Lithosphere structure beneath the Philippine Sea plate and adjacent regions caused by eastward mantle flow

The Philippine Sea Plate (PSP) is the largest trench-arc-basin system in the Western Pacific, which is completely surrounded by convergent boundaries and characterized by a complex evolution of back-arc systems and marginal basins involving lithospheric processes. The azimuthally anisotropy structure can be employed to reflect the deformation histories of the lithosphere mantle and help us to reveal the influencing factors of the marginal basins formation especially the West Philippine Basin (WPB). However, seismic anisotropic velocity structures have not been established reasonably due to lack of seismic stations in the interior of the PSP. In this study, high resolution 3-D shear-wave velocity structures and azimuthal anisotropy of the lithosphere mantle beneath the PSP are estimated using the continuous waveform data recording by broadband passive OBS stations and seismic stations surrounding the PSP. Strong and consistent N-S fast directions in the crust beneath the southern WPB are approximately perpendicular to the magnetic anomaly strips and subparallel to the seafloor spreading direction, which may be contributed to the remnant anisotropy accompanying with oceanic crust formation induced by the seafloor spreading. Whereas, relatively weak and inconsistent azimuthal anisotropy beneath the northern WPB might indicate that the oceanic crust was modified controlled by the subduction of the Pacific Plate. Prominent E-W fast direction in the lithosphere mantle beneath the northern WPB may be responsible for the eastward mantle flow triggered by the retreating of the Pacific plate along a stationary Marianas trench since the Miocene. While NE-SW fast direction beneath the southern WPB may correspond to the deformation filed controlled by the interactions both the eastward mantle flow and the collision between the Caroline Plate and PSP. Furthermore, anisotropy images reveal that lithosphere deformation and low-velocity zone in the asthenosphere beneath the marginal basins around the PSP including the Japan Sea and the South China Sea are controlled by the eastward mantle flow as a result of the retreat of the Pacific plate.