The Residual of the Oki-Daito Plume in the Lower Mantle with the P wave Arrival-time Delays

Abstract:

The mantle plumes rising from different depths relate to the seismic slow anomaly in the Earth’s interior and is significant to understand the geodynamic process. Deep mantle plumes often result as volcano chains or oceanic island chains. The previous EM-2 Oki-Daito mantle plume acted on the Philippine Sea Plate at ~50-35 Ma [Ishizuka et al., 2013]. In order to detect the residual of the Oki-Daito plume in the lower mantle beneath the Philippine Sea Plate, P wave arrival-time delays of seven earthquakes occurred at the subduction zones surrounding the plate and recorded by the Chinese Digital Seismic Network and High Sensitivity Seismograph Network of Japan, are used to map the deep fine structure by using the ray-tracing method [Rawlinson and Sambridge, 2004]. A slow anomaly is found at depths between ~837 and 1007 km beneath the central west Philippine Sea Plate, with the P-wave velocity decrease of 0.35%-0.45% and the related temperature increase of ~117-150K. The slow anomaly should be the residual of Oki-Daito plume. In addition, local fast anomaly of ~200km thickness is also found in the mantle transition zone beneath the northern west Philippine Sea Plate, with the P-wave velocity increase of 1.8-2.0%, which should be the stagnant slab of the Pacific Plate [Fukao et al., 2009]. We also analyze the possible earlier interaction between the Oki-Daito plume and stagnant slab, or the cause of the slow anomaly in the lower mantle. Compared to seismic tomography results [Huang and Zhao, 2006; Fukao and Obayashi, 2013], the slow anomaly obtained by the cross constraint of dense seismic ray-paths shows similar velocity perturbation, but much smaller scales and higher resolution. The phenomenon means that the lower mantle is more homogeneous than that shown in seismic tomography.