Lithospheric structure beneath the fossil ridge in the South China Sea from OBS
data
Ba Manh Le and Ting Yang
School of Ocean and Earth Science, Tongji University, PR China
E-mail: 87manhlb@tongji.edu.cn
The central sub-basin of South China Sea (SCS) is a tectonically complex area. There
exists a fossil mid-ocean ridge with a seamount chain formed above and a subduction
zone developed perpendicularly in the east. Due to the limitation of seismic data
coverage, the lithospheric structure beneath this sub-basin remains obscured. A
passive-source OBS array deployed in the region allows us to image its lithospheric
structure. We selected 60 earthquakes (M>5.5) recorded in 9 OBS stations and 2
stations in the Luzon Island, and measured the amplitudes and phases at periods
ranging from 15s to 100s for fundamental mode of Rayleigh waves. We then use this
data to inverse the phase velocity by using the modified version of the two-plane
wave method with finite frequency kernels, through which a 3D model of shear wave
velocity is derived. Our results show an alongated low velocity anomaly sits beneath
the seamount chain at depths from 30 km to ~70 km. A weaker low velocity anomaly
to the north coincides in position with the older ridge prior to the ridge jump at ~25
Ma. Compared to the normal mid-ocean ridge, the low velocity anomaly beneath the
fossil ridge situates at much deeper position, which may have important implications
on the thermal evolution and the magmatic processes of this post-spreading ridge. Our
model also images a high velocity belt along the Manila trench, which is clearly
related to the subducted South China Sea plate. However, at the location of the fossil
ridge subduction, the high velocity belt has a clear gap, suggesting the ridge and
seamount subductions have complex consequences dynamically and geochemically in
the Luzon island.