Low-velocity layers in the northwestern margin of the South China Sea: Evidence from receiver functions of ocean-bottom seismometer data

摘要

The receiver function method was applied to teleseismic recordings of an active-source ocean-bottom seismometer array in the northwestern margin of the South China Sea. Forward modeling of the receiver functions was used to derive velocity interfaces beneath the stations. Two main groups of low-velocity layers were imaged. The first group in the reef area, with a thickness of 7-10 km, exhibits a seaward uptrend of the buried depth but poor continuity. The second group, with a thickness of 3-4 km, is more continuous along the Zhongsha Trough; the buried depth decreases asymmetrically from the flanks to the rift center. A high shear wave velocity layer of similar to 4.0 km/s is imaged at the crust bottom beneath the Zhongsha Trough, with thickness of similar to 3 km. The average Vp/Vs in the Cenozoic sediment is 2.0-3.0 and was affected by high fluid activity induced by intensive faults. The Vp/Vs of the crust varies between 1.77 and 1.88, suggesting an intermediate to basic composition under the continental extension. Together, these results suggest a two-stage evolution model of the continental crust. In the first stage, the crust deformed in a ductile manner and thus created considerable shearing structures with high anisotropy that caused the obvious decrease of the shear wave velocity. In the second stage, the shearing structures connected asymmetrically, driven by detachment faults in the upper crust and upwelling of the lower crust. The Zhongsha Trough then formed, probably with magmatic underplating due to lithospheric extension or rising of hot mantle materials.