In the past 30 years, the comprehensive study on drilling and refraction/reflection seismic data of the Iberian-Newfoundland magma-lean extensional continental margin in the North Atlantic has greatly improved the understanding of the extensional breaking of the passive continental margin^[1,2]. A new extensional breaking model of the continental margin has been put forward, which emphasizes that the passive continental margin lithosphere experienced the uniform extension in the early upper crust rupture, the strain concentration and towards to ocean migration in the middle and late extension, the brittle-ductile extension and detachment thinning of the lithosphere, the outcropping of serpentine mantle peridotite and the final rupture and separation of the lithosphere, and the formation of a new ocean basin^[1,2], as a result, the continental margin fault-depression basins have the evolution and distribution characteristics of zonation on the plane and stage in time from continent to ocean. Fault depression basins in the early stage of extension appear as narrow rifts controlled by brittle upper crustal fractures and high-angle faults^[3]. In the middle and later stages of extension, wide and deep fault depressions with large subsidence and horizontal fault throw were formed in the necking zone and distal zone of hyperextension zone in lithosphere. This model of extension and breaking of continental lithosphere and formation and evolution of continental margin basin has also been introduced into the study of extension and breaking process of northern continental margin of South China Sea, hence the academic and industrial circles have better understandings on the structures and forming mechanisms of deep-water basins on the continental margin of South China Sea^[4,5,6,7].

In recent years, with the deepening of exploration in Pearl River Mouth Basin, a large number of deep-water wells have been drilled there, and high-resolution 3D seismic data of the basin has been processed and interpreted. These data reveal that this basin has some different characteristics in crust lithosphere structure and basin structure from Iberia-Newfoundland magma-lean extensional continental margin. Preliminary examination shows that the detachment fault reflecting obvious extension and thinning of the crust isn’t one type merely extending to the Moho, rather detachment faults of different scales can be seen in upper crust, middle crust and crust-mantle interface, showing the characteristics of multi-level development. Although influenced by the North Atlantic magma-lean continental margin model, the role of magma participation is not considered in the fault structure analysis of the Pearl River Mouth Basin, in the drilling data of strata in fault depression period of the basin, a large amount of igneous rock has been found^[8]. Through well-seismic correlation, magmatic intrusions and eruptive rocks of various occurrences such as cap rock, bedrock and volcanic mechanism have been revealed on the high-resolution 3D seismic profiles. Meanwhile, the pre-existing structural belt, different lithology and a large number of faults formed by Mesozoic subduction continental margin make the basement structural strength of the Cenozoic Pearl River Mouth Basin very complicated. A large amount of evidence shows that magmatism and pre-existing faults^[9] also participated in the extension and breaking process of the crustal lithosphere, reforming the structure of the faulted basin strongly, and leading to the diversification of structural styles and complex structure of the basin. These are different from the Iberian-Newfoundland magma-lean extensional continental margin in the North Atlantic developing in the stable craton background.

Based on the existing seismic and drilling data and the latest knowledge, we tried to reveal the formation mechanism of diversified faulted structures in the extensional continental margin with the participation of magmatism and preexisting structures by analyzing the partial structural characteristics of the faulted depressions in the Pearl River Mouth Basin.

In this paper, the analysis of structural transformation characteristics of extensional fault depression focuses on magmatism, and on the basis of Ye et al.^[9,10], the characteristics of preexisting faults are analyzed. When discussing the structural mechanism of continental margin extensional fault depression, magmatism and preexisting structures are considered together.

The Pearl River Mouth Basin is a Cenozoic continental margin hydrocarbon-bearing faulted basin located in the northern South China Sea (Figs. 1 and 2). The basin is on the crustal lithosphere gradually thinning from north to south towards the oceanic basin. The basin has the sedimentary filling framework characteristics of lower continental fault depression (Paleocene Shenhu Formation, Eocene Wenchang Formation and Enping Formation) and upper marine depression (Oligocene Zhuhai Formation, Miocene Zhujiang Formation, Hanjiang Formation and Yuehai Formation, Pliocene Wanshan Formation and Quaternary Holocene formation). Zhu I Depression is in the proximal zone with less crustal extension, consisted of Enping Sag, Xijiang Sag, Huizhou Sag, Lufeng Sag and Hanjiang Sang. Baiyun Sag, which belongs to Zhu II Depression, is located in the necking zone where the continental crust is strongly stretched and thinned, and it is a giant sag with the largest basin area and the thickest sedimentary strata, and Liwan Sag is located in the distal zone with crustal hyperextension. These sags are subdivided into several sub-sags according to the controlling faults. Controlled by the tectonic evolution of continental crust thinning degree extensional faulted structures and continental sedimentary environments during Paleocene and Eocene, Baiyun Sag and Zhu IV depression zone have gradually evolved into deep-water sedimentary environment of continental slope since Oligocene^{[4-5, 11]}. Different from the orderly distribution of the faulted depressions formed in the Iberian-Newfoundland extensional continental margin of the North Atlantic^[1,2,3], the faulted depressions in the Pearl River Mouth Basin at the northern continental margin of the South China Sea have diverse structures.

The crustal hyperextension area where the Baiyun and Liwan Sag are located has different crustal thickness and fault depression structure from the intense extensional necking zone and distal zone of the North Atlantic magma-lean continental margin. In the Baiyun main sag, the crystalline crust is only about 6 km thick, and a crust-mantle detachment fault system develops near the Moho surface (Fig. 3). The faults have a horizontal displacement of up to 40 km, rotation and tilting in the upper walls obviously. Thick strata deposited in the fault depression period, with a total thickness of 12 km (from de-compaction recovery). The Baiyun main sag is a wide and deep fault depression with composite structure controlled by crust-mantle detachment fault^{[4, 12]}, where from bottom to top half-grabens controlled by brittle tension and high angle faults, and fault depressions controlled by brittle-ductile detachment faults in crust-mantle, and depressions with ductile thinning subsidence in lower crust stack over each other. But the eastern Baiyun Sag has some differences in crustal structure and characteristics from the Baiyun main sag. In this area, the crust is about 16 km thick, and there is an upper crust detachment fault depression^[12]. The fault displacement is smaller and almost the same in width with Baiyun main sag, suggesting great ductile extension and thinning degree. The main fault plane is slope-flat, the subsidence and sedimentation centers migrated from south to north, and the sedimentary filling is thinner than that in Baiyun main sag. In the southwest step-fault zone of Baiyun Sag, the fault system turns into low angle at the depth of 3-5 km in the upper crust^[11], that is, the detachment faults occur in the upper crust instead of Moho. In the Baiyun West subsag the crust is 12 km thick, and the strata in the half graben controlled by high-angle fault in the early stage of extension are thicker. The subsag had inter-crust detachment faults developing in the later stage, but smaller crustal extension and thinning. Liwan Sag is located to the south of Baiyun Sag, where the crust is about 11 km thick now, and most of the basement surface may be a ductile shear surface with extensional detachment, magmatism and gravity slip.

According to Iberia-Newfoundland magma-lean extensional continental margin model in North Atlantic, narrow strip-shaped half grabens controlled by brittle high-angle faults should develop in the proximal zone with weak crustal extension, but a few fault depressions controlled by low-angle detachment faults have been discovered in recent years in Pearl River Mouth Basin of the proximal zone in northern South China Sea. The research shows that these detachment faults, usually reformed by preexisting faults or magmatism, have detachment planes in the upper crust with shallow buried depth, and give rise to detachment fault depressions with small aspect ratio.

Igneous rocks developed during the fault depression period have been encountered in Lufeng 13, Lufeng 22, Main Xijiang, East Yangjiang, Huizhou 10, Xijiang 23, Haifeng 33 subsags and Enping sag, etc. These subsags all are influenced by upper crust detachment faults, indicating that the strength of the upper crust in the subsags is affected by magma upwelling.

Through 3D seismic analysis, Ye et al. pointed out that some faults in the Pearl River Mouth Basin were obviously activated and developed on the basis of Mesozoic pre-existing faults, and controlled the evolution of fault depression structure^{[9, 10]}. For example, the sag-controlling fault in Enping sag is a low-angle normal fault, which goes down into the crust and connects with Mesozoic thrust faults. Obviously, the preexisting faults, as relatively weak surfaces, are more likely to be reactivated in the later extensional stress field and evolved into extensional faults.

Up to now, drilling in the basin reveals that there were several stages of volcanic activities in the fault depression period, among which the most concentrated period was 41-43 Ma, corresponding to Eocene upper and lower members of Wenchang Formation boundary. Lithologically, magmatic rocks in the fault depression consist of mainly tholeiitic basic magma eruption, and some dacite and andesite volcanic rocks, belonging to calc-alkaline rock series^{[8, 13-14]}. Based on the interpretation and analysis of seismic facies, it is found that volcanic seismic facies appear from the upper member of Wenchang Formation to Enping Formation of Eocene in Baiyun Sag and Liwan Sag of the deep-water area. Previous studies also showed that magmatism generally occurred from the upper Enping Formation to the lower Zhuhai Formation in Liwan Sag^{[12, 15]}, and tuffaceous and pyroclastic materials were observed in core slices of several exploration wells.

The seismic profile in Fig. 4 is located from the south side of Xijiang 36 subsag to the north edge of Dongsha uplift. Detailed well-seismic correlation and seismic interpretation show that the right side of the profile is a Mesozoic remnant fault depression (orange area) and the left side is a Cenozoic fault depression. During the sedimentary period of Shenhu Formation and lower Wenchang Formation (Tg-T83), the depression-controlling fault of Cenozoic depression was fault F2, and the point A in Fig. 4 is the strata rise due to magma intrusion. On the T83 interface, the strata on both sides bulged onlap, indicating that the magmatism occurred after the deposition of the lower member of Wenchang Formation. There are wavy upward reflections in the basement of the fault depression, and some of them enter the fault depression, indicating strong magma intrusion. The study suggests that it is magma intrusion that led to the uplift, warping and denudation of the early fault depressions including F2 fault and Mesozoic fault depressions, and caused the depression-controlling fault F2 during the deposition of lower Wenchang Member to shift to fault F1 during the deposition of upper Wenchang Member. Extending into the basement, Fault F1 and F2 may still belong to the same fault (dotted line in Fig. 4).

Fig. 5 is a seismic profile passing through Lufeng 13 subsag, which shows the strong reformation of magmatic activity to the early fault depression. Exploration well F14 is located on the bulge of the gentle slope of the fault depression, and basalt was encountered in the T84 of the lower member of Wenchang Formation, indicating that magmatism occurred in the late stage of sedimentation of the lower member of Wenchang Formation. On the seismic profile, this set of basalt is connected with the uplift reflection of the basement, and the uplift reflection wave group can extend down the basin boundary fault (the first thick line fault on the left) to the bottom of the profile. It is inferred that the wavy reflection in this basement is magmatic rock invaded into the crust during the sedimentary transition period of the upper and lower members of Wenchang Formation. The seismic profile clearly shows that the intrusion pass through the middle part of the fault, and this part is lifted and bent, forming a seat-like fault shape on the whole. The upper and lower parts of the fault with weak magma intrusion disturbance have steep attitude and basically the same dip angle, which indicates that the boundary fault was a high-angle slab fault in the early stage. The gentle slope side of the fault depression is abnormally distorted, tilted, uplifted and denuded. The above characteristics indicate that the basement was subjected to magmatic intrusion after the half graben (Tg-T84, in the depositional period of the middle and lower parts of the lower member of Wenchang Formation) controlled by the high-angle slab-shaped normal fault was filled with sediments, which on the one hand led to the rotation and tilting of the fault depression, as a result, the attitude of the boundary fault became gentler, and the gentle slope of the fault depression deformed, warped and denuded; on the other hand, the high temperature caused by magmatism made the crust ductile, and extended and detached along the gentler boundary fault, further strengthening the rotation of the hanging wall. According to several exploration wells in Lufeng 13 subsag, the subsag after magmatism had rapid subsidence and expansion of basin boundary outward, in which middle-deep lacustrine source rock deposited widely with the maximum thickness of 200 m (encountered strata)^[16,17].

The extension of Lufeng 13 subsag experienced early brittle fracture of crust, forming a half graben controlled by high-angle fault, and magmatism in the middle and late period. Due to the magmatism, the strength of the crust weakened, resulting in the detachment of brittle-ductile deformation of upper crust. It is not difficult to understand that once magmatism occurred, the crustal strength would change. Under the same extensional strength, local rapid and obvious extensional detachment would occur, and the upper wall of the detachment fault would have rapid rotation and warping, which would result in rapidly subsidence in the subsag and wide and deep undercompensated sedimentary space, thus resulting in the sedimentary environment of middle-deep lake.

Fig. 6 shows the seismic profile of Lufeng 22 subsag, which is also a typical upper crust detachment fault depression reformed by magmatism. The dip angle of the subsag-controlling fault becomes gentler in the shallow part of the upper crust. The fault depression has a long axis of about 35 km, residual width of about 15 km, strata of about 3 500 m thick, and horizontal displacement of the fault of 8 km. Volcanic rocks (gray tuff) of 42.7 Ma and 41.4 Ma (U-Pb age) were drilled on the uplifts on the east and west sides of the subsag. 100m thick basalt of Wenchang Formation was encountered at the bottom of the exploratory well in the subsag, which appears as high impedance and strong amplitude reflections on seismic profile. According to the basalt strong reflection seismic facies (purple area) revealed by drilling, two volcanism periods were identified between T85-T83 strata in the fault depression. Upward wavy reflections can be seen in the basement. Combined with the basalt encountered in drilling, it is speculated that these wavy reflections are magma bodies intruded into the basement. The filling of strata in the fault depression period can be divided into three sections according to the structure: The lower section is the lower part of Wenchang Formation (Tg-T85), which deposited in the dustpan-shaped half graben (controlled by typical high-angle fault in the early stage, see the following), with top surface eroded. The middle section is the upper part of the lower member of Wenchang Formation (T85-T83), in this depositional period, the subsag-controlling fault turned lower in angle, and the gentle slope on the right side of the subsag strongly tilted, uplifted and denuded, resulting in obvious angle unconformity; the left side of the subsag near the fault tilted and subsided, with sediments increasing in thickness; the sedimentation center deviated from the bottom of the fault and was close to the middle of the subsag, indicating that the subsag had subsidence contributed by crust thinning below the detachment plane, which is a typical filling feature of detachment; the seismic facies of two stages of volcanic rocks are in this section. The upper section is the upper member of Wenchang Formation-Enping Formation (T83-T70), and the formations have more distinct depression features, which is the manifestation that the crust mainly subsided due to ductile stretching and thinning.

It is concluded through comprehensive analysis that in the early stage (before the formation of T85 interface), the upper crust was still characterized by brittle exten-sional fracture, the stretching led to high-angle faulting and the formation of dustpan-shaped fault depression. During the middle stage magmatism, the upper crust decreased in strength and had ductile extension, the high-angle fault evolved into a low-angle detachment fault, bringing about the detachment fault depression (T85-T83). The detachment caused the upper wall of the fault to rotate and tilt, especially the two stages of magma emplaced into the stratum all corresponded to the rapid tilting of the upper wall. Well F22 reveals that hundreds of meters thick mid-deep lacustrine high-quality source rocks deposited on the basalt at the bottom of the well, which indicates that the magmatic activity accelerated the extensional detachment, and led to the rapid subsidence of the sag, so undercompensated lacustrine deposits developed for a long period. In the late stage (after the formation of T83 interface), the subsidence made the depression sink more obviously, and the tilting denudation at the right side of the subsag continued, indicating that the continuous intrusion of magma made the extension and thinning process of fault footwall continue.

Fig. 7 shows the dip profile of Huizhou 10 subsag. The typical feature of this sag is that the main boundary fault extends to the deep formation and turns gentler, appearing as listric shape, while the gentle slope of the subsag has strong tilting and lifting. A series of synthetic faults on the gentle slope are also in listric shape, but they are cut by two steeper and later active faults on the right side. Combined with the rock cap indicated by the wave-like upward reflection in the basement and the bedrock indicated by the strong reflection layer in the sedimentary layer, it is considered that this fault depression pattern deviating from the typical half graben structure is caused by the reformation of magmatism. Seismic profile interpretation shows that magmatism occurred in about half of the gentle slope side of the early half graben, which resulted in strong tilting and uplifting of the subsag structure and strata but corresponding strong tilt settlement on the side close to the main control fault. A 2 500-m thick stratum deposited between the T83-T72 interfaces, in which there are 2-3 high impedance seismic reflection layers that may reflect volcanic eruption lava entering the stratum. During magmatism, the fault depression caused by tilting subsidence was much smaller in width than the original half graben. Although magmatism led to changes in structural styles of the faults and depression, the horizontal extension was not large, and accordingly, the strata filling the subsag show no characteristics of depression. This subsag had no obvious subsidence and uplift after the formation of T72 interface, indicating that magmatic activity stopped after the formation of T72 interface, and it has no depression subsidence period representing deep ductile extension like that in Lufeng 22 subsag. In the sedimentary period of the lower member of Wenchang Formation (Tg-T83) before magmatism, this subsag was a typical half graben. Later, during the sedimentary period from upper member of Wenchang Formation to the lower member of Enping Formation (T83-T72), magma activities continued on the gentle slope side of this fault depression, and the subsag structure was reformed considerably.

The controlling fault of Baiyun main subsag is a crust- mantle detachment system that almost breaks through the crust and reaches the Moho surface. Deep and large detachment controlled the development of the wide and deep fault depression, and the subsag structure hasn’t been reformed by magmatism significantly^[4] (Fig. 3). But with the detachment planes becoming shallower, the detachment faults on the east and west wings of Baiyun main subsag belong to detachment faults in the upper crust. In the east part of Baiyun Sag, the crust is about 16 km thick, and the faults are in slope-flat shape. Drilling reveals that the upper Wenchang Formation and Enping Formation contain tuffaceous and pyroclastic rocks, and seismic profiles show common wavy reflection and strong reflection axis in the basement, these are the main evidences of magmatic activity during rift period. The seismic profile in Fig. 8 shows that there are still steep and straight fault planes extending to the deep crust in Baiyun east subsag (dotted line in Fig. 8), and the lower member of Wenchang Formation below T83 interface has the characteristic of half-graben structure controlled by high-angle fault, indicating that the upper crust of the lower member of Wenchang Formation (Tg-T83) in the early stage of extension had mainly brittle fracture during deposition, resulting in the half-graben controlled by high-angle faults. Two stages of angle-reducing processes of faults can be identified on the seismic profile, which are manifested as the shift of the plane of the subsag-controlling fault in the crust from "1" track to "2" and "3" tracks with lower angles, and the hanging wall stratum gradually tilted and uplifted correspondingly, resulting in two stages of denudation angle unconformities (T83 and T70 interfaces) (Fig. 8), reflecting two stages of magmatism after the formation of T83 interface made the crust ductile, and the high angle subsag-controlling faults evolved into low angle detachment faults. Comparing the two small fault depressions A and B in Fig. 8 shows Zhuhai Formation in the fault depression B is missing in fault depression A (inside the red arrow), and Wenchang-Enping Formations in the fault depression A were strongly uplifted and denuded, with significant uplifting than those in the fault depression B. On both sides of F1 fault, the strata below T70 interface are almost the same in thickness, which indicates that F1 fault was active only in the later period, and Zhuhai Formation (T70-T60) was pulled nearly up-right, indicating that the uplift on the right side of the fault was completed during the sedimentary period of Zhuhai Formation, and the driving force of the uplift was deep magmatism. Compared with the uplift of the fault depression A, the fault depression B showed subsidence, where the Zhuhai Formation has the sedimentary characteristics of depression, which is the manifestation of local crust turning ductile and thinner under magmatism. Three stages of magma activities can be identified in this area: namely, those in the formation period of T83 interface, the formation period of T70 interface and the deposition of the lower member of Zhuhai Formation. The former two stages correspond to the track of fault plane change and the unconformities of T83 and T70, while the later stage is mainly reflected by the whole uplift of the footwall of the fault, making most of Zhuhai Formation missing in the fault depression A and south of it.

The time of magma activities can be determined by the evolution of filling structure of strata. As shown in Fig. 9, T83 is a regional angle unconformity surface, and the flattened T83 is assumed to be the stratum surface at that time. It is not difficult to see that Liuhua 29 low bulge had not yet formed before the formation of T83 interface, and magma intruded after T83 interface, and the controlling fault of Liwan 3 subsag changed from a high- angle fault to a low-angle detachment fault, and the upper wall of the fault had strong rotation and warping. The strata between T83 and T60 overlap on Liuhua 29 low bulge. The strata above T60 interface are draping sediments, and this structural evolution shows that Liuhua 29 low bulge was formed during the depositional period of T83-T60. Combined with the analysis of Fig. 8, it can be inferred that the magma upwelling in the deep crust also occurred during the deposition of T83-T60. The zircon samples obtained from volcanic rocks at depth of 2755-2 810 m and 3 200-3 240 m of Well H29 have minimum age records of 38.8±0.5 Ma and 43.3±0.7 Ma respectively, which are consistent with the interface ages of T80 and T83. Combined with the unconformity of the T83 interface in Well L4 indicating detachment caused by magma upwelling, it is inferred that the magma activities occurred after 43 Ma in the eastern Baiyun fault depression.

In the southwest step-fault zone of Baiyun Sag, the main boundary faults show high-angle linear fault plane reflection within the depth range of the upper crust, but some faults have low-angle detachment at the upper crust level, which may also be closely related to magma intrusion. Drilling has revealed magmatism in the fault depression period there^[18].

Recently, magmatism in the deeper crust from the south of Baiyun Sag to Liwan Sag has been studied in detail^{[12, 15, 19-21]}. In this area, the crust is about 11 km thick at present, and a large number of magmatic bodies have been identified in seismic reflection of the basement and strata, which mainly occurred in extension and breaking stages of middle and late rifting, and are mainly laccoliths, sills and underplating. The basement of Liwan Sag is not the unconformity interface before the continental margin breaking, and most of it may be a ductile shear surface with extensional detachment and gravity sliding origins^[22,23]. The intrusions of massive hot magma intensified the ductile extension and deformation of the crust, making ductile flow change stronger and buried depth of ductile layer shallower. The early brittle fault depression was small in scale, which was mainly a "mini basin" flexible deformation structure between magmatic arching and magmatic intrusions. According to the analysis of the filling structure of "the mini basin", the arch tension formed by magmatism occurred from the late rifting to the early deposition of Zhuhai Formation after rifting (Fig. 1b), indicating that there was still magmatic activity after the rifting.

It can be seen that different parts of Baiyun Sag had noticeable differences in initial crustal structure and the participation degree of magma in extension period. Magma strongly participated in the extension of eastern Baiyun-Liwan Sag, leading to different characteristics of detachment surface depth, rotation and horizontal extension displacement of upper fault blocks, and structural evolution styles of fault depressions.

Different from the typical Atlantic passive continental margin, the Cenozoic northern continental margin of the South China Sea developed on the background of Mesozoic subduction continental margin. According to geophysical data and regional geological analysis, Zhou et al.^[24] proposed that the Pearl River Mouth Basin in the northern part of the South China Sea is located in the continental margin structural belt of late Mesozoic paleo-Pacific subduction to Eurasian plate. Xu et al.^[20], based on the zircon U-Pb dating and geochemical tectonic environment discrimination of granitoids drilled in the Pearl River Mouth Basin, identified the nearly NE-trending Late Jurassic-Early Cretaceous magmatic arc from Dongsha Uplift to Panyu Low Uplift in the north of Chaoshan Sag in the South China Sea. Combined with the characteristics of the forearc environment of the Late Jurassic radiolarian silicalite deposits in MZ-1 well, it is proposed that the northern South China Sea is located in the tectonic belt of the ancient Pacific plate subduction during the Late Jurassic-Early Cretaceous. Ye et al. identified the late Cretaceous compressional-extensional- compressional structural fault system on the basement of Pearl River Mouth Basin and granitic crust formed from Late Jurassic to Early Cretaceous (161.6-101.7 Ma) and related to continental margin arc^[10].

It is not difficult to understand that the complexity of lithosphere thickness, lithology, rheology and fault distribution in the pre-existing subduction continental margin in the northern South China Sea will inevitably lead to the difference of lithosphere strength, which will further complicate the continental margin extensional fault structure. First of all, the zoning pattern of the NE-trending depression belt and uplift belt in the Cenozoic Pearl River Mouth Basin is consistent with the trend of the Mesozoic subduction belt^[24,25], which indicates that the subduction structural belt controls the extensional fracture belt. Secondly, the complex Mesozoic pre-existing faults directly controlled and influenced the development of Cenozoic faults and fault depression structures. There have been many studies on this part. For example, Ye et al. think that the development of depression-controlling faults in Enping Sag and Huizhou 26 Sag is related to the activation of Mesozoic thrust faults^[9]. Obviously, the pre-existing fault, as a relatively weak surface, can be easily used in the later extension stress field and become an extension fault. After further research, the author's team has found that the depression-controlling faults in several sags (such as Enping Sag, Xijiang Main Sag, Xijiang 36 Sag, Huizhou 26 Sag, Lufeng 15 Sag and Baiyun Southwest Fault Step Area, etc.) are related to the activation of preexisting Mesozoic faults.

Generally speaking, there are many types of depression-controlling faults in the Pearl River Mouth Basin (Fig. 10), such as high-angle faults, activation of preexisting faults, magma-reformed upper crust detachment faults, inter-crust detachment faults and crust-mantle detachment faults. Among these types, the upper crust detachment faults influenced by preexisting structures and magmatic activity are commonly seen in the northern South China Sea, which show the particularity of the fault depression structure in the Pearl River Mouth Basin. These pre-existing faults and magmatism are different in the location, intensity and sequence of occurrence and development in the fault depressions, which will also make the fault structure and evolution have various styles. The diversity of structural styles, the complexity of evolution process and the differences between them are the important characteristics of fault structure and sedimentary filling in the Pearl River Estuary Basin.

In order to better explore the formation mechanism of structural diversity of extensional faults on the northern continental margin of the South China Sea, it is necessary to first understand the types and formation mechanism of fault structures on the Atlantic continental margin. The extensional continental margins on both sides of the Atlantic Ocean originated in the stable supercontinental craton, and include two types, i.e., magma-rich and magma-lean. (1) Magma-rich extensional lithosphere melted rapidly due to the rapid emergence of magmatism. Under the main effect of magma, the continental margin characterized by volcanic rock filling (SDR) obliquely to the sea was formed. The magmatic continental margin, with short extension time, has a clear boundary between the continent and the ocean, and a relatively narrow transition zone between the ocean and the land. (2) In the magma-lean continental margin represented by Iberia, the preexisting craton crustal lithosphere has consistent layered rheological structure; the lithosphere gradually stretched, detached and thinned until pinched out, forming wedge-shaped continental margin. The detachment faulting is the main mechanism of crustal lithosphere thinning. Stretching at an ultra-slow speed for a long time, the magma-lean extensional continental margin has broader transition zone between ocean and continent and unclear boundary between continent and ocean^{[2, 26-27]}. The magma-lean extensional continental margin has lithosphere strength characterized by layered brittleness and ductility, so a continental margin extensional structure with a sequence change from the proximal zone to the hyper-extensional zone from land to sea would be formed during the extensional detachment process of uniform extension and concentrated strain migration. The proximal zone has fault depressions controlled by high-angle faults developing. With strong subsidence in the early period, these fault depressions could have undercompensated deep lacustrine sediments depositing. In the late stage of extension, as the strain concentration migrated to the hyper-extensional zone, and the proximal zone had weak activity, thus only fluvial-shallow lacustrine sediments developed. After experiencing early high-angle faulting, the necking zone and the distal zone in the hyperextension zone had detachment and thinning between the crust and mantle due to concentrated strain (Fig. 10d, 10e), giving rise to large-scale detachment fault depressions. The early fault depressions were controlled by high-angle faults, and the later ones were controlled by low-angle faults between crust and mantle, with the structural and morphological characteristics of fault depression and depression. As a result of migration of strain concentration, in the late stage, opposite to the deformation weakening in the proximal zone, the hyper-extensional zone had strain concentration in the late stage of rifting, and strengthened subsidence, giving rise to deep detachment fault depressions which provided conditions for forming large deep lake basins^[28]. Therefore, uniform extension, migration of strain concentration, detachment thinning, lithosphere pinch-out, and finally ocean basin expansion result in sequential (in zoning, formation time and migration) fault depressions in the continental margin area. This is the development and evolution process and structural features of continental margin formed by the mechanisms of extension, detachment and thinning.

Overall, the northern continental margin of the South China Sea, like the general passive continental margin, also has the crust and lithosphere thinning and pinching out toward the sea, and structural deformation and basin development migrating and deforming from land to sea. But this research reveals that the continental margin and basin structure in the South China Sea have diversity in development and evolution under the background of this overall development and evolution pattern.

For example, in Zhu I Depression, the proximal zone with weak crustal extension, there are many upper crustal detachment faults controlled by magmatism and/or pre-existing faluts, while the proximal zone of the Atlantic magma-lean continental margin only develops fault depressions controlled by high-angle faults. The Baiyun Sag in the hyperextension area of the upper crust has also developed the detachment fault depression of the upper crust. The hyperextension area of the Atlantic magma-lean continental margin is mainly controlled by the detachment fault between the crust and mantle. The author thinks that it is mainly controlled by the background of pre-existing subduction continental margin, which is the fundamental reason different from the Atlantic extension continental margin developed in the supercontinent craton.

The Cenozoic extensional breaking in the northern South China Sea occurred in the craton margin, and the subduction continental margin background is Mesozoic. In the late Mesozoic, the NW Pacific subduction zone distributed in the northern South China Sea controlled the tectonic pattern of the Pearl River Mouth Basin in the Cenozoic^[24]. As the crust put together by trenches, arcs and basins and preexisting faults on the active continental margin serve as the structural brittle-ductile transition surface, the lithosphere has heterogeneity in lithology, structure, thickness, properties and thermal state and differences in rheology. These characteristics would affect and reform the structural style and subsag structure of Cenozoic extensional strain to various degrees; in addition, it is particularly important that the subduction of Mesozoic and Cenozoic continental margin plates made the mantle rich in fluids and recirculated materials^[19], and the decompression and melting of Cenozoic lithosphere resulted in magma upwelling along fissures or weak belts, forming magma bodies distributed dispersedly in the crust, so crustal strength had local dissimilation at different depths, which accelerated the differential of extensional thinning. Que Xiaoming et al. (2013) studied the origin of 35.5Ma basalt-andesite depositing in the late Enping Formation in Well BY7 in southwest of Baiyun Sag, and concluded that the volcanic rock there was the melting product of mantle source material with some continental crust components, which was related to lithospheric subsidence of pre-Cenozoic subduction plates, and deep mantle upwelling might play an important role in the process of stretching and breaking of the South China Sea^[18]. As mentioned above, magmatic rocks of fault depression period commonly encountered in the Pearl River Mouth Basin and a large number of magmatic rocks shown by seismic data (Fig. 1b) all indicate magma activities in the South China Sea, especially during the middle and late stage of extension and thermal subsidence after rifting of fault depression basins in the continental margin.

The magmatism would make the lithosphere detach and thin rapidly, and even melt quickly, which would inevitably change the structural deformation pattern of the crust, bringing about detachment structures in different depths and strata (Fig. 10), thus reforming the fault depression structure and sedimentary filling pattern. As Wang et al. pointed out, "for a long time, the origin of passive ancient marginal basins has always been based on the Atlantic model, but these geological features of tectonic evolution in the northern margin of South China Sea tell us that, we should think out of the box of the Atlantic model and have our own perspective in understanding the fault depression basins in the northern South China Sea"^[26].

The intracontinental fault depression developed in the typical craton is mainly controlled by high-angle faults due to the strong layered crust strength (Fig. 10a). The Atlantic continental margin fault depressions formed in the craton rifting, due to the strong extension of the continental margin, developed intra-crust detachment rift and crust-mantle detachment rift (Fig. 10d, 10e). Generally, On the northern margin of the South China Sea in the background of subduction, five types of fault depressions can be identified: typical high-angle fault depressions controlled by high-angle faults (Fig. 10a), detachment faults in upper crust controlled by reactivated preexisting faults (Fig. 10b), detachment faults in upper crust reformed by magma (Fig. 10c), inter-crust detachment faults (Fig. 10d), and crust-mantle detachment faults (Fig. 10e). It is characterized by the widespread development of pre-existing faults and magmatism-controlled detachment faults in the upper crust of Pearl River Mouth Basin, which is different from the fault depressions in Atlantic continental margin. However, due to the differences in the location, intensity and sequence of occurrence of the development of pre-existing faults and magmatism in the fault depressions, the specific structure and evolution and sedimentary fillings will have a variety of styles, which need to be clarified after in-depth study.

From the research results of this paper, the structure of the northern margin and basin of the South China Sea is far more complex than the previous understanding. It is not only different from the typical intraplate faulted basin mainly controlled by brittle fracture and high-angle fault, but also different from the Atlantic continental margin faulted basin developed under the cratonic background. The petroleum geological exploration research of the continental margin faulted basin needs to deeply study the basin control mechanism of the continental margin lithosphere structural evolution, and deeply analyze the influence of preexisting structure and magmatism on the structural characteristics of the continental margin extensional fault depression. Only in this way, and by avoiding blind analogy and simple application, we can correctly understand the structure and evolution, sedimentary filling and hydrocarbon generation of the depression.

Three types of detachment faults with different structural levels developed in the northern continental margin of the South China Sea: crust-mantle detachment, inter-crust detachment and upper crust detachment faults. Among them, upper crust detachment faults are mainly affected by magmatism and preexisting structures, different types of detachment faults control diverse fault depression structures and, in general, there 5 types of structures are identified.

There are several fault depressions controlled by low-angle detachment faults in the upper crust in Zhu I Depression located in the proximal zone. These detachment faults are usually affected by preexisting faults and/or magmatic emplacement, and have detachment planes in the upper crust at shallow buried depths. Baiyun and Liwan sags located in the hyper-extension area show different subsag structure characteristics. The Baiyun main subsag with weak magmatism is a composite wide-deep fault depression controlled by crust-mantle detachment system; whereas in the eastern and southwestern step-fault zone of Baiyun Sag, extensive magmatism happened after the deposition of the lower member of Wenchang Formation (T83), consequently, the crust had ductile extensional deformation, resulting in the development of detachment faults located in the upper crust, and fault depressions shallower than in Baiyun main sag. The depth of the detachment plane determines the depth of the fault depression, and the degree of ductile extension determines the width of the fault depression.

The detachment fault depression has structure and sedimentary filling different from fault depression controlled by slab fault. The fault depression controlled by slab fault is usually in narrow rift valley shape, while the detachment fault depression is in ellipse shape with a small ratio of length to width, and features significant rotation and tilting of the upper wall. The tilting rotation of the upper wall of the detachment fault leads to the decrease of the axial provenance supply, while the increase of the gentle slope provenance, resulting in the development of large delta deposits and lacustrine deposits in the subsidence area of the fault depression. The detachment fault controlled by magmatism would lead to rapid detachment and rotation of the upper wall, giving rise to deep-seated undercompensated deep lacustrine sedimentary environment.

The lithosphere strength of Atlantic extensional continental margin developed on the background of supercontinent stable craton is characterized by layered structure, and the continental margin fault depressions developed on this lithosphere change orderly in structure from land to ocean with the extension and thinning of lithosphere. The Mesozoic active continental margin background made the fault depressions at the Cenozoic continental margin of the northern South China Sea complex and diverse in structure inevitably. The preexisting structure and magmatism destroyed the consistency of crustal strength, and had a strong control and influence on the structure of subsags in the Pearl River Mouth Basin at the northern continental margin of the South China Sea. These factors cause the particularity and diversity of basin structures there.

In the writing of this paper, Professor Ren Jianye of China University of Geosciences (Wuhan) gave us valuable guidance and help. We express our sincere gratitude.