Xisha Trough Research Articles

Detrital zircons record the provenance and source-to-sink process in the northwestern margin of the South China Sea

The northwestern continental margin of the South China Sea, extending from the broad shelf and canyon-growth slope across the Xisha Trough, represents a relic crustal rift that eventually connects to the abyssal plain. However, the provenance and source-to-sink process of sediments in this special topography remains unclear. In this study, we present the detrital zircon U-Pb geochronology of surface sediments from the northwestern margin of the South China Sea to identify the sediment provenance, track the transport pathway, and understand the source-to-sink process in this margin. The results showed that detrital zircons exhibit a wide range of U-Pb ages from 3,062.9 to 41.5 Ma and are characterized by peaks centered on 140–154 Ma, 240–258 Ma, 425–452 Ma, and 738–991 Ma. Statistical analysis indicated that the clastic sediments on the shelf closely resemble those sourced from the Pearl River system. Conversely, sediments on the slope and abyssal plain demonstrated characteristics indicative of multiple sources, primarily originating from the Pearl River, with a minor contribution from the Red River. Further quantitative study revealed a progressive increase (from 8.2% to 43.2%) in the prevalence of Red River-derived sediments with ascending water depth on the shelf and slope, whereas sediment influx in the deepest sector remains predominantly sourced from the Pearl River. This study emphasizes the controls of topography and currents in modulating the transportation of coarse-grained and fine-grained sediments within the source-to-sink system and provides a favorable solution for reconstructing the sedimentary process in continental margins.

Controls on terrigenous sediment supply to northwestern South China Sea based on a sediment trap record at Xisha Trough

The South China Sea (SCS) is the largest marginal sea in the western Pacific and comprises an ideal natural laboratory for studying deep-sea processes. Previous investigations in the region have primarily focused on surface currents and to a lesser extent on intermediate currents, while observations of bottom currents are scarce, hindering our understanding of deep ocean sediment transport and deposition mechanisms. Here, we carried out grain size measurement and end-member modelling analysis (EMMA) on samples collected by a sediment trap at a water depth of 1500 m in the Xisha Trough in the northwestern South China Sea. Grain size data shows that terrigenous material has particle sizes ranging from 0.3 μm to 745.9 μm and a peak at 4–11 μm. The EMMA result suggests that finer sediment accounting for an average of 48.2% of the sediment represents material transported by deep water currents (End-member 1; EM1), and this is primary sediment source in the region. Coarser sediment contributing to 32.9% of the sediment is the second most significant source (EM3), representing the deposits of surface currents influenced by monsoons and deposited close to the source. A third sediment type (EM2) constituting approximately 18.9% of the sediment exhibits a wide range of grain sizes and rapid changes, indicating transport in the bottom nepheloid layer triggered by anticyclonic eddies. Our study identifies three primary sediment transport mechanisms to the northwestern SCS, bridging present-day observations with paleoceanography and providing a crucial basis for future research on paleoceanographic reconstructions.

Gravity-Seismic Joint Inversion of Lithospheric Density Structure in the Qiongdongnan Basin, Northwest South China Sea

Abstract Qiongdongnan Basin (QDNB), located at the northwestern corner of the South China Sea (SCS), is a key juncture between the extensional tectonic regime in the northern continental margin and the shear tectonic regime in the western continental margin. Analyzing the crustal density structure and tracking the thermodynamic controlling factors are effective approaches to reveal the nonuniform breakup process of the northwestern SCS. Herein, focusing on the obvious tectonic deformation with distinct eastern and western parts in the QDNB, we present the crustal density structures of five profiles and identify the high-density anomaly related to the synrifting mantle underplating and postrifting magmatic intrusions. The crustal density model was constructed from the Bouguer gravity anomaly, ocean bottom seismic profiles, and multichannel seismic reflection profiles. The northern part of QDNB, with normal crustal density, lower surface heat flow of <55 mW/m2, and limited extension factor of 1.25–1.70, is recognized as the initial nonuniform extension continental crust. The mantle underplating beneath the QDNB is identified as a high mantle density of 3.30–3.40 g/cm3 and a high lower crustal density of 2.92–2.96 g/cm3, which is usually recognized by the high-velocity layers in the northeastern margin of SCS. The magmatic intrusions are identified as the high-density bodies ranging from 3.26 g/cm3 at the base to 2.64 g/cm3 at the top, which become stronger from the west to east. The central part of Xisha Trough is featured by the cooling of the heavily thinned lower crust in the final continental rifting stage, which is close to the cold and rigid oceanic crust. Lateral variations in the deep magmatic anomaly should be the crucial factor for the nonuniform breakup process in the northwestern margin of SCS.

Morphological and Sulfur-Isotopic Characteristics of Pyrites in the Deep Sediments from Xisha Trough, South China Sea

Morphological and Sulfur-Isotopic Characteristics of Pyrites in the Deep Sediments from Xisha Trough, South China Sea

Potential triggers and development processes of submarine landslides in the Xisha Trough Basin, South China Sea

Both submarine landslides and submarine canyons can transfer large amounts of terrigenous quantities from the continental shelf to the deep sea. In some cases, they can have a close relationship to their spatial distribution, formation and evolution. Submarine landslides on the shelf–slope can provide early depressions for submarine canyons, representing the early stage of submarine canyons. The continuous downcutting process during the evolution of submarine canyons can steepen the gradient of the canyon sidewalls, which can lead to the occurrence of submarine landslides. However, the role of submarine canyons in the initiation of slope failure has been overlooked. In this study, high-resolution multibeam bathymetry data, submarine image data and 2-D seismic data were used to investigate two adjacent submarine landslides developed on the northern slope of the Xisha Trough and the spatiotemporal relationship of these landslides to the Central Canyon. We identified a steep downslope between the submarine landslides and the Central Canyon. The Central Canyon formation age was much earlier than those of the submarine landslides. The Central Canyon is filled with modern submarine landslide deposits. The characteristics of the multiple failure scarps indicate retrogressive failure of the submarine landslides. We suggest that the submarine landslides developed on the northern slope of the Xisha Trough Basin have finally been captured by the Central Canyon. We determined that the emptying of sediments at the end of the northern slope of the Xisha Trough Basin caused by the development of the Central Canyon caused the end of the northern slope of the Xisha Trough Basin to lose support, which may have affected the slope stability and could have even played a role in the initiation of submarine landslides. Consequently, the slope area along the Central Canyon may be susceptible to slope instability and slope failure processes.

Sea level controls on the provenance of fine-grained sediments in the Xisha Trough, northwestern South China Sea over the last ~30 ka

The Xisha Trough situated in the northwestern South China Sea (SCS), plays a pivotal role in transporting sediments from the Red River into the deep-sea basin of the SCS, and thus is a critical place to examine variations in source-to-sink process of the Red River-derived sediments associated with past climate and oceanography changes. In this study, we analyzed clay minerals, grain size composition, and specific XRF core scanning elements of sediment core XT-1, retrieved from the northern slope of the Xisha Trough, to assess changes in sediment provenance and their primary influencing factors in the studied area over the last ~30 ka. The results show that clay minerals are primarily composed of illite+chrolite with a mean of 89%, followed by kaolinite with a mean of 9%; smectite occurs as a minor component. The mean grain size and XRF core scanning elements exhibit a three-phase change that corresponds well to variations in sediment source since the last glacial period. Over the last 29.8 ka, sea level fluctuations are the primary factor controlling changes in provenances and their contributions in the northern slope of the Xisha Trough. During the last glacial from 29.8 to 20.2 ka with lower sea levels, both the Red River and Taiwan were the primary contributors of fine-grained sediments in the lower continental slope of the Xisha Trough. However, with marine transgressions from the last deglaciation, Taiwan gradually became the dominant source of fine-grained sediments, followed by the Red River. Additionally, during the Last Glacial Maximum and the early to middle Holocene periods, the strengthened deep-water current (DWC) and East Asian summer monsoon, respectively, exerted strong impacts on the contribution of Taiwanese sediments to the Xisha Trough. The changing pattern of primary provenance in the studied core is roughly in line with those observed in the northern SCS, while differing from those in the southern regions of the western Xisha Trough. This discrepancy is likely caused by varying influences of the DWC on these regions.

Active pathways of anaerobic methane oxidization in deep-sea cold seeps of the South China Sea.

Anaerobic oxidation of methane (AOM) is critical for controlling methane emissions from deep-sea cold seeps. Microbial groups associated with different AOM processes have been briefly investigated, but the AOM activities associated with the utilization of different electron acceptors in the deep-sea methane seeps remain largely unknown. Here, surface sediments (0-4 cm) from two cold seeps (Site F/Haima) and the Xisha trough in the South China Sea (SCS) were incubated to measure AOM potentials driven by nitrite, nitrate, and sulfate as different electron acceptors with isotopically labeled methane gas (13CH4), and the population shifts of microbial communities along the incubation were investigated by high-throughput sequencing and quantitative polymerase chain reaction based on both the 16S rRNA gene and functional genes. Nitrite and nitrate were preferentially used prior to sulfate during the incubation, and nitrate-/nitrite-dependent anaerobic methane oxidation (Nr-/N-DAMO) contributed more to the methane flux in both the cold seeps and the trough. The highest rates of DAMO and sulfate-dependent anaerobic methane oxidation (SAMO) were detected at Site F, consistent with the transcript abundance of mcrA and dsrB genes during the incubation, and might be explained by the in situ active functional groups. The former process might be explained by Campylobacteria, which could couple sulfide oxidation and nitrate consumption to further participate in the N-DAMO process, while the latter process might be due to the higher proportions of Desulfobacterota. This study elucidates the diversity and potential activities of major microbial groups in the DAMO and SAMO processes in the deep-sea cold seeps using isotopic tracing incubations and provides direct evidence for the occurrence of Nr-/N-DAMO as a previously overlooked microbial methane sink in the deep-sea hydrate-bearing sediments in the SCS. IMPORTANCE Cold seeps occur in continental margins worldwide and are deep-sea oases. Anaerobic oxidation of methane is an important microbial process in the cold seeps and plays an important role in regulating methane content. This study elucidates the diversity and potential activities of major microbial groups in dependent anaerobic methane oxidation and sulfate-dependent anaerobic methane oxidation processes and provides direct evidence for the occurrence of nitrate-/nitrite-dependent anaerobic methane oxidation (Nr-/N-DAMO) as a previously overlooked microbial methane sink in the hydrate-bearing sediments of the South China Sea. This study provides direct evidence for occurrence of Nr-/N-DAMO as an important methane sink in the deep-sea cold seeps.

Differences in Thermo-Rheological Structure between Qiongdongnan Basin and Pearl River Mouth Basin: Implications for the Extension Model in the Northwestern Margin of the South China Sea

This study combines surface heat flow, multi-channel seismic reflection profiles, and ocean-bottom seismometer (OBS) profiles to determine the thermo-rheological structure of the Qiongdongnan Basin (QDNB) and Pearl River Mouth Basin (PRMB), with the aim of researching the west–east variation of the passive continental margin rifting. Based on the initial lithospheric rheological model of a jelly sandwich-1 (JS-1) regime, the current architecture of the continental margin is identified to be the result of a non-uniform extension. Due to the decoupled crust–mantle relationship caused by the weak lower crust, the non-uniform extension led to the rupture of the mantle lithosphere before the crust. The central Xisha Trough falls into the JS-2 regime with only one brittle load layer, which is close to the rigid oceanic lithosphere of the Northwest Sub-basin (NSB). The high-velocity layers (HVLs) and detachment faults beneath the Xisha Trough are considered to be the result of the cooling of a thinned lower crust with mantle underplating during the middle stage of continental margin rifting. A seaward-increasing trend of lithospheric rheological strength is exhibited across the PRMB, from the crème brûlée-1 (CB-1) regime at the continental shelf to the JS-2 regime at the NSB. Unlike the HVLs of the Xisha Trough, the lower crustal HVLs beneath the eastern PRMB formed during the late stage of continental margin rifting due to the mantle lateral flow. The absence of HVLs beneath the western PRMB may indicate that the mantle lateral flow demonstrates a limited impact.

Biogeographic distributions of microbial communities associated with anaerobic methane oxidation in the surface sediments of deep-sea cold seeps in the South China Sea.

Cold seeps are oasis for the microbes in the deep-sea ecosystems, and various cold seeps are located along the northern slope of the South China Sea (SCS). However, by far most microbial ecological studies were limited to specific cold seep in the SCS, and lack of comparison between different regions. Here, the surface sediments (0-4 cm) from the Site F/Haima cold seeps and the Xisha trough in the SCS were used to elucidate the biogeography of microbial communities, with particular interest in the typical functional groups involved in the anaerobic oxidation of methane (AOM) process. Distinct microbial clusters corresponding to the three sampling regions were formed, and significantly higher gene abundance of functional groups were present in the cold seeps than the trough. This biogeographical distribution could be explained by the geochemical characteristics of sediments, such as total nitrogen (TN), total phosphorus (TP), nitrate (NO3 -), total sulfur (TS) and carbon to nitrogen ratios (C/N). Phylogenetic analysis demonstrated that mcrA and pmoA genotypes were closely affiliated with those from wetland and mangroves, where denitrifying anaerobic methane oxidation (DAMO) process frequently occurred; and highly diversified dsrB genotypes were revealed as well. In addition, significantly higher relative abundance of NC10 group was found in the Xisha trough, suggesting that nitrite-dependent DAMO (N-DAMO) process was more important in the hydrate-bearing trough, although its potential ecological contribution to AOM deserves further investigation. Our study also further demonstrated the necessity of combining functional genes and 16S rRNA gene to obtain a comprehensive picture of the population shifts of natural microbial communities among different oceanic regions.

Quantitative assessment of dissolved inorganic carbon cycling in marine sediments from gas hydrate-bearing areas in the South China Sea

Quantitatively assessing dissolved inorganic carbon (DIC) cycling in methane-rich marine sediments is essential for understanding the contributions of different carbon sources to the global carbon cycle. Here, the data of δ13C-DIC, SO42−, DIC, Ca2+, Mg2+, PO43−, and a reactive transport model were used to analyze the DIC and methane source and calculate the DIC budget in the shallow sediments at two methane seep sites in the Xisha Trough (CL27A) and to the west of the Haima cold seeps in the Qiongdongnan Basin (CL44). Model results show that methane contributed to 92.75% and 79.95% of DIC sources through anaerobic oxidation of methane (AOM) versus only 7.25% and 12.90% by organic matter via organoclastic sulfate reduction (OSR) and methanogenesis at both sites. However, the methane sources vary between the two sites. External thermogenic methane that is decoupled from contemporaneous organic matter deposition drives the AOM at CL27A. A combination of upwardly diffusing external biogenic methane and internal methane derived from in situ methanogenesis drives the AOM at CL44. Internal methane flux from depth reveals the deep methanogenic zone influences the DIC cycling in the shallow sediments at CL44. It is accompanied by the upward diffusive DIC flux that can be identified by the δ13C-DIC data when modeling, which contributes to the remaining 7.15% of the DIC source. Neglecting this DIC source that is set as a fixed value at the lower boundary will misunderstand the DIC cycling in the shallow sediments. This study identified and highlighted the influence of external thermogenic methane and the methanogenic zone on the DIC cycling in marine sediments, and demonstrated assessing DIC cycling accurately in methane-rich marine sediments is important to estimate its contribution to oceanic carbon cycling.

New method for the reconstruction of sedimentary systems including lithofacies, environments, and flow paths: A case study of the Xisha Trough Basin, South China Sea

Existing seismic facies analysis schemes, including traditional seismic facies analysis, seismic slice analysis, and automatic classification of seismic facies, can reconstruct the sedimentary systems of basins; however, they are not ideal for basins with few boreholes and many rock types. Herein, we propose a new method for reconstructing sedimentary systems using seismic data, considering the Xisha Trough Basin in the Northern South China Sea as the research object to demonstrate the principles, methods, and technologies of this novel scheme. Based on the six commonly used seismic parameters, three additional seismic parameters (smoothness, neatness, and special waveform) and one geological marker (provenance) were added to improve the recognition of lithofacies. The scanned seismic interpretation technique was used to describe the distribution of sandstone and other skeleton lithofacies; sedimentary environments were determined according to the coastline, shelf break line, and slope toe line; flow path systems were determined according to the paleotopography, incised valleys, and sandstone distribution; and the sedimentary systems of the trough basin were reconstructed through the superposition of skeleton lithofacies, depositional environments, and flow paths. Results show that the sedimentary system reconstructed using the new method contains three elements, namely, lithofacies, the environment, and flow path, which fully reflect the concept of the sedimentary system defined by Fisher and McGowen (1967) ; this overcomes the inherent defects of previous schemes, such as poor lithofacies recognition and the uncertainty of types and boundaries of sedimentary bodies. This method enabled identifying subdivided seismic lithofacies directly from seismic profiles and enhanced the ability to obtain sedimentary information from seismic data. The proposed reconstruction method can be used in areas without boreholes or with many rock types and is significant for marine geology research and oil and gas exploration conducted in the deep sea.

The genome of a new anemone species (Actiniaria: Hormathiidae) provides insights into deep-sea adaptation

Deep-sea (deeper than 200 m) creatures are poorly understood taxa that live in high-pressure, dark, and cold environments. Here, we provide a high-quality genome assembly of a new deep-sea anemone species (Paraphelliactis xishaensis sp. nov.) living at a depth of 3230 m in the Xisha Trough in the South China Sea. Through comparative genomic analyses, we revealed molecular signatures of deep-sea environment adaptation including the rapid evolution of genes involved in cytoprotective osmolyte metabolism, membrane function, translational activity, and cytoskeletal processes in deep-sea anemone, along with parallel amino acid substitutions in genes between the deep-sea anemone and deep-sea mussel to cope with high hydrostatic pressure or low temperature. Finally, the inferred demographic events of this deep-sea anemone coincide with major post-Pliocene climate transformations, implying that global climate changes also affect creatures in deep-sea zones. Together, these results extend our understanding of the adaptation and evolutionary history of deep-sea creatures.

Seismic Imaging of an Intracrustal Deformation in the Northwestern Margin of the South China Sea: The Role of a Ductile Layer in the Crust

AbstractThe continental margins of the South China Sea (SCS) have undergone episodic rifting since the Cenozoic, and there are ongoing debates surrounding the processes of crustal deformation and seafloor opening. In this work, we present a P‐wave velocity model extending from the north of Xisha Trough to the Zhongshanan Basin in the northwestern SCS margin, using ocean bottom seismometer data of the wide‐angle seismic profile OBS2013‐1. The results show that the crust thins symmetrically across the western Xisha Trough, from more than ∼20 km at the flanks to ∼10 km in the central valley where the sedimentary layers thicken to over 6 km. In the Zhongsha Trough, closer to the deep basin, the upper crust is detached in a ∼20 km wide region and the lower crust has seismic velocities increased by more than ∼0.3 km/s. The top boundary of the lower crust is located at a depth of ∼13 km across the Zhongsha Trough, and a ∼5 km thick midcrustal ductile layer is imaged. A ∼50 km wide ocean‐continent transition region beneath the Zhongshanan Basin characterizes a ∼6 km thick continental crust underlain by serpentinized and magnetized upper mantle. These observations, together with plate reconstructions based on gravity and magnetic analysis, suggest that deformation of the continental margin was controlled by a ductile crustal layer. Magmatism, associated with the early stage oceanic accretion, has mixed with the highly extended continental crust. Developments of the failed rifted basins were controlled by the westward propagation of the continental breakup.

Seismic Recognition and Origin of Miocene Meishan Formation Contourite Deposits in the Southern Qiongdongnan Basin, Northern South China Sea

AbstractResearch into the contourite deposits in the Upper Meishan Formation of the southern Qiongdongnan Basin in South China Sea is weak; their characteristics, distribution and original geological conditions are not clear. Using geological and geophysical methods including seismic and drilling data, based on seismic reflection characteristics, geometrical configuration description, and wave impedance inversion, two types of contourite deposits are recognized. Contourite deposits have blurred boundaries between each deposit and disordered internal seismic reflections; They are mound‐shaped only in transverse section, and banded in the longitudinal direction. Type I contourite deposits are conical, with medium‐high amplitude, low‐continuity, low‐frequency mound‐shaped seismic facies, and subparallel‐chaotic reflections internally. These deposits are conical with sharp tops, the canal between mounds is V‐shaped and deep. The western wing is gentle and the eastern wing is steep, with the slope toe mostly between 10° and 20°, and width height ratio about 1–2. Type II contourite deposits are flat, exhibiting medium‐amplitude, medium‐continuity, low‐frequency mound‐shaped seismic facies, with subparallel weak reflections internally. Their mounds are flat with gently arced tops, with shallow canals between. The slope toe is between 5° and 10°, with a width height ratio of about 2–5. The wave impedance value of these contourite deposits is 4.6 kg/m3×m/s to 6.8 kg/m3×m/s, about 5.8 kg/m3×m/s on average, which is presumed to represent marly‐calcareous clastic sediments. The contourite deposits mainly develop beneath the slope break at the margin of the fault‐controlled platform in the Southern Uplift zone of the basin. In plane view, they are distributed approaching a west‐to‐east direction, and in section, lie in low‐lying areas near the faults at fault‐controlled terraces of the Southern Uplift zone, with a paleo‐current direction nearly west‐to‐east. The paleotectonic setting of the gentle monoclinic platform was favorable for the development of such contourite deposits. The intensification of the Mid‐Miocene deepest bottom current gave rise to the contourite‐forming currents around the Southern Uplift zone in the northern South China Sea, which flow from Hainan Island to the Xisha Trough in a nearly west‐to‐east direction leading to the contourite deposits developing in the late Mid‐Miocene transgressive environment, with multiple slow sea‐level fall cycles.

Deep seafloor plastics as the source and sink of organic pollutants in the northern South China Sea

Large plastic litter (as opposed to microplastics and plastic pellets) could adsorb organic pollutants and thus pose a serious threat to the marine environment. We report high levels of polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) adsorbed to plastic litter sampled from depths of 1800–3100 m in the Xisha Trough region of the northern South China Sea (NSCS). ∑PCBs on plastics ranged from 126.9–142.1 ng/g, with tri-PCBs accounting for 92–97% of the total PCB concentrations in all samples. Levels of ∑OCPs varied from 4280 to 5351 ng/g (average 4690 ng/g), with a total of 19 compounds detected in the seven samples. While no parent DDT (dichlorodiphenyltrichloroethane) was detected, op′-DDE (metabolite of DDT) was most abundant, with concentrations ranging from 947.5–1551.7 ng/g. ∑CHLs (heptachlor + heptachlor epoxide A + heptachlor epoxide B + cis-chlordane + trans-chlordane) ranged from 1083.1–1263.7 ng/g (mean 1153 ng/g) and accounted for 24% of ∑OCPs. Various compositional ratios of HCH (hexachlorocyclohexane) and DDT metabolites improved our understanding of the sources and transport pathways of OCPs. The total absence of DDT may be a “ghost indicator” of no recent DDT inputs into the oceans. There could well be inputs of DDT, but only as the degraded metabolites DDE and DDD when they are adsorbed to seafloor plastic litter. A comparison of HCH isomer ratios in seafloor plastics with technical HCH ratios revealed that HCHs were possibly not from early residues but from later inputs. An ecological risk assessment of the contaminants indicated a high risk from ∑DDTs, p,p-DDE, and γ-HCH in all the sampled locations. Finally, we propose a descriptive model depicting the movements and transportation of PCBs and OCPs from the ocean surface to seafloor plastics in the NSCS.

Anaerobic methane oxidation coupled to denitrification is an important potential methane sink in deep-sea cold seeps

Microbes play a crucial role in mediating the methane flux in deep-sea cold seep ecosystems, where only methane-related microbes have been well studied, while the whole microbial community and their ecological functions were still largely unknown. Here, we utilized metagenomic data to investigate how the structure and metabolism of microbial community shift in the reduced sediment habitats along the spatial scales. Microbial communities in cold seeps and troughs formed two distinct clades likely driven by environmental factors, such as total sulfur, total phosphate and NO3−, rather than geographical proximity. The predominance of Methanosarcinales reflected a high potential for methane production. In addition to the already well-reported ANME-1/SRB consortia, prevalence of bacterial Methylomirabilis and archaeal Methanoperedens as important performers in the n-damo process with respective of nitrite and nitrate as respective electron acceptor was observed in deep-sea hydrate-bearing regions as well. Aerobic methane oxidization was conducted mainly by type I methanotrophs at Site F (Formosa Ridge), but also via the n-damo process by Methanoperedens and Methylomirabilis in the Haima seep and Xisha Trough, respectively. Based on the high abundance of those denitrifying-dependent methane oxidizers and their related functional genes, we concluded that the previously overlooked n-damo process might be a major methane sink in cold seeps or in gas hydrate-bearing sediments if nitrate is available in the anoxic zones. The signature of isotopic labeling would be essential to confirm the contribution of different anaerobic methane oxidizing pathways in deep-sea cold seep ecosystems.

Numerical modeling of failed rifts in the northern South China Sea margin: Implications for continental rifting and breakup

Failed rifts record important information of continental extension and breakup process in the northern South China Sea (SCS) margin. The Tainan Southern Depression and the Baiyun Sag to the east are characterized with lower-crust high-velocity anomalies (LCHVA), and intracrust detachment faults, whereas the Xisha Trough to the west develops on a larger scale with crust-cutting normal faults and absence of LCHVA. These contrasts indicate different rifting processes between the northeastern and northwestern SCS. 2D numerical modeling is performed to understand the formation mechanism of these failed rifts. Two types of mechanisms are proposed: I) syn-rift competitive type and II) rift migration type with a half extension rate of 2 cm/yr and 1.5 cm/yr, respectively. In type I, two rifts develop initially on the shoulders of the weak zone, but they compete with each other during extension. One rift becomes dominant to furnish the final breakup, whereas the other one is abandoned. The crust structure of this type fits the observations in the Baiyun Sag and the Tainan Southern Depression. However, in type II, only one rift develops at the beginning. The initial rifting center will migrate and the final continental breakup will occur at a place far from the initial rifting location. In this type, normal faults cut through whole crust and wide extensional margins will form, such as observed in the Xisha Trough. Our results suggest that the depth-dependent extension of the SCS is strongly heterogeneous, resulting primarily from varying extensional rates.

Rift Structure and Sediment Infill of Hyperextended Continental Crust: Insights From 3D Seismic and Well Data (Xisha Trough, South China Sea)

AbstractThree‐dimensional seismic and well data from the deepwater Xisha Trough are used to investigate the rift structure and sediment infill of a region formed adjacently to the initial oceanic ridge of the South China Sea (SCS). The high‐quality data permitted a detailed analysis of features such as (1) detachment faults soling out at the Moho, (2) rotated and thinned continental blocks covered by thick sediment, and (3) changes in the location of basin depocenters resulting from detachment faulting. During the continental rifting phase (Eocene to earliest Oligocene), faulting was broadly distributed in the Xisha Trough and resulted in the generation of isolated grabens/half grabens filled by proximal sediment sources. During continental breakup in the Northwest Ocean Sector of SCS (Oligocene), extension became restricted to a narrow region where highly tilted continental blocks and thin crust were formed. Sediment was, at that time, fed to distal depocenters, which are presently bounded by listric faults rooted in a basal detachment. Later in a second stage (early Miocene), synchronously with continental breakup in the Southwest Ocean Sector of the SCS, the study area was blanketed by thick sediment. During the two continental breakup events, the hyperextended Xisha Trough was affected by closely spaced, small‐scale faults rather than large basement‐related structures. Our study highlights the effect of continental breakup as a way to broaden sediment influx from multiple sources into deepwater basins. As a corollary, this work recognizes two distinct breakup sequences in the Xisha Trough and concludes on their geodynamic significance to the SCS.

Deep crustal structure imaging of gravity and magnetic anomalies in the Xisha Trough, China

The deep structure of the Xisha Trough (China) is of great significance for describing its tectonic attributes. In this paper, the multi-scale segmentation for anomaly separation and the iteration method for downward continuation are used to obtain the information about high-resolution gravity anomaly and magnetic-source gravity anomaly, which is then conducted with apparent density imaging inversion. According to the analysis of anomalous characteristics, it is considered that the aeromagnetic anomaly in the Xisha Trough (China) should be caused by the reverse magnetization of big tilting angle. The magnetic anomaly is conducted with magnetic-source gravity conversion and apparent density imaging inversion, whose results show that the apparent density imaging of magnetic-source gravity has a good comparability with the apparent density imaging of the Bouguer gravity anomaly; thus, it can reflect the homology characteristics of gravity and magnetism in trough center. At the same time, the Moho bathymetric map of the Xisha Trough (China) based on gravity anomaly inversion indicates that the crusts on the north and south sides of the trough have similar structures. The homology characteristics of gravity and magnetism and the similar and comparable characteristics of crustal structure can reflect that the trough is a typical extensional rift structure and also reveal that the processes of mantle upwelling, crust thinning, generation of tensional fault, and basalt eruption occur in the trough evolution.

Multi-scale Apparent Magnetization Inversion Imaging Method Based on the Theoretical Pseudo-gravity Anomalies

Abstract The complex superposition of magnetic anomalies on the ground caused by multi-source magnetic geologic bodies makes it difficult to invert their geometries and positions. According to the theoretical pseudo-gravity anomalies of the Poisson formula of the gravity and magnetic field, this paper converts the magnetic anomaly dipole source into the centroid anomaly of unipolar source of the gravity, adopts the multi-scale interpolation cutting method to separate the anomaly and utilizes the iterative method to downward continue the multi-scale anomaly to the close-to-source depth of the top interface of each magnetic source so as to acquire the high-resolution magnetic anomaly near the top interface of each source. This magnetization inversion of the anomaly can greatly improve the inversion accuracy and speed. However, the continuation space is always required to meet the first type of boundary conditions of the Laplace equation, which leads to the fact that the close-to-source level may not really reflect the real depth of the anomaly which contains the equivalent effect of depth. Therefore, the method is essentially a fast apparent magnetization imaging method which describes the multi-source and multi-scale geological bodies. The increased vertical resolution may help to better define the magnetic interface morphology, and it can also obtain the physical property distribution of the multi-scale magnetic source geological body in three-dimensional space. The method is demonstrated on the synthetic models and on an aeromagnetic data set from the Xisha Trough, South China Sea.