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The South China Sea (SCS) located at the intersection of the three intercontinental plates of Eurasia, IndiaAustralia, and the Pacific Oceanis, is a typical marginal sea basin formed by the seafloor spreading under the tectonic background of plate convergence. Many crustal-scale studies indicate that the SCS basin has undergone asymmetric spreading, multi-phase ridge jumps, and intense post-spreading volcanic activity. Due to the lack of seismic data in the oceanic basin of the SCS, it remainsunclear about the scale and basin control of the Zhongnan fault, the magma source depth of the SCS basin, and the transport channel after the cessation of seafloor spreading. Phase velocity derived from ambient noise surface wave tomography may provide useful information to shed light on the mechanisms of the aforementioned problems. From October 2019 to July 2020, a 3D Ocean Bottom Seismometers (OBS) passive seismic observation experiment was carried out by the Second Institute of Oceanography, Ministry of Natural Resources (SIOMNR) in a broad area of the SCS. Based on the seismic ambient noise data recorded by 16 OBSs in the SCS basin, we inverted the phase velocity images over a period range of 1020 s using ambient noise surface wave tomography. Our results indicate that the Zhongnan fault zone is a lithospheric-scalefault, which played a regulating role in the last oceanic ridge transition of the SCS basin from the East Subbasin to the Southwest Subbasin. In addition, the low-velocity body in the north flank of the Southwest Subbasin extends from the post-spreading seamounts on the ocean crust to the uppermost mantle (i.e., about 1030 km), which indicates an oblique magma migration during the postspreading volcanism.
Cheng et al. (Fri,) studied this question.