Northern South China Sea Margin Research Articles

Tectono-thermal evolution from the proximal to hyperextended domains of the northern South China Sea margin since initial rifting

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.

Mesozoic structural evolution of the Northern South China Sea margin using potential field modelling

This study conducts a comprehensive geophysical inspection of the Northern South China Sea Margin (NSCSM), a complex and pivotal area for unraveling the tectonic history of Southeast China. We employ structural interpretation of the gravity and magnetic data by applying processing techniques and 2D forward modelling to delineate and subdivide the primary subsurface structures, and also to identify the sedimentary basins. Our findings reveal that the NSCSM exhibits a succession of extensional forearc basins above a heterogeneous basement, including a Mesozoic Andean-type magmatic arc and a Western Pacific-type marginal oceanic basin. It has substantial variations in crustal densities and susceptibilities across the NSCSM, reflecting its intricate geological history. The integrated interpretation of this study highlights the substantial impact of the Indosinian and Yanshanian thrusting and orogenies. These geological processes have played a significant role in deformation partitioning, leading to the development of elongated troughs and basins intersected by numerous prominent structures trending in the NE, ENE, and EW directions. The Indosinian faults provide insights into the boundary between these structures, which formed due to the Dian-Qiong Suture and the Paleo-Pacific Plate subduction. These tectonic processes have major structural trends, particularly impacting the offshore forearc region in the NE and EW directions, characterized by deep basins overlying thin crust. Our study also identifies multiple minor trends, including horse-tails and secondary fault systems of the Yanshanian origin, which inherited from the regional tectonics. These trends suggest that post-orogenic stresses and shearing have played a role in rotational and ridge jump processes during the opening and closing phases of the South China Sea (SCS). Furthermore, an uneven relationship exists between the calculated magnetic anomaly and the thickness of Cenozoic sediments, with a significant increase in sediment depth observed in the southern NSCSM compared to both crustal thickness and its heterogeneity.

A prospect analysis of the Upper Oligocene sandstone reservoirs of the Wenchang 9 block, Pearl River Mouth Basin, northern South China Sea margin

Faults can act as a migration pathway and/or a lateral seal in structural traps in the Wenchang 9 block (WC9B) oil field, affecting the potential for enhanced hydrocarbon recovery in the field. In this study, three-dimensional (3D) seismic survey and well data are used to characterize Upper Oligocene fan delta - tidal flat heterogeneous reservoirs and investigate fault seal potential in two new structural traps interpreted in the WC9B oil field. Five lithofacies are identified in the Upper Oligocene Zhuhai Formation based on the well log analysis and core description. The fan delta front and sand-rich tidal flat sandstone of facies F1 in reservoir unit A of the middle Zhuhai member is characterized by the best porosity and permeability. The RMS amplitude attribute analysis and facies modeling help to predict gradual upward progression of the F1 fan delta deposits from the proximity of the Shenhu Uplift to the NW into the basin for ∼ 7–8 km during accumulation of the middle member. The NE-dipping conjugate normal faults of lower throw in the hanging wall of the NW-dipping main normal basement faults contributed to the formation of hydrocarbon-bearing structural traps and side seals. Two new structural traps in the middle member of Zhuhai Formation interpreted in the WC9B oil field. The traps are 3-way dip closure bounded by the NE-dipping normal faults of the conjugate set that are characterized by good sealing potential in the interval of the middle member. The results of this study contribute to the optimization of the enhanced hydrocarbon recovery of the WC9B oil field.

Slope instability and submarine-moat initiation: Insights from the northern South China sea margin

Submarine landslides that occur on continental margins can reshape the seafloor morphology and form steep slide scarps. Although the emplacement processes of submarine landslides have been well studied, the development of post-slide deposition has long been overlooked. Here, we use high-resolution multibeam bathymetric and three-dimensional seismic data from the northern South China Sea margin to decode how slide scarps affect bottom currents and generate associated bottom current-related features. Distinct features have been identified within the slide scars, such as sediments waves and drifts. In parallel, multiple longitudinal negative topographies are distinguished along the scarps, undercutting into underlying strata with width and depth up to 240 m and 20 m, respectively. These incisional structures are interpreted as moats, formed following the emplacement of landslides, and running along the foot of scarps that parallel to contours. The origin of these moats could thus represent an interaction between slide scars and contourite deposition. We suggested that escarpments act as morphological barriers, regionally affecting the velocity and direction of bottom currents, leading to an intensification of their velocities and their erosive ability. Furthermore, we propose a co-evolution model of submarine landslides and related moats, which elucidates and emphasizes the significance of scarps in moat initiation and evolution. Our results demonstrate that the interaction between escarpments and bottom current flow can lead to topographic remodeling and sedimentary processes on varying continental margins.

Crustal structure of the Xisha block and implications for along-strike segmentation in the northern South China Sea continental margin

Continental blocks complicate the pre-rift lithospheric inheritance of passive continental margins, hence exert unique influences on rifting process. Along-strike rifting contrasts of the continental margins may reflect the pre-rift crustal heterogeneity and segmented rifting processes. A wide-angle seismic profile acquired on the Xisha Block, the northwestern margin of the South China Sea (SCS), provides the first constrains on the along-strike crustal structures of the continental block. Seismic imaging of the profile reveals a continental crust with relatively uniform thickness of about 22 km and the thickness ratio of the upper crust to lower crust at 0.8. Crustal stretching factor is approximately 1.5, and high-velocity anomalies are absent in the lower crust. Crustal velocities of the upper and lower crust are approximately 5.4–6.5 km/s and 6.6–6.9 km/s, respectively. The results indicate that the Xisha Block underwent slight and uniform extension during rifting, and the crust has not suffered from strong magmatic activities that formed extensive magmatic underplating. As a rigid continental block, the Xisha Block has retarded the rifting onset and shortened the process from lithospheric rifting to breakup in the northwestern SCS margin. Hypothetic models for the rheological heterogeneity of the inherited crust are proposed tentatively to interpret the contrasting distribution of the high velocities in the lower crust along the northern SCS margin.

Subsurface structures and nature of seafloor mounds in the northern South China Sea margin: Implications for Mesozoic hydrocarbon exploration

The Chaoshan Depression in the northern South China Sea (SCS) margin hosts thick (ca. 4–9 km) Mesozoic strata including two sets of hydrocarbon source rocks (the Late Triassic-Early Jurassic and Late Jurassic marine facies sequences). It was predicted to be petroliferous but unproven. Numerous submarine mounds are distributed on its seafloor, but whether they are mud volcanoes or magmatic volcanoes remains controversial due to poor knowledge of the subsurface structures. To reveal the nature of these mounds and the Mesozoic hydrocarbon potential in the Chaoshan Depression, multi-channel seismic (MCS) and ocean bottom seismometer (OBS) data along line CS-L1 across three seafloor mounds are processed and analyzed. The MCS profile reveals the existence of fuzzy reflection zones (FRZs) within the Mesozoic strata beneath these mounds. The overlying sedimentary sequences appear uplifted and deformed. The OBS velocity results exhibit that these FRZs feature significantly lower velocities than their host strata with velocity differences ranging from ∼0.25 km/s to 0.71 km/s, likely resulting from vigorous fluid charging rather than magma intrusion. Thus these FRZs are identified as mud-fluid diapirs. Between these diapirs and the overlying seafloor mounds, numerous high-angle faults exist, serving as vertical fluid feeder conduits. Additionally, several bright spots, acting as indicators of gas reservoirs, are observed near these faults. Moreover, bottom simulating reflectors with reverse polarity are imaged within the shallow strata around the seafloor mounds on an adjacent MCS profile, indicating rich gas trapping and/or gas hydrate accumulation. Therefore, we propose that these seafloor mounds are mud volcanoes with fluids coming from the underlying diapirs. As the diapirs extend downward into the Jurassic and Triassic strata, the hydrocarbon gases feeding the mud volcanoes and diapirs are reasoned to originate from the deep Mesozoic source rocks, implying significant Mesozoic hydrocarbon potential in the Chaoshan Depression of the SCS.

Revisiting the South China Sea MORBs: Mg isotope and whole-rock geochemical constraints

Mid-Ocean Ridge Basalts (MORBs) from the South China Sea (SCS) have attracted great attention from geologists in the past decade, but their petrogenesis still remains controversial. Here, we show that the Mg isotopes of MORBs from Integrated Ocean Drilling Program (IODP) Sites U1431, U1433 and U1434 are heterogeneous and present the mechanisms causing such heterogeneity. The MORBs from Sites U1431, U1433 and U1434 have δ26Mg values ranging from −0.35‰ to −0.21‰, from −0.34‰ to −0.13‰, and from −0.30‰ to −0.20‰, respectively. Together with all the published data, we found the MORBs at Site U1431 with high MgO contents (> 9 wt%) usually have δ26Mg values that are generally lower than the normal mantle (−0.25‰ ± 0.04‰), whereas some of the Site U1433 MORBs are higher than the normal mantle. Shallow-level geological processes like seawater alteration have negligible effects on the Mg isotope fractionation of our samples. Elemental ratio indicative of the degree of partial melting (like Sm/Nd) from the same site also shows no relationship with the δ26Mg variations. Olivine is proposed to have light Mg isotopes. Considering the high MgO (> 9 wt%) contents of low-δ26Mg samples and the petrographic evidence, we propose that their slightly light Mg isotopic compositions of U1431 MORBs may result from the enrichment of olivine, rather than reflecting the mantle source. Moreover, low Ce/Pb ratios and most variable δ26Mg values of some U1433 MORBs suggest that lower continental crustal materials have contributed to their source. As all the previously published MORBs are evolved, we use the Revpet to estimate the mantle melting conditions of SCS MORBs, including the Site U1500 samples recovered from the northern margin of SCS. Compared with the global MORBs data, there is no thermal anomaly in SCS MORBs. Together with our new Mg isotope data, therefore, we propose that the role of the Hainan mantle plume in the opening and spreading of the SCS may be insignificant.

Rock physics diagnostics to characterize early diagenetic processes in hemipelagic calcareous ooze in the northern South China Sea margin

Rock physics diagnostics to characterize early diagenetic processes in hemipelagic calcareous ooze in the northern South China Sea margin

Interactions between pre‐existing structures and rift faults: Implications for basin geometry in the northern South China Sea

AbstractThe northern South China Sea (SCS) margin evolved from the Mesozoic convergent to Cenozoic divergent continental margin, and thus, it developed on a heterogeneous crystalline basement with inherited Mesozoic structures. Pre‐existing structures and their interactions with rift faults have historically not been described or interpreted in the intensely stretched Baiyun sub‐basin. Large‐scale 3D seismic reflection data allow us to identify four types of Mesozoic tectonic fabrics within the basement and explain their genesis: (1) Thin, isolated and north‐dipping seismic reflections 1, interpreted as thrust faults representing orogenic processes. Tilted thick seismic reflections 2 are formed by reactivation of seismic reflections 1 during post‐orogenic extension, which are all related to the NW‐ward subduction of the palaeo‐Pacific plate. (2) Thin, isolated and shallowly dipping seismic reflections 3 and low‐amplitude, semi‐transparent and chaotic seismic reflections 4 represent the low‐angle thrust system and the associated nappe units, which are related to the shift from NW‐ to NNW‐ward subduction of the paleo‐Pacific plate. Subsequently, we investigate the structural interaction between Mesozoic intra‐basement and Cenozoic rift structures. Syn‐rift, post‐rift and long‐term faults are developed in Cenozoic strata, and quantitative statistical and qualitative analyses revealed two main types of structural interactions between them and underlying intra‐basement structures: (1) Rift faults develop with inheritance of intra‐basement structures, including fully and partially inherited faults. (2) Rift faults modify intra‐basement structures, although they are controlled by intra‐basement structures at an earlier stage. Finally, our results reveal the control of pre‐existing structures on the geometry of the Baiyun sub‐basin, especially the selective reactivation of NE‐trending shear zones (SR2), which are influenced by the regional stress field and the width and dip of the shear zones.

Initiation and evolution of an isolated submarine canyon system on a low-gradient continental slope

Submarine canyons are important conduits transferring large volumes of sediment, nutrients, and pollutants from the continental shelf to deep-water basins. However, the mechanisms initiating submarine canyons and the factors influencing their evolution are still poorly understood. Here, we use multibeam bathymetry and two-dimensional seismic reflection data to investigate the origin and development of a submarine canyon system on the northern South China Sea margin. Our results show a submarine canyon system lying at a water depth of 400–1200 m on a relatively low-gradient (<0.5°), open continental slope. At the bottom of this canyon system, buried canyons undercut a mass-transport complex (MTC 1), whose top surface is early Pliocene in age. No other modern or buried canyons, channels and gullies are observed outside the area spanned by MTC 1. Such an observation demonstrates that pre-existing slide scars can capture gravity flows by providing accommodation space for sediment transported onto the lower continental slope, thus facilitating the development of pre-existing channels above MTCs. Lateral accretion packages identified on the southwest walls of several submarine canyons suggest they migrated northeastward due to the influence of contour currents. In addition, the presence of several basal erosional surfaces and smaller-scale MTCs in the canyons confirms they have undergone multiple cut-and-fill cycles, which were likely controlled by relative sea-level changes. The relative high sea level recorded at present ultimately led to the preservation of the studied canyon system on the continental slope. The results not only demonstrate the crucial role of submarine landslides in the initiation of submarine canyons, but also highlight how relative changes in sea level influence the evolution of submarine canyons on low-gradient continental slopes.

How do fault systems and seafloor bathymetry influence the structure and distribution characteristics of gas chimneys?

AbstractGas chimneys are key pathways for geofluid vertical migration; therefore, deciphering their formation and evolution is crucial for hydrocarbon exploration and geohazard risk assessment. However, the influences of ambient conditions (e.g. bathymetry, tectonics, sediment supply flux) on gas chimney development have not been thoroughly investigated. Using high‐resolution 3D seismic data, we have identified 59 gas chimneys beneath the Shenhu Slope (a gas hydrate test production area on the northern South China Sea margin), 35 of which intersect faults. Above fault interfaces, internal seismic structures are dominated by chaotic discontinuous reflections. Internal structures below interfaces display more continuous reflections, which are also apparent in gas chimneys, not intersecting faults. A higher degree of chaotic or discontinuous seismic reflections may indicate more fragmented networks. This may occur due to increased fluid flow along faults and concomitant fluid overpressure. The present‐day undulating seafloor comprises the inter‐canyon (IT) region, intra‐canyon (IN) region and flat slope (FD) region downstream of canyons. Gas chimneys beneath IT and IN regions exhibit elongated elliptical shapes in the plane, with the long axis azimuth (sub‐) parallel to the main strike of canyons. Chimneys beneath the IT region have larger heights than those beneath the IN and FD regions. Thicker sediment in the IT region corresponds to a higher overburden pressure, which may induce stronger overpressure in the subsurface reservoir region. This overpressure may promote chimneys gathering in the IT region. Canyons' main directions are likely to limit hydraulic fracturing due to maximum gradient boundaries between overlying sediment stress fields. This study provides insights into gas chimney distribution, morphology and structure evolution in relation to bathymetry and fault conditions. It contributes to an improved understanding of how geofluids migrate in marginal ocean basins.

New Insights Into the Rift‐To‐Drift Process of the Northern South China Sea Margin Constrained by a Three‐Dimensional Wide‐Angle Seismic Velocity Model

AbstractA three‐dimensional (3D) P‐wave seismic velocity (Vp) model of the crust in the northern South China Sea margin drilled by IODP Expeditions 367/368/368X has been obtained with first‐arrival travel‐time tomography using wide‐angle seismic data from a network of 49 OBSs and 11 air‐gun shot lines. The 3D Vp distribution constrains the extent, structure and nature of the continental, continent to ocean transition (COT), and oceanic domains. Continental crust laterally ranges in thickness from ∼8 to 20 km, a ∼20 km‐width COT contains no evidence of exhumed mantle, and crust with clear oceanic seismic structure ranges in thickness from ∼4.5 to 9 km. A high‐velocity (7.0–7.5 km/s) lower crust (HVLC) ranges in thickness from ∼1 to 9 km across the continental and COT domains, which is interpreted as a proxy of syn‐rift and syn‐breakup magma associated to underplating and/or intrusions. Continental crust thinning style is abrupter in the NE segment and gradual in the SW segment. Abrupter continental thinning exhibits thicker HVLC at stretching factor (β) &lt; ∼3, whereas gentler thinning associates to thinner HVLC at β &gt; ∼4. Opening of the NE segment thus occurred by comparatively increased magmatism, whereas tectonic extension was more important in the SW segment. The Vp distribution shows the changes in deformation and magmatism are abrupt along the strike of the margin, with the segments possibly bounded by a transfer fault system. No conventional model explains the structure and segmentation of tectonic and magmatic processes. Local inherited lithospheric heterogeneities during rifting may have modulated the contrasting opening styles.

Geodynamic processes control sediment routing: Insight from the Earth surface evolution of the northern South China Sea margin and SE Tibetan Plateau

Geodynamic processes control sediment routing: Insight from the Earth surface evolution of the northern South China Sea margin and SE Tibetan Plateau

Large-scale igneous intrusion emplacement as a trigger for fluid seepage on the northern South China Sea margin

Widespread mud eruptions have been identified along the northern margin of the South China Sea. However, the ultimate driving force for initiation of the eruptions remains to be discussed. Here, we investigated the discovered hydrothermal vents and the presence of gas/fluid occurring above deep-seated igneous intrusions and volcanic edifices and consider them as a likely trigger. Using a one-dimensional (1D) numerical model, SILLi 1.0, we studied the thermal effect of a 100-m-thick dolerite sill complex emplaced at 1100 °C and 3.8 km depth within the organic-rich Triassic series (∼3.5 wt% total organic carbon). The modeled scenarios also account for variations in sill count and thickness and the effects of erosion. The calculated CH4 produced from contact metamorphism reaches a peak of ∼700 kg/m2/yr shortly after emplacement of the first sill in the complex. When scaled to a sill size of 100 m by 25 km2, i.e., a sill volume of 2500 km3, ∼42−550 Mt of CH4, depending on individual sill thickness, have been produced within 10,000 yr of sill emplacement from sedimentary organic matter in the aureole, with peak generation of ∼17.5 Mt CH4/yr just after emplacement. The newly discovered vents are suggested to have formed as a direct result of sill emplacement and by overpressure generated in the sediments around the sill by hydrothermal fluids and gases liberated during organic matter degradation. Combining data from seismic images with the likely production of carbon gas released from contact metamorphism with igneous intrusions, we suggest that volcanically induced gas release is common along the South China Sea margin. This study supports the important role of magmatism in driving regional-scale gas/fluid seepage. Our findings also suggest that the hydrothermal vents and intrusive magmatic bodies provide postemplacement fluid focusing pathways to shallower strata or the seafloor over long time scales.

Interactions Between Depositional Regime and Climate Proxies in the Northern South China Sea Since the Last Glacial Maximum

AbstractSedimentary deposits from the northern South China Sea (SCS) can provide important constraints on past changes in ocean currents and the East Asian summer monsoon (EASM) in this region. However, the interpretation of such records spanning the last deglaciation is complicated because sea‐level change may also have influenced the depositional processes and patterns. Here, we present new records of grain size, clay mineralogy, and magnetic mineralogy spanning the past 24 kyr from both shallow and deep‐water sediment cores in the northern SCS. Our multi‐proxy comparison among multiple cores helps constrain the influence of sea‐level change, providing confidence in interpreting the regional climate‐forced signals. After accounting for the influence of sea‐level change, we find that these multi‐proxy records reflect a combination of changes in (a) the strength of the North Pacific Intermediate Water inflow, (b) the EASM strength, and (c) the Kuroshio Current extent. Overall, this study provides new insights into the roles of varying terrestrial weathering and oceanographic processes in controlling the depositional record on the northern SCS margin in response to climate and sea‐level fluctuations.

An approach to determine brittle upper crustal thinning: Insights into crustal extension discrepancy in the central part of Qiongdongnan Basin

Summing fault heaves is the most commonly used method to evaluate upper crustal thinning. However, since fault deformation width (W) is often assumed as a constant in the range of 75–150 km, the stretching factor estimated from fault geometry (βf) accompanies significant uncertainty. Here we propose a new approach to determine brittle stretching factors on the foundation of numerical analyses of W and further compare our results to previous methods, with specific reference to the central part of Qiongdongnan Basin, South China Sea (SCS). Our results suggest that the value of W is generally less than 80 km and mostly less than 50 km in the northern SCS margin. We confirm that applying an overestimated value of W can lead to an underestimated amount of fault-related extension and overstatement of extension discrepancy in the rifted margin. Results also indicate an inverse discrepancy with our new method in the southeast of the basin. The difference in syn-rift sediment thickness across the rifted margin likely drove the lower crust flow causing a transition between inverse and positive extension discrepancies.

Evolution of pull-apart basins with overlapping NE-trending strike-slip fault systems in the northern South China Sea margin: Insight from numerical modeling

A pull-apart basin is a significant type of oil-bearing basin. Its formation mechanism is a hot and challenging issue in geodynamic research that involves the interaction of three-dimensional faults and stress states. In particular, the systematic and quantitative interpretation of the three-dimensional mechanism is not clear. Therefore, we use ASPECT simulation software to analyze how the main dynamic parameters affect the evolution of the basins. The following features were recognised: (1) The greater the degree of overlapping between parallel strike-slip faults, the longer the time of rapid subsidence at the initial stage, but the larger the range of subsidence and the shallower the depth of subsidence at the final stage. (2) All models generate two small basins at the beginning, but they connected together at different stages. The antithetic end-member models of different spacing faults subsided rapidly at the initial stage. However, the two independently developed small basins were connected with time. (3) Based on the comprehensive comparison of numerical modeling results with the evolution of strike-slip faulting in the Pearl River Mouth Basin (PRMB) in the northern South China Sea margin, this paper proposes that the 120° overlapping releasing stepover pattern is likely dominant at the early stage. Furthermore, this study finds a linear relationship between the strike-slip displacements and the depths of basin depocenter. Based on this relationship, the displacements of master strike-slip faults on both sides of the Baiyun Sag of the PRMB are less than 3.5 km. It can further help to understand the formation mechanism of the PRMB.

The paleo-lithospheric structure and rifting-magmatic processes of the northern South China Sea passive margin

The northern South China Sea has complex rifting-magmatic styles, which are significantly distinct from the Atlantic end-member passive margins that initiated rifting in a craton, and was argued to originate from a subducted setting. Here we propose a new kinematic method to calculate the paleo-lithospheric structure and reconstruct the rifting processes along a ∼ 400 km seismic line in the northern South China Sea margin. The detailed paleo-tectonic setting and its linkage with the rifting-magmatic processes are explored. Seismic-gravity analysis reveals a “boudin-type” passive margin with a wide distal domain (>200 km) comprising discrete hyperextended zones of the Baiyun Sub-basin, Liwan Sub-basin and a narrow continent-ocean transition (<30 km). The calculated pre-rift lithospheric thickness declines abruptly from ∼ 100–80 km in the proximal margin to ∼ 40 km in the distal margin, implying that the pre-rift margin was affected by a flat to steep slab of the subducted paleo-Pacific. The steep slab lying beneath the proximal margin formed an arc where the high-density lower crust probably has a pre-rift age. The flat slab beneath the Baiyun Sub-basin to the southern conjugate margin formed a forearc. As the slab steepened and retreated to beneath the Baiyun Sub-basin in rift phase 1 (∼56–38 Ma) and Liwan Sub-basin in rift phase 2 (∼38–33 Ma), the mantle convection and slab dehydration weakened the above lithosphere successively, promoting seaward migration and younging of strong faulting, magmatic underplating, intrusion, and eruption. When slab broke suddenly at ∼ 33 Ma in the continent-ocean transition, the deep hot asthenosphere upwelled to rupture the lithosphere abruptly and induced intense magmatism. Modelling results further show that the paleo-crustal thickness and basement erosion are both fundamentally important in assessing and understanding the crustal thinning.

Deep and surface driving forces that shape the Earth: Insights from the evolution of the northern South China sea margin

The sediments and their temporal and spatial distribution on Earth are the archives of interaction between deep and surface processes. Numerous simulations have attempted to decipher the evolution of landscape and sedimentary basins. However, it remains a challenge to couple paleogeographic evolution with deep geodynamics, especially at the continental margins such as the Northern South China Sea Margin (NSCSM). Here we employ a numerical simulating tool (Badlands) for the tectono-sedimentary evolution of the NSCSM to test and compare our restored interpolated paleo-bathymetries with previously published paleogeography. Based on the subsequent reconstruction of cumulative sediment thicknesses at the distal boundary of the continental margin, we found that the erodibility and elastic thickness of the lithosphere are the best-fitted and the most promising parameters to isostatically deal with deep dynamics in controlling regional topography. According to our findings, the Early Oligocene and Early Miocene are two stages of high erodibility and fast uplifting in the source provenances or weathering regions as well as lithospheric strengthening. In contrast, the lower values of elastic thickness indicate the Eocene rifting and Late Miocene reactivation of extension due to its lithospheric weakening, relative to the post-rift thickening.Plain language summary: Transient deep and surface processes actively participated in the geography of Earth’s landscape and sediment distribution. It takes millions of years for these processes and remains challenging to investigate the past landscape evolution. However, in the era of numerical modelling and more advanced research, it is now possible to solve many scientific questions. This study also attempted to find paleogeographic core findings based on the combined effect of deep and surface processes using a numerical simulation tool, Badland. Finally, we establish an erosion as a surface leading factor and an elastic thickness of lithosphere and mantle convection as the deep dynamic factors, to effect the Earth’s surface landscape and link with the accumulation of sediments.

NE-trending transtensional faulting in the Pearl River Mouth basin of the Northern South China Sea margin

The joint tectonic interaction to the Pearl River Mouth Basin (PRMB) between the collision of the Indo-Australia and Eurasian plates and the subduction of the Pacific Plate (the Philippine Sea Plate) made the present-day complex structures. The basement of the northern South China Sea (SCS) margin has an affinity to the South China Block basement, which has experienced multi-stage tectonic events. The divisions of tectonic units and events, and dynamics of the PRMB are always controversial. Based on the structural analysis to seismic interpretation, and taking the Xijiang, Baiyun and Liwan sags as examples, we conclude that the PRMB sub-basins are transtensional basins. The PRMB can be subdivided into three Cenozoic structural layers in profiles from bottom to top: Early Cenozoic graben, Mid-Cenozoic half-graben and Late Cenozoic depression. In the early Cenozoic (∼55–45 Ma), the Izanagi-Pacific Mid-ocean Ridge subducted beneath the East Asian Continental Margin, and the study area entered the NNW-SSE diffusive extension stage, forming the ENE-trending wide rift. In the middle Cenozoic (45–25 Ma), the Paleo-Pacific Plate subducted from NNW to WNW under the East Asian Continental Margin, and the Mesozoic structure was activated, showing NE-NNE-trending dextral faulting. In the late Cenozoic (since 25 Ma), the Philippines Sea Plate wedged WNW-ward, and the South China Sea began to subduct eastward under the Philippines Sea Plate. Combined with the extrusion of the Indochina Block, the regional stress field was dominated by the NNE-SSW-directed extension, and the NW-trending sinistral structures developed.