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We analyzed original and published seismic data to investigate the distribution and characteristics of subsurface fluid flow features in the gas hydrate provinces along the northeastern South China Sea (SCS) continental slope, including Jiulong Ridge, Horseshoe Ridge, Pointer Ridge, and Formosa Ridge. Numerous features indicating the presence/migration of hydrocarbons, such as bottom simulating reflections (BSRs), bright spots, gas chimneys, and fluid flow through faults, are identified. The results reveal that the hydrofracturing-induced gas chimneys act as the primary conduits for the overall focused fluid flow in the study area, though fluid flow along permeable faults is also observed at Pointer Ridge. Thirty-three gas chimneys identified are categorized into two types depending on their capability for focused fluid flow migrating into the gas hydrate stability zone (GHSZ). The type-I chimneys that transport fluids into the GHSZ contribute to gas hydrate formation and may even lead to seafloor seepage. Buried by a considerable thickness of sediments, the type-II chimneys cannot directly contribute to generating gas hydrate and surface seepage. Our results suggest that the sedimentary processes and fluid accumulation significantly control the development of gas chimneys in the study area. Since the focused fluid flow conduits that indicate overpressured fluids are critical pathways that feed gas into the GHSZ and form gas hydrates, utmost attention should be paid to them during hydrocarbon exploration. [ABSTRACT FROM AUTHOR]

Radiometric dates of key rock units indicate that a remnant Late Mesozoic ocean of the Huatung Basin is still preserved today east of the South China Sea (SCS). We integrate regional geology with a Cretaceous oceanic basement in the vicinity of the Huatung Basin to reconstruct the Huatung Plate east of the Eurasian continent. Results of geophysical investigations, four expeditions of deep-sea drilling and a renaissance of regional geology allow us to propose a hypothesis that the mechanism responsible for the SCS opening was raised from strike-slip fault on the east. The hypothesis suggests that the SCS opening could highly relate to the strike-slip faults inherited from Late Mesozoic structures onshore–offshore the SE Cathaysia Block to develop rhombic-shaped extensional basins en echelon on the thinned Eurasian continental crust in the Early Cenozoic. It was followed by sinistral strike-slip movements along the boundary between the Eurasian Plate and the Huatung Plate driven by oblique subduction of the Huatung Plate to the northwest coupled with slab-pull force by southward subduction of the Proto-SCS to open up the triangle-shaped oceanic East Sub-basin in the Early Oligocene (33/34 Ma). The spreading ridge then propagated southwestward in the step-over segment between the Zhongnan-Lile and the Red River strike-slip fault systems to open the triangle-shaped oceanic Southwest Sub-basin by 23 Ma. The plate boundary fault was subsequently converted into the Manila Trench when the Eocene Sierra Madre arc of the Huatung Plate had moved from the south to its present latitude by the Middle Miocene. [ABSTRACT FROM AUTHOR]

Taiping Island, also known as Itu Aba, is the largest natural terrestrial landmass in the South China Sea and is centrally located. Using bathymetry and marine multi-channel seismic data, we explored the seismic stratigraphic features of the offshore and isolated carbonate platform north of Taiping Island. The western flank of the carbonate platform is characterized by an intercalation between high-amplitude and low-amplitude reflections, showing the landward and seaward migration of the platform foreslope deposits. In addition, there are two offshore carbonate build-ups that are underlain by normal faults. Six sequence boundaries and five depositional sequences caused by eustatic sea level cycles are identified and correlated with the eustatic sea level change chart. Although the evolution of the seismic sequences is partly controlled by local tectonics, the overall stacking pattern of the sedimentary strata in our study area reveals five third-order cycles and one second-order cycle, which is in accordance with the eustatic sea level chart. Additionally, the formations of the Western Taiping Seamount Group and the Zhenghe-Daoming Trough are preliminarily analyzed based on seismic data. [ABSTRACT FROM AUTHOR]

Temperature measurements carried out on 9 hydrocarbon exploration boreholes together with Bottom Simulating Reflectors (BSRs) from reflection seismic images are used in this study to derive geothermal gradients and heat flows in the northern margin of the South China Sea near Taiwan. The method of Horner plot is applied to obtain true formation temperatures from measured borehole temperatures, which are disturbed by drilling processes. Sub-seafloor depths of BSRs are used to calculate sub-bottom temperatures using theoretical pressure/temperature phase boundary that marks the base of gas hydrate stability zone. Our results show that the geothermal gradients and heat flows in the study area range from 28 to 128°C/km and 40 to 159mW/m2, respectively. There is a marked difference in geothermal gradients and heat flow beneath the shelf and slope regions. It is cooler beneath the shelf with an average geothermal gradient of 34.5°C/km, and 62.7mW/m2 heat flow. The continental slope shows a higher average geothermal gradient of 56.4°C/km, and 70.9mW/m2 heat flow. Lower heat flow on the shelf is most likely caused by thicker sediments that have accumulated there compared to the sediment thickness beneath the slope. In addition, the continental crust is highly extended beneath the continental slope, yielding higher heat flow in this region. A half graben exists beneath the continental slope with a north-dipping graben-bounding fault. A high heat-flow anomaly coincides at the location of this graben-bounding fault at the Jiulong Ridge, indicating vigorous vertical fluid convection which may take place along this fault. [ABSTRACT FROM AUTHOR]

Based on the multi-channel seismic (MCS) and gravity data offshore north of Taiping Island (Itu Aba Island) in the Spratly Islands (Nansha Islands), we revisited the crustal structures in the northern part of the southern margin of the Southwest (SW) Sub-basin of the South China Sea (SCS). The MCS data suggest that the basement structural highs in the southwest margin of the SCS are dominated by both fault blocks and volcanic basement structures that probably formed along with the basement faults. The gravity modeling results reveal that these volcanic basement structures were probably associated with the high velocity or density lower crust (HVDLC), suggesting that the southern margin of the SW Sub-basin serves as an intermediate margin. Based on the tectonic features in the SCS margins, including (1) the transition between the magma-poor margins of the Northwest (NW) Sub-basin and intermediate margins of the East and SW Sub-basins; (2) the cessation of the seafloor spreading in the NW Sub-basin being succeeded by a major ridge jump event, which was very likely to reflect an active mantle upwelling event; (3) the extensive distribution of the HVDLC beneath in the well-investigated northern margin of the East Sub-basin and as explored in the SW Sub-basin in this study; and (4) basement faulting and volcanism shown in this study that may be related to the HVDLC and hyper-extension, we proposed a conceptual model to interpret the origin of the HVDLC in the southern margin of the SW Sub-basin. The margin of the SCS basin was magma-poor at the time it occurred. An active mantle upwelling event caused by small-scale mantle convection, possibly related to the influence of the nascent Hainan mantle plume, was formed subsequently, resulting in a southward ridge jump and the HVDLC beneath the current margins of the East and SW Sub-basins. The East and SW Sub-basins continued to spread, latitudinally dividing their HVDLCs as the margins separated accordingly. Afterwards, the hyper-extension in the distal margins may also be responsible for a crust weakening zone wherein the subsequent volcanic events could occur. [ABSTRACT FROM AUTHOR]

We re-examined the cessation of the South China Sea seafloor spreading by using radioactive dating and fossil assemblage of the ETO, and the Te-derived age. The youngest available age for the fragment of the South China Sea is ∼15 Ma and is more consistent with those suggested by previous studies. Moreover, recent IODP Expedition preliminary results suggested that the hemipelagic claystone layer stratigraphically bounded by igneous sequences of massive lava flow rock is Early Miocene in age. Therefore, the previous interpretations are more inclusive than those provided by Barckhasen et al. (2014) and indicate that a slow spreading ridge was more plausible to have occurred during the seafloor spreading of the South China Sea. [ABSTRACT FROM AUTHOR]