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The southwestern depression of the Qiongdongnan Basin (QDNB) hosts thick Cenozoic sediments and awaits major hydrocarbon discovery. Multichannel seismic (MCS) profile CFT2011 across the southwestern QDNB reveals a ∼60-km-wide fuzzy reflection zone (FRZ) within the sediments, but its origin and distribution remain unclear. Here ocean bottom seismometer (OBS) data of Line CFT2011 are processed with focus on the velocity structures by traveltime tomography inversion and analyzed together with the coincident and adjacent MCS profiles. The OBS velocity results show that the giant FRZ features lower velocity with difference up to 1.5 km/s and smaller vertical velocity gradient than the surrounding sedimentary sequences at the same depth, likely resulting from enhanced fluid infilling. The MCS profile exhibits that the giant FRZ is about 3–9-km thick and extends from the Paleogene strata rich in organic matters upward to the lower Pleistocene sediments. Within the shallow overlying sediments, multiple bright spots with reverse polarity are imaged and their reflection amplitudes increase with offset, consistent with the features of gas-charged sediments. They are probably shallow gas reservoirs with gases sourced from the deep FRZ. Therefore, the FRZ is proposed to be a giant gas-charged zone, which probably contains lots of hydrocarbon gases migrated vertically from the deep Paleogene source rocks through the boundary faults of the depressions and the minor fractures generated under overpressure. This FRZ is also imaged on the adjacent MCS profiles MCS-L1 and MCS-L2 with the width of about 40 km and 68 km, respectively. It is roughly estimated to cover an area of ∼1 900 km2 and host a volume of ∼11 400 km3 assuming an average thickness of 6 km, implying huge natural gas potential in the sedimentary depression of the southwestern QDNB of the South China Sea. [ABSTRACT FROM AUTHOR]

The Zengmu Basin located in the shallow water area of the southern South China Sea, is rich in oil and gas resources, within which faults and mud-diapir are developed, but it is unknown whether oil and gas migrate to the seafloor surface. The newly collected multibeam data across the Zengmu Basin reveal a large number of depressions, with depths of 2–4 m, widths of several tens of meters, large distribution range of 1.8–8 km along survey line, up to ∼50 km, and their backscatter intensity (-26 dB) is much greater than that of the surrounding area (-38 dB). Combined with the developed mud-diapir and fracture structures, and abundant oil and gas resources within this basin, these depressions are presumed to be pockmarks. Furthermore, more than 110 mono-sized small circular pockmarks, with a depth of less than 1 m and a width of 5 m, are observed in an area of less than 0.03 km2, which are not obliterated by sediment infilling with high sedimentation rate, implying an existence of unit-pockmarks that are or recently were active. In addition, seismic profiles across the Zengmu Basin show characterization of upward migration of hydrocarbons, expressed as mud-diapir structures, bright spots in the shallow formation with characteristics of "low frequency increase and high frequency attenuation". The sub-bottom profiles show the mud-diapir structures, as well as the gas-bearing blank zones beneath the seafloor. These features suggest large gas leaking and occurrence of large amounts of carbonate nodules on the seafloor. This indicates the complex and variable substrate type in the Zengmu Basin, while the area was once thought to be mainly silty sand and find sand. This is the first report on the discovery of pockmarks in the Zengmu Basin; it will provide basic information for submarine stability and marine engineering in China's maritime boundaries. [ABSTRACT FROM AUTHOR]

Seafloor spreading can be explained by different dynamic mechanisms, magmatically or tectonically dominated. The Southwest Sub‐basin, located at the southwest tip of the propagating seafloor spreading of the South China Sea, remains unclear for its spreading regime owing to poor and debated knowledge of the crustal structures. Here two multichannel seismic lines and one oceanic bottom seismometer line across this subbasin are reprocessed and analyzed with focus on crustal imaging. The sediments are usually 0.5–1.0 km thick over the abyssal basin and slightly thicker in the few grabens. The thickest (3.3 km) sediments are found in the fossil spreading center. The basement is fairly rough and highly faulted by ubiquitous crustal faults. The fossil spreading center is characterized by a deep median valley, similar to that of magma‐poor spreading cases. Both multichannel seismic lines show a few intermittent and diffusive reflectors at 1.5‐ to 3.6‐km depth below the fragmented basement. These reflectors, correlating well with the velocities of 6.8–7.2 km/s obtained from velocity inversion of oceanic bottom seismometer data, are interpreted as Moho reflections. Thus, the inferred crustal thickness is only 1.5–3.6 km excluding the sediments, which is much thinner than usual. The underlying mantle with velocities lower than 8.0 km/s might be serpentinized with water infiltration along these crustal faults. Therefore, it is proposed that the spreading of the Southwest Sub‐basin was tectonically dominated. Key Points: Intermittent and diffusive Moho reflections are imaged in the Southwest Sub‐basin of the South China SeaThe crust is unusually thin (1.5–3.6 km) and highly faulted, and the underlying mantle with low velocities is likely serpentinizedThe spreading of the Southwest Sub‐basin is inferred to be tectonically dominated with poor magma supply [ABSTRACT FROM AUTHOR]