. Beneath the Eurasian Plate. Therefore, isolated velocity anomalies are thought to be remnants of the subducting Izanagi plate.

“NW-E FVDs in the two isolated fast anomalies are believed to reflect the flow field of the Pacific lower mantle around 40 Ma because the two isolated fast anomalies are surrounded by an amorphous mantle flow field and are not affected by the conditions of the present-day lower mantle field. Mantle flow,” the study said. said first and corresponding author Professor Fan Jianke from the Institute of Oceanology, Chinese Academy of Sciences (IOCAS).

“Our study shows that seismic anisotropy is more widespread in the lower mantle than previously thought,” Professor Fan said. “These observations also provide important and independent seismic evidence of past deformation in the lower mantle, which may help us better understand the geodynamic properties of the lower mantle.”