Xinong Xie

Evidence for episodic expulsion of hot fluids along faults near diapiric structures of the Yinggehai Basin, South China Sea

Abstract Diapiric structures are well developed and occur in most of the central part of the Yinggehai Basin, on the western side of the South China margin. A strong thermal anomaly due to hot fluid flows occurs in the diapiric zone, as evidenced from vitrinite reflectance ( R o ), clay mineral transformation, and fluid inclusion homogenization temperatures. This anomaly results from hydrothermal fluid flow along vertical faults from overpressured compartments into the overlying Late Miocene and Quaternary sand-rich layers. The magnitude of thermal anomaly is related not only to the distance to which the vertical fault is hydraulically open, but the permeability of rocks interconnected with the faults. Intense heat transfer for convection of fluids occurs in the sand-rich intervals adjacent to vertical faults. Abnormal organic-matter maturation, together with rapid transformation of clay minerals, which occurs at certain intervals within the present-day normally pressured system and normal conductive temperatures in a diapir, can be used to identify palaeo high pressure zones. Abnormal high temperatures measured from a drill-stem test in a diapir can be inferred to be the results of recent expulsion of hydrothermal fluid flow. The results of this study suggest that thermal fluid expulsion along faults plays an important role in the modification of thermal regimes, the enhancement of organic-matter maturation, and rapid transformation of clay minerals, as well as the accumulation of hydrocarbons in diapiric structures of the Yinggehai Basin, South China Sea.

Depositional characteristics and spatial distribution of deep-water sedimentary systems on the northwestern middle-lower slope of the Northwest Sub-Basin, South China Sea

Based upon 2D seismic data, this study confirms the presence of a complex deep-water sedimentary system within the Pliocene-Quaternary strata on the northwestern lower slope of the Northwest Sub-Basin, South China Sea. It consists of submarine canyons, mass-wasting deposits, contourite channels and sheeted drifts. Alongslope aligned erosive features are observed on the eastern upper gentle slopes (<1.2° above 1,500xa0m), where a V-shaped downslope canyon presents an apparent ENE migration, indicating a related bottom current within the eastward South China Sea Intermediate Water Circulation. Contourite sheeted drifts are also generated on the eastern gentle slopes (~1.5° in average), below 2,100xa0m water depth though, referring to a wide unfocused bottom current, which might be related to the South China Sea Deep Water Circulation. Mass wasting deposits (predominantly slides and slumps) and submarine canyons developed on steeper slopes (>2°), where weaker alongslope currents are probably dominated by downslope depositional processes on these unstable slopes. The NNW–SSE oriented slope morphology changes from a three-stepped terraced outline (I–II–III) east of the investigated area, into a two-stepped terraced (I–II) outline in the middle, and into a unitary steep slope (II) in the west, which is consistent with the slope steepening towards the west. Such morphological changes may have possibly led to a westward simplification of composite deep-water sedimentary systems, from a depositional complex of contourite depositional systems, mass-wasting deposits and canyons, on the one hand, to only sliding and canyon deposits on the other hand.

Geochemistry of Pore Water and Associated Diagenetic Reactions in the Diapiric Area of Yinggehai Basin, Northwestern South China Sea

This study examined the geochemical features of pore water in the diapiric area of the Yinggehai Basin, northwestern South China Sea, and illuminated the origin and evolution of basin fluids. Pore water with low salinity occurs in marine sediments in the diapiric area even without meteoric water infiltration. The presence of low-salinity water within deep, overpressured compartments is assumed to be due to smectite-illite transformation. Howerver, in shallow portions (less than 2 000 m) of diapiric areas with normal pressure, pore water has a much wider variation and much lower salinity than that in the overpressured intervals. Its total dissolved solid (TDS) content is ∼5 336 to 35 939 mg/L. Moreover, smectite and chlorite content sharply decreases as kaolinite and illite content increase in shallower intervals. The geochemical variation of pore water in diapiric structures indicates the expulsion of low-salinity, overpressured fluids along vertical faults. Strong injection of hot fluids from deep overpressured sediments results in rapid clay mineral transformation in shallow reservoirs. Consequently, fluid mixing due to fluid expulsion from deeper overpressured deposits leads to variation in salinity and ionic composition as well as some diagenetic reactions. This includes transformation of clay minerals caused by the higher temperatur of deeper hot fluids, e.g., the transfromation of smectite to illite and chlorite to kaolinite. Therefore, variations in salinity and ionic compositions in various pressured systems provide a clue to flow pathways and associated diagenetic reactions.

Seismic features and origin of sediment waves in the Qiongdongnan Basin, northern South China Sea

Sediment waves have been documented around the world for several decades, and their origins are still debated because of their various characteristics in different settings. Based on numerous high-resolution seismic profiles and two boreholes, sediment waves are identified in deepwater areas of the eastern Qiongdongnan Basin, and their distribution and seismic features are illustrated. Combined with the bathymetry, the potential origins of these sediment waves are discussed. Drilling in the central canyon revealed that the channel infill comprises some along-slope fine-grained turbidites, which are good reservoir for gas plays. The sediment waves are distributed on the banks of the central canyon and their seismic features indicate that most of them are caused by turbidity current overflows along the canyon. Although previous researches on these sediment waves suggested that they were of westward-flowing contourite origin, detailed topographic map derived from the seafloor reflector on seismic data shows that there is a N–S trending ridge at the east part of sediment wave zones, which could block and divert the bottom current. According to the geometry of sediment waves, the flow thicknesses across the entire wave field are calculated as 280–560xa0m, and the current velocity falls in the range of 30–130xa0cm/s, which would favor a fine-grained composition and could be a good reservoir because of the better sorting of turbidites than contourites or other gravity flow deposits.

Post-rift tectonic reactivation and its effect on deep-water deposits in the Qiongdongnan Basin, northwestern South China Sea

The post-rift evolution of extensional basins is traditionally thought to be dominated by thermal subsidence due to cessation of the major fault activity during the post-rift stage. The Qiongdongnan Basin, which is located in the northwestern continental margins of the South China Sea, has exhibited significant deviations from typical post-rift characteristics. In the basin, a distinct tectonic reactivation occurred since the Late Miocene (11.6xa0Ma). Three notable aspects of the observed tectonic reactivation during the post-rift stage include, (1) pre-existing fault reactivation, (2) multiple large-scale magmatic intrusions, and (3) rapid post-rift subsidence. During this period the basin infill significantly changed in depositional environments shifting rapidly from littoral-neritic to bathyal-abyssal environments since Late Miocene. The pre-existing fault activity along the No. 2 fault of the basin resulted in the formation of initial shelf breaks and led to the development of continental slope. In addition, the pre-existing faults along the Central Depression zone created a small sub-basin with distinctive axial negative topography characteristics formed between structural highs. These geomorphological changes led to the formation of the Central Canyon. Large-scale magmatic intrusions occurred along the fault zone in the Central Depression of the basin during the post-rift stage. Those deviations, as evidenced from pre-existing fault reactivation, magmatic intrusions, and rapid post-rift subsidence in the Qiongdongnan Basin is believed to be related to the Hainan Plume event.

Controlling factors on the submarine canyon system: A case study of the Central Canyon System in the Qiongdongnan Basin, northern South China Sea

Based on an integrated analysis of high-resolution 2D/3D seismic data and drilling results, this study analyzes the tectonic-sedimentary evolution of the Qiongdongnan Basin (QDNB) since the late Miocene, and discusses the controlling factors on the formation and development of the Central Canyon System (CCS). The sediment failures caused by the relative sea level falling might have discharged deposits from the slope to the canyon. The two suits of the infillings, i.e., turbidites and mass transport complex (MTC), were derived from the northwestern source and northern source, respectively. The sediment supplies, which differ significantly among different areas, might have led to the variations observed in the internal architectures. Tectonic transformation around 11.6 Ma had provided the tectonic setting for the CCS and formed an axial sub-basin in the central part of the Changchang Depression, which could be called the rudiment of the CCS. The tectonic activity of the Red River Fault (RRF) at about 5.7 Ma might have strengthened the hydrodynamics of the deposits at the junction of the Yinggehai Basin (YGHB) and the QDNB to trigger a high-energy turbidity current. The MTC from the northern continental slope system might have been constrained by the Southern Uplift, functioning as a barrier for the infillings of the CCS. Thanks to a sufficient sediment supply during the Holocene period and the paleo-seafloor morphology, the relief of modern central canyon with the starving landform in the eastern Changchang Depression might have been accentuated by deposition of sediments and vertical growth along the canyon flanks, where collapse deposits were widely developed. Corresponding to the segmentation of the CCS, the forming mechanisms of the canyon between the three segments would be different. The turbidite channel in the head area had likely been triggered by the abundant sediment supply from the northwestern source together with the fault activity at about 5.7 Ma of the RRF. The formation and evolution of the canyon in the western segment were caused by combined effects of the turbidite channel from the northwestern source, the MTC from the northern continental slope, and the paleo-seafloor geomorphology. In the eastern segment, the canyon was constrained by the tectonic transformation occurring at approximately 11.6 Ma and the insufficient sediment supply from the wide-gentle slope.

Difference in full-filled time and its controlling factors in the central canyon of the Qiongdongnan Basin

Based on the interpretation of high resolution 2D/3D seismic data, sedimentary filling characteristics and fullfilled time of the Central Canyon in different segments in the Qiongdongnan Basin of northwestern South China Sea have been studied. The research results indicate that the initial formation age of the Central Canyon is traced back to 11.6 Ma (T40), at which the canyon began to develop due to the scouring of turbidity currents from west to east. During the period of 11.6–8.2 Ma (T40–T31), strong downcutting by gravity flow occurred, which led to the formation of the canyon. The canyon fillings began to form since 8.2 Ma (T31) and were dominated by turbidite deposits, which constituted of lateral migration and vertical superposition of turbidity channels during the time of 8.2–5.5 Ma. The interbeds of turbidity currents deposits and mass transport deposits (MTDs) were developed in the period of 5.5–3.8 Ma (T30–T28). After then, the canyon fillings were primarily made up of large scale MTDs, interrupted by small scale turbidity channels and thin pelagic mudstones. The Central Canyon can be divided into three types according to the main controlling factors, geomorphology-controlled, fault-controlled and intrusionmodified canyons. Among them, the geomorphology-controlled canyon is developed at the Ledong, Lingshui, Songnan and western Baodao Depressions, situated in a confined basin center between the northern slope and the South Uplift Belt along the Central Depression Belt. The fault-controlled canyon is developed mainly along the deep-seated faults in the Changchang Depression and eastern Baodao Depression. Intrusion-modified canyon is only occurred in the Songnan Low Uplift, which is still mainly controlled by geomorphology, the intrusion just modified seabed morphology. The full-filled time of the Central Canyon differs from west to east, displaying a tendency of being successively late eastward. The geomorphology-controlled canyon was completely filled before 3.8 Ma (T28), but that in intrusion-modified canyon was delayed to 2.4 Ma (T27) because of the uplifted southern canyon wall. To the Changchang Depression, the complete filling time was successively late eastward, and the canyon in eastern Changchang Depression is still not fully filled up to today. Difference in full-filled time in the Central Canyon is mainly governed by multiple sediment supplies and regional tectonic activities. Due to sufficient supply of turbidity currents and MTDs from west and north respectively, western segment of the Central Canyon is entirely filled up earlier. Owing to slower sediment supply rate, together with differential subsidence by deep-seated faults, the full-filled time of the canyon is put off eastwards gradually.

True volumes of slope failure estimated from a Quaternary mass-transport deposit in the northern South China Sea

Submarine slope failure can mobilize large amounts of seafloor sediment, as shown in varied offshore locations around the world. Submarine landslide volumes are usually estimated by mapping their tops and bases on seismic data. However, two essential components of the total volume of failed sediments are overlooked in most estimates: a) the volume of sub-seismic turbidites generated during slope failure and b) the volume of shear compaction occurring during the emplacement of failed sediment. In this study, the true volume of a large submarine landslide in the northern South China Sea is estimated using seismic, multibeam bathymetry and ODP/IODP well data. The submarine landslide was evacuated on the continental slope and deposited in an ocean basin connected to the slope through a narrow moat. This particular character of the sea floor provides an opportunity to estimate the amount of strata remobilized by slope instability. The imaged volume of the studied landslide is ~1035±64 km3, ~406±28 km3 on the slope and ~629±36 km3 in the ocean basin. The volume of sub-seismic turbidites is ~86 km3 (median value) and the volume of shear compaction is ~100 km3, which are ~8.6% and ~9.7% of the landslide volume imaged on seismic data, respectively. This study highlights that the original volume of the failed sediments is significantly larger than that estimated using seismic and bathymetric data. Volume loss related to the generation of landslide-related turbidites and shear compaction must be considered when estimating the total volume of failed strata in the submarine realm.

New insights on the origin of the basement of the Xisha Uplift, South China Sea

The study of basement geochronology provides crucial insights into the tectonic evolution of oceans. However, early studies on the basement of the Xisha Uplift were constrained by limited geophysical and seismic data; Xiyong1 was the only commercial borehole drilled during the 1970s because of the huge thickness of overlying Cenozoic strata on the continental margin. Utilizing two newly-acquired basement samples from borehole XK1, we present petrological analysis and zircon uranium (U)-lead (Pb) isotope dating data in this paper that enhance our understanding of the formation and tectonic features of the Xisha Uplift basement. Results indicate that this basement is composed of Late Jurassic amphibole plagiogneisses that have an average zircon 206Pb/238U age of 152.9±1.7 Ma. However, the youngest age of these rocks, 137±1 Ma, also suggests that metamorphism termination within the Xisha basement occurred by the Early Cretaceous. These metamorphic rocks have adamellites underneath them which were formed by magmatic intrusions during the late stage of the Early Cretaceous (107.8±3.6 Ma). Thus, in contrast to the Precambrian age (bulk rubidium (Rb)-strontium (Sr) analysis, 627 Ma) suggested by previous work on the nearby Xiyong1 borehole, zircons from XK1 are likely the product of Late Mesozoic igneous activity. Late Jurassic-Early Cretaceous regional metamorphism and granitic intrusions are not confined to Xisha; rocks have also been documented from areas including the Pearl River Mouth Basin and the Nansha Islands (Spratly Islands) and thus are likely closely related to large-scale and long-lasting subduction of the paleo-Pacific plate underneath the continental margins of East Asia, perhaps the result of closure of the Meso-Tethys in the South China Sea (SCS). Controversies remain as to whether, or not, the SCS region developed initially on a uniform Precambrian-aged metamorphic crystalline basement. It is clear, however, that by this time both Mesozoic compressive subduction and Cenozoic rifting and extension had significantly modified the original basement of the SCS region.

Sedimentary evolution of the Central Canyon System in the Qiongdongnan Basin, northern South China Sea

Abstract This study elucidates sedimentary evolution history of the Central Canyon System (CCS), a large axial submarine canyon in the Qiongdongnan Basin (QDNB), northern South China Sea. The geomorphological characteristics and infill architectures of the CCS are summarized based on the analysis of two- and three-dimensional seismic data. Based on a comparative analysis of the CCS in different segments and evolutionary stages and in consideration of the tectono-sedimentary conditions of the QDNB four stages of the sedimentary evolution of the CCS can be divided, i.e., initial development stage in the Late Miocene (11.6–5.7xa0Ma), erosion - infilling stage in the Early Pliocene (5.7–3.7xa0Ma), tranquil infilling stage in the Late Pliocene (3.7–1.81xa0Ma), and rejuvenation stage since the Pleistocene (1.81xa0Ma to present). In the late Middle Miocene (~11.6xa0Ma), the rudiment of CCS was developed by a regional tectonic transformation in the eastern part of the basin. In the Early Pliocene, the CCS was further developed from west to east and restrained in the central depression belt of the basin due to abundant sediment supplies from the northwestern and northern provenances, the blocking effect of the southern uplift belt, and the restrictive geomorphological features of the eastern part of the basin. In the Late Pliocene, changes in the sedimentary environment resulted in the development of the CCS in the eastern part of the basin only. Since the Pleistocene, the joint action of climatic factors and geomorphological features of the eastern part of the basin led to the rejuvenation of the CCS.