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Temporal variability in bottom water structures has been observed in 37‐h high‐resolution temperature data at the ocean bottom up to 67.4 m on the continental shelf of the northern South China Sea. The water temperature is generally hot (cold) in the downslope (upslope) flow, but rapid temperature change is associated with the cross‐slope flow intensity changes. Downslope, wandering, and upslope phases are classified depending on the flow directions. Ellison scale method is used to evaluate the turbulent mixing. Intensive dissipation is observed in the wandering phase, the vertically integrated dissipation Eɛ ∼ 322 mW/m2 and the eddy diffusivity 〈Kz〉 in 0.01–0.15 m2/s, which is attributed to the critical interaction between the semidiurnal tides and the slope boundary. Eɛ is gradually reduced to 38.1 mW/m2 in the upslope phase, in which the topography is subcritical to the internal tides, while 〈Kz〉 is occasionally promoted to 0.077 m2/s owing to the weaker stratification. The bottom mixed layer is persistently exhibited, but highly intermittent around 13.9 m and varies from 0.2 m to greater than 67.4 m. Its thickness depends strongly on the water stratification and seems not to be directly linked with the turbulent mixing. The bottom unstable layer occurs mainly in the period slightly before and in the first half of the upslope flow phase. The induced convection makes a substantial contribution to the turbulent mixing when the overall mixing is relatively mild. In the intense mixing, even though the convectively driven mixing becomes much stronger, its contribution is less significant. Plain Language Summary: The ocean bottom boundary layer (BBL) is a main area of interaction of physical, geochemical and biological processes. It is characterized by a bottom mixed layer (BML). A mooring equipped with densely distributed thermistors near the seafloor was deployed at a site on the continental shelf in the northern South China Sea for 37 h. The temporal variability of BBL was examined with high‐resolution temperature loggers (sampling rate of 8 Hz). It was observed that the bottom water structures are modulated by internal tides and the bottom current. The bottom mixed layer was persistently exhibited in accompaniment with the occasional appearance of an unstable bottom layer (UL). The thickness of BML was highly intermittent and varied from 0.2 m to greater than 67.4 m. The active bottom mixing was observed either in the energetic dissipation induced by the critical interaction of semidiurnal tide and slope topography or in the weak stratification induced by strong upslope flow. The influence of the convective flow to the bottom mixing was evaluated. It was found that convection makes a substantial contribution to bulk mixing when the bottom mixing is relatively mild, and its contribution is less significant with intense bottom mixing. Key Points: A mixed layer is persistently exhibited with dense‐distributed measurements near bottom, occasionally accompanied with the convective layerActive bottom mixing occurs either in energetic dissipation under critical interaction of tides and topography or in weak stratificationConvection makes substantial contribution to bottom mixing when mixing is mild, and its contribution is less significant with intense mixing [ABSTRACT FROM AUTHOR]

The South China Sea (SCS) is one of the classical example of a non-volcanic passive margin situated within three tectonic plates of the Eurasian, Indo-Australian and Philippine Sea plate. The development of SCS resulted from interaction of various types of plate boundaries, and complex tectonic assemblage of micro blocks and accretionary prisms. Numerous models were proposed for the formation of SCS, yet none can fully satisfy different aspects of tectonic forces. Temporal and geographical reconstruction of Cretaceous and Cenozoic magmatism with the isochrones of major basins was conducted. Our reconstruction indicated the SE margin of Asia had gone through two crustal thinning events. The sites for rifting development are controlled by localized thermal weakening of magmatism. NW-SE extension setting during Late Cretaceous revealed by magmatism distribution and sedimentary basins allow us to allocate the retreated subduction of Pacific plate to the cause of first crustal thinning event. A magmatic gap between 75 and 65 Ma prior to the initiation of first basin rifting suggested a significant modification of geodynamic setting occurred. The Tainan basin, Pearl River Mouth basin, and Liyue basins started to develop since 65 Ma where the youngest Late Cretaceous magmatism concentrated. Sporadic bimodal volcanism between 65 and 40 Ma indicates further continental extension prior to the opening of SCS. The E-W extension of Malay basin and West Natuna began since late Eocene followed by N-S rifting of SCS as Neotethys subducted. The SCS ridge developed between Pearl River Mouth basin and Liyue basin where 40 Ma volcanic activities concentrated. The interaction of two continental stretching events by Pacific followed by Neotethys subduction with localized magmatic thermal weakening is the cause for the non-volcanic nature of SCS. [ABSTRACT FROM AUTHOR]

Abstract The Late Cretaceous is a significant geologic period in the South China Sea area, as it marks the tectonic transition from the early Paleo-Pacific Plate subduction to the subsequent Cenozoic rifting related to the opening of the SCS. However, the Late Cretaceous tectonic evolution of the South China Sea area is yet to be clearly defined, and it is at present largely debated in the literature. In this paper, mainly on the basis of ~240,000 km2 newly acquired 3D seismic reflection data, 94 industrial wells and existing U Pb ages of basement granitoids, we carried out an integrated interpretation, mapping and analysis of the pre-Cenozoic (> 66 Ma) structures within the basement of the Cenozoic rift basin in the northern South China Sea area, that can be crucial for a broad understanding of the Late Cretaceous tectonic evolution of this region. Results of this analysis suggest that three Late Cretaceous fault systems can be identified in the northern South China Sea margin: 1) the WNW-striking thrust system, 2) ENE-striking extensional fault system, and 3) ENE-striking thrust system. These three fault systems are considered to have developed in the Late Cretaceous since they are superimposed on the well dated Late Jurassic to Early Cretaceous (161.6–101.7 Ma) arc-related granitoids that define most of the basement of the Cenozoic rift basin within the study area. This new structural evidence, combined with a review of previous studies, led us to propose a renewed multi-phase geodynamic evolutionary model of the South China Sea area during the Late Cretaceous. The first tectonic event that can be recognized generated the WNW-striking thrust system and is, in this paper, interpreted to have formed as a result of a sinistral transpressional event that took place in the South China Sea area and adjacent areas around 100 Ma at the Early-Late Cretaceous boundary. This transpressional event was likely related to intense oblique convergence between the Paleo-Pacific Ocean Plate and the Eurasia Plate. The second tectonic phase that was identified in this paper (~100 Ma to ~72 Ma) is defined by an extensional event that generated the largely ENE-striking extensional fault systems and associated basins, which is interpreted to be related to the back-arc extension that took place in this period in response to the slab roll back and high-angle subduction of the Paleo-Pacific Plate. This extensional phase eventually led to the opening of the Proto-South China Sea on the SE margin of the proto-South China Block, and it should be distinguished from the Cenozoic rifting phases that are related to the opening of the South China Sea. The third and last tectonic phase that was recognized in this paper is a compressional event that took place in the late stage of the Late Cretaceous (~72 Ma to ~66 Ma) and was responsible for the development of the ENE-striking thrust system. We suggest that this post-subduction (post-Yanshanian) compressional event can be interpreted to have developed in response to process of ridge push related to the sea floor spreading of the Proto-South China Sea. Highlights • Two Late Cretaceous (K 2) thrust systems and one extensional fault system are identified in the northern SCS area • A sinistral transpressional event at the Early-Late Cretaceous boundary in response to a change in plate configuration • An episode of regional extension related to the back-arc extension in the early K 2 in the SCS area • A previously unrecognized compressional event caused by ridge push process of the Proto-SCS opening in the late K 2 • A renewed multi-phase geodynamic evolutionary model of the SCS area during the Late Cretaceous [ABSTRACT FROM AUTHOR]

The northern South China Sea (SCS) continental margin was proposed to be an active margin during the Mesozoic. However, only a few papers discussed the Mesozoic structural evolution in this region. Here, we provide information based on the seismic profile interpretations with age control from biostratigraphic studies and detrital zircon U-Pb dates of well MZ-1-1 in the western Dongsha-Penghu Uplift of the northern SCS continental margin. The industrial seismic profiles reveal evidence for structural inversion as represented by folds and high-angle reverse faults, formed by reactivation of pre-existing normal faults. The inversion event likely started after the Early Cretaceous, and developed in Late Cretaceous, but ceased before the Cenozoic. The areal extent of the structural inversion was restricted in the western Dongsha-Penghu Uplift and was approximately 100 km in width. Based on the paleogeographic reconstruction of SCS, the structural inversion was likely formed by a collision between the seamount (volcanic islands) swarm of the current North Palawan block (mainly the Calamian Islands) and the northern SCS continental margin around Late Cretaceous. [ABSTRACT FROM AUTHOR]