A Wide Angle Ocean Bottom Seismometer Experiment Across Liyue Bank, the Southern Margin of the South China Sea

AbstractThe P wave velocity model of wide angle seismic profile‐OBS973‐2 that extends in NW‐SE direction for 369 km long from the northeastern Liyue Bank to the central sub‐basin is established by the trial‐and‐error 2D ray‐tracing method. This profile model is compared with that in the northern continental margin for the discussion of the conjugate relationship between the northern margin and the southern margin of the South China Sea (SCS). The velocity model consists of six layers, i.e. three sediments layers, upper crust, lower crust and upper mantle. The velocities of the three sediments layers are 1.8~2.0 km/s, 2.0~2.7 km/s and 3.5~4.0 km/s, respectively. The sediment layers are much thinner in general or even absent in some parts. There are some small volcanoes on the top of crust in Liyue Bank and P wave velocities increase downward from 5.5~6.4 km/s in the upper crust (9~10 km thick) to 6.6~7.1 km/s in the lower crust (11 km thick). In the transition zone and sea basin P wave velocities increase downward from 5.9~6.1 km/s in the upper curst (4~5 km thick) to 6.6~6.9 km/s in the lower crust (2~4 km thick). The crust in the transition zone and sea basin shows some tectonic effects of stretching and thinning. The velocity structure of the model indicates that its continental crust is a standard one but its oceanic upper crust is of relatively higher velocity than standard one. The seaward rising of the buried depth of Moho is 23 km in Liyue Bank and 8~12 km in the sea basin. The velocity beneath Moho is 8.0 km/s in the sea basin obviously smaller than 8.2 km/s in Liyue Bank.The comparison of profile OBS973‐2 with a previous profile OBS2006‐1 in the northern margin shows the much more similarity between them. This result and detailed discussion suggest a possibility of conjugate relationship of Liyue Bank with Zhongsha Massif, separated by the central seamount chain of the SCS. Furthermore, the full rates of seafloor spreading of the northwestern sub‐basin and the southwestern sub‐basin are calculated.