Petrology, geochronology and geochemistry of granitoids in the Baiyun Sag, Pearl River Mouth Basin: implications for Paleo-Pacific Plate subduction along offshore SE China

The Pearl River Mouth Basin (PRMB) is located on the northern margin of the South China Sea (SCS). Heat flow measurements indicate that the PRMB is a typical ‘hot basin' characterized by a high background heat flow. However, the tectono-thermal evolution of the PRMB and the mechanisms involved are still controversial due to the different input parameters used in tectono-thermal models, small datasets and the limited area of the basin studied. We used tectono-thermal modelling with a multi-stage finite stretching model, constrained by extensive well data, to systematically analyse the tectono-thermal evolution of the PRMB since the onset of rifting. The study area was expanded relative to previous studies to cover both the proximal and hyperextended regions of the northern SCS margin, providing a more comprehensive understanding of its tectono-thermal history. Numerous wells and seismic data covering the entire basin were used, along with appropriate input parameters, to address the gaps in previous studies and to enhance the accuracy of the results. Our findings indicate that the PRMB underwent two phases of heating, primarily due to thinning caused by lithospheric extension during rifting. By the end of rifting, the Northern Depression Zone and the Kaiping Sag had reached peak heat flow, while the Panyu Lower Uplift and Baiyun Sag saw their highest heat flow since the initial rifting. The PRMB experienced thermal decay during the post-rift stage, but a significant reheating event occurred from 23.03 to 13.82 Ma, likely due to northwards lower crustal flow from the Baiyun Sag. The Panyu Lower Uplift and the Baiyun Sag had reached their peak heat flow by 13.82 Ma and the heat flow generally increased by 1–3 mW m −2 in the other studied areas during this stage. The basin's thermal decay slowed at 5.33 Ma and localized heating events linked to magmatic activities in the southern Dongsha Uplift were observed. In general, the proximal domain of the northern SCS margin reached peak heat flow by the end of rifting, whereas the hyperextended domain reached peak heat flow by 13.82 Ma. The heat flow in the hyperextended domain was generally higher than that in the proximal domain as a result of the thinner crust of the hyperextended domain caused by intense multiple lithospheric detachment–extensional thinning processes. During the rifting stage, the lithospheric extensional thinning process was the most important factor influencing the tectono-thermal evolution of the northern SCS margin, while lower crustal flow and magmatism were the most important factors during the post-rift stage.

Characteristics of geothermal field and tectono-thermal evolution in Baiyun Sag, Pearl River Mouth Basin, China

There is a controversy regarding the source area and weathering intensity of Oligocene to Lower Miocene sediments in the Baiyun Sag of the Pearl River Mouth Basin. Detrital modes, geochemistry, heavy mineral data, and detrital zircon U–Pb ages were analysed to understand the provenance, source area weathering, and tectonic setting of Oligocene to Lower Miocene sandstones in the Baiyun Sag. Petrographic data reveal that the sandstones were derived from a recycled orogen. With increasing transport distance, sediment maturity increased correspondingly. The Oligocene to Lower Miocene sandstones in the Baiyun Sag have low compositional maturity and have experienced low to moderate weathering. The sediments of the Zhuhai Formation have experienced relatively strong weathering intensity compared with those of the Enping and lower Zhujiang formations. Due to low degree of illitization and differences in the geochemical compositions of the sediments, the continental island arc data from different boreholes in the Baiyun Sag are more irregularly distributed in the A‐CN‐K plot. The modal composition (e.g., quartz, feldspar, and lithic fragments) and geochemical indices (Al2O3/TiO2 ratio; Al2O3‐TiO2 diagram; discriminant F1‐F2 diagram; Eu/Eu*, La/Sc, La/Co, Th/Sc, and Th/Co ratios; Hf‐La/Th and La/Sc‐Co/Th diagrams; and rare earth element patterns) indicate that the sediments in the Baiyun Sag are mainly derived from felsic source rocks. Although the uplift around the Baiyun Sag includes multistage magmatism and basic rocks, it has little influence on the sediment in the depression, and no sediments were transported by the Han River to the Baiyun Sag. Geochemical data and detrital zircon U–Pb ages suggest that the Pearl River was formed in the Early Oligocene and became the main source for the sediments in this area. The river carried felsic material from the South China Coast forming a large delta on the northern slope of the Baiyun Sag. The ratios of Eu/Eu*, La/Sc, La/Co, Th/Sc and Th/Co, and heavy mineral data indicate that the sediments transported by the Pearl River to the Baiyun Sag were changed from the arc‐dominated coastal zone to the interior of the South China Block during the Late Oligocene to Early Miocene and even reached the south‐eastern margin of the Tibetan Plateau (West Yunnan Plateau). The La‐Th‐Sc and Th‐Sc‐Zr/10 diagrams suggest that the provenance of these sandstones is evolved continental island arc or an active continental margin setting.

Thermal maturity of crude oils in the Baiyun sag, Pearl River Mouth basin, South China Sea: Insights from biomarkers，aromatic hydrocarbon and adamantane compounds

The Pearl River Mouth Basin has experienced multi-stage tectonic activity and developed complex fault systems since the Cenozoic. Limited by the available time-domain seismic data, previous studies have focused on the overall structure and evolution of the basin and much is still unknown about the boundary faults. We analysed the boundary fault system of the Baiyun Sag using 3D depth-migrated seismic reflection data, which provided new insights into the basin structure and migration of the depocentre. Several large boundary faults pass through the Tg basement in the Baiyun Sag. The activity of these faults has been affected by the rotation of the stress field in the region of extension and by magmatism in the basin. There are differences in the structure of the Baiyun Sag from west to east. The western basin is controlled by steeply dipping faults, whereas the eastern basin is controlled by wide, gentle, low-angle faults, resulting in a detachment basin. The four stages of basin evolution can be divided into: (1) an initial synrift stage; (2) a strong synrift stage; (3) a transition stage; and (4) a post-rift stage. The Baiyun Sag has been affected by changes in the intensity of fault activity and the depocentre has gradually migrated from south to north.

The late Cenozoic deep-water channel system in the Baiyun Sag, Pearl River Mouth Basin: Development and tectonic effects

Organic geochemistry of source rocks in the Baiyun Sag of the Pearl River Mouth Basin, South China Sea

The Baiyun Sag is the deepest sag in the Pearl River Mouth Basin in northern continental margin of South China Sea, with the maximum sediment thickness over 12.5 km above the basement including >6.5 km sediments above the 30 Ma breakup unconformity. According to the theoretical models for the rifted basins, the post-rift subsidence is driven solely by the thermal contraction and can be calculated as the function of the lithospheric stretching factor. A method combining the forward modeling and reverse backstripping was designed to estimate lithospheric stretching factor. Using the 2D forward modeling based on the flexural cantilever model, we simulated the multi-rifting process of the Baiyun Sag with constrain of the backstripped profiles. By doing this the lithospheric stretching factor was obtained, and then the theoretical post-rift subsidence was calculated. The calculated theoretical subsidence was much smaller than the observed subsidence given by backstripping. Along the 1530 line in the Baiyun Sag, the anomalous post-rift subsidence is over 2 km in the sag center, and varies slightly to the north and south edges of the sag. This suggests that the anomalous post-rift subsidence continues beyond the sag both in the continental shelf to the north and in the continental slope to the south. The sensitivity tests in the forward modeling process indicate that only the use of low-angle faults (⩽13°) can we simulate the shape of the backstripped profile.

The Zhujiang Formation in the Baiyun Sag, Pearl River Mouth Basin, China, is formed primarily in a deep-water continental slope environment. Its chronostratigraphic framework is based on biostratigraphy and sequence stratigraphy, and its geological dating is based on micropaleontological data. This makes it difficult to obtain precise absolute ages for various geological events. In this study, gamma ray (GR) well log data from Wells Y1, Y2, and Y3 were used as paleoclimate proxies, and spectral and wavelet analyses were used to conduct cyclostratigraphic research. The results show that the Milankovitch cycles were preserved in the Zhujiang Formation in the Baiyun Sag. Stratigraphic cycles controlled by 405 and 95 ka orbital eccentricity, 40.4 ka orbital obliquity, and 23.5 ka orbital precession cycles can be identified; the signal of stratigraphic cycles controlled by the 405 ka long eccentricity cycle is the strongest. The floating astronomical time scale is constructed based on 405 ka orbital eccentricity cycle tuning of the GR series. The precise durations of the Zhujiang Formation in Wells Y1, Y2, and Y3 are 7.13, 6.93, and 7.18 Ma, and the average deposition rates are 4.68, 5.91, and 5.33 cm/ka, respectively. The Zhujiang Formation was divided into 17 fourth-, 76 fifth-, and 174 sixth-order cycles using the 405, 95, and 40.4 ka orbital periods as the dividing scales, respectively. This study provides a quantitative method for high-precision isochronous stratigraphic division and correlation in deep-water sedimentary systems.

Using seismic geomorphology and detrital zircon geochronology to constrain provenance evolution and its response of Paleogene Enping Formation in the Baiyun Sag, Pearl River Mouth Basin, South China sea: Implications for paleo-Pearl River drainage evolution

The Pearl River Mouth Basin is one of the most favorable areas for gas exploration on the northern slope of the South China Sea. Differences of fault patterns between shelf and slope are obvious. In order to investigate the tectonic evolution, five series of analogue modeling experiments were compared. The aim of this study is to investigate how crustal thickness influences fault structures, and compare this to the observed present-day fault structures in the Pearl River Mouth Basin. The initial lithospheric rheological structure can be derived from the best fit between the modeled and observed faults. The results indicate. (1) Different initial crustal rheological structures can produce different rift structures in the Pearl River Mouth Basin. (2) We also model that the Baiyun Sag in the southern Pearl River Mouth Basin may have had a thinned crust before rifting compared to the rest of the basin. (3) The thickness ratio of brittle to ductile crust in southern Pearl River Mouth Basin is less than normal crust, suggesting an initially hot and weak lithosphere. (4) Slightly south of the divergent boundary magma may have taken part in the rifting process during the active rift stage.

Considerable post-breakup extensional deformation is recorded in the continental margins of the South China Sea (SCS). To recognize the nature and origin of the significant deformation during the syn-spreading stage (32–15.5 Ma) in the SCS, we comprehensively analyzed the geometry and kinematics of the faults and contemporaneous magmas in the Baiyun sag, northern margin of the SCS, using high-resolution regional three-dimensional seismic data. The kinematic analyses indicate that the faults in the Baiyun sag are recently formed following the onset of seafloor spreading in the SCS. The faults exhibit multiple episodes of growth history, with three active episodes, 32–29, 23.8–21 and 18.5–16.5 Ma, separated by periods of inactivity. Four volcanic groups comprising 98 volcanic mounds have been identified and described, located separately in the northwestern, the central, the southeastern and the northern slope areas. The occurrence of multiple palaeo-seafloors, complemented by the biostratigraphic and K-Ar dating data, reveals multiple extrusive events of the syn-spreading magmas in the Baiyun sag, with three active periods of 23.8–21, 18.5–17.5 and 17.5–16.5 Ma. This study confirms that the normal faulting has a shared genetic origin with the contemporaneous magmatism during the syn-spreading stage in the deep-offshore Baiyun sag, northern margin of the SCS. The episodic fault growth and magmatic extrusive events reveal that the Baiyun sag has undergone at least three episodic tectonic events during the syn-spreading stage, which evolved in response to the multi-stage seafloor spreading of the SCS.

Sequence stratigraphy and sedimentary characteristics of the shelf-edge delta and slope fan systems in the Late Oligocene, Baiyun Sag, Pearl River Mouth Basin, China

Post-rift submarine volcanic complexes and fault activities in the Baiyun Sag, Pearl River Mouth Basin: New insights into the breakup sequence of the northern South China Sea

Post-rift faulting controlled by different geodynamics in the Pearl River Mouth Basin, northern South China Sea margin