Bohai Bay Basin is a Cenozoic polycyclic superimposed fault basin developed above the basement of the North China Craton, Bohai Sea is a part of it^[1]. It is surrounded by the Yanshan fold belt, the Taihang Mountain orogenic belt, the Jiaoliao uplift belt, the Qinling-Dabie orogenic belt and other tectonic units^[2]. Since the late Paleozoic, the North China plate where the Bohai Sea is located has been jointly affected by the northern Siberian plate, the southern Yangtze plate, the eastern Pacific plate and the Indian plate, and witnessed very active deep mantle movement and strike-slip fault activities^[3,4,5]. In the Mesozoic, the Indosinian north-west-west/ nearly east-west tectonic system and Yanshanian north- north-east tectonic system superimposed to form the basement tectonic frame zoning east-west and segmenting north-south. The oblique subduction of the Cenozoic Pacific plate formed the back-arc mantle uplift extension and strike-slip pull-apart, which became the main driving force for the development and evolution of the Bohai Sea. The two tectonic stresses, shear and tension, worked together to give rise to the tectonic style of the symbiotic strike-slip and extension of the Bohai Sea^[6,7]. Compared with the land part of the Bohai Bay Basin, the tectonic characteristics of symbiotic strike-slip and extension in the Bohai Sea are mainly manifested in two aspects: (1) Two large strike-slip faults in eastern China (north-north-east Tanlu strike-slip fault, north-west-west Zhangjiakou-Penglai strike- slip fault) converge and develop in the Bohai Sea, causing the strike-slip fault to develop extremely well^[8,9,10]; (2) Influenced by the continuous extension, the Bohai Sea becomes the thinnest area of the continental crust in eastern China^[11]. This special geological background has resulted in the complexity of the origin and the fault system of the Bohai Sea, which has become the focus for many scholars to study and discuss for a long time.

Different scholars have different concepts regarding the origin of the Bohai Sea. The most controversial ones are the two division schemes of the Rift Basin and the Strike-Slip Pull-Apart Basin. As the Tanlu fault zone runs through the eastern Bohai Sea and had strong strike-slip activity in the Mesozoic and Cenozoic, and the Bohai Bay Basin is generally in reversed “S” shape, so some scholars have summarized it as a pull-apart basin^{[12,13,14,15]}. However, the basin has the obvious two-layer structure of the Paleogene fault depression and the Neogene depression in the filling structure, and it also reflects the characteristics of the rift basin, therefore, many scholars believe that it should be more suitable as a rift basin^{[16,17,18,19,20,21]}. At present, the understanding of strike-slip faults and their associated structures is relatively mature, and it mainly includes pull-apart basins, negative flower shaped structures, en echelon folds and pull-type bend and dual structures^[22]. However, according to the geological characteristics of the pull-apart basin, the long edges on both sides of the pull-apart basin are restricted by strike-slip faults, and the long axis direction of the basin is mostly parallel to the direction of the strike-slip faults, which is obviously inconsistent with the pattern that the depression of the Bohai Sea is axially controlled by faults in different directions. According to the extensional fault depression model, the two major faults, Tanlu and Zhangjiakou-Penglai in the Bohai Sea show strong strike-slip property, the basin has various types of strike-slip structures, especially affected by the strike-slip activity in the Neogene, the fault systems are widely developed, which is different from the characteristics of the non-development of fault and the exponential attenuation of the subsidence rate in traditional rift basins after the depression period^[23]. The Bohai Sea has experienced three major stages of tectonic movements, the Paleogene fault depression, the Neogene depression and the neotectonic movement since the late Neogene (12 Ma ago-present). In particular, during the Neotectonic period, strike-slip activities in the Bohai Sea were intense, as a result, fault systems developed, and strike-slip structures formed by the joint effects of various tectonic stresses came about extensively. As the exploration degree and knowledge of Bohai Oilfield increase, it is found that neither the strike-slip pull-apart mode nor the extensional fault depression mode can accurately explain the current tectonic features of the Bohai Sea.

The Bohai Sea is a Cenozoic complex superimposed fault basin formed under the control of the dual power sources of the mantle convection extension and the oblique strike-slip pull-apart under the subduction background of the Pacific plate, which has obvious characteristics of extensional fault depression, and varying degrees of strike-slip reformation in different parts^{[24,25,26,27,28]}. However, the superimposition and intensity ratio of multiple tectonic stresses vary considerably in different evolution stages and different regions, resulting in the fact that the strike-slip faults widely developed in the Bohai Sea are different from the traditional typical strike-slip structures caused by a single shear stress and have obvious characteristics caused by multistage structure superimposition and multiple tectonic stresses. Based on the basin-forming background of the dual power sources in the Bohai Sea, and combined with the 3D seismic structure interpretation, physical modeling of structure and test data, the superimposition characteristics and genetic mechanism of the strike-slip faults in the Bohai Sea were studied, and the differential time-space superimposition effects of the strike-slip fault were mainly discussed in this paper. This aims to reveal the intrinsic relationship between the tectonic features of the strike-slip fault zonation and hydrocarbon accumulation in the Bohai Sea, and to guide the evaluation of hydrocarbon exploration potential and the selection of favorable zones.

Under the extension, contraction and torsion of the crust, the tectonic activities of the geologic body are dominated by the extension stress, extrusion stress and horizontal shear stress. The mode, intensity and duration of these tectonic stresses are usually unfixed, the final tectonic deformation may be the combined response to stresses induced by power sources of different natures or different directions in the same period, or may be the result of the superimposition of stresses of different natures or different directions in different periods. During the long geological history, the three stresses might coexist in the spatial domain or superimpose on the time scale, resulting in a variety of tectonic response types, for example, the strike-slip extensional (transtensional) structure is the result of the coexistence of the shear stress and the tensile stress in the spatial domain, the compressional strike-slip (transpressional) structure is the result of the coexistence of extrusion stress and shear stress in the spatial domain, and the inverted structure is the result of the superimposition of tensile stress and extrusion stress on the time domain. Therefore, both the stress superimposition sequence in different time domains and the superimposed stress intensity ratio in the same spatial domain will lead to significant differences in the final tectonic response characteristics.

As the most important stress forms running throughout the Cenozoic basin evolution of the Bohai Sea, the superimposition effect of strike-slip and extension is the most common in the Bohai Sea, and has the strongest control effect on hydrocarbon accumulation. The differential superimposition effects of strike-slip and extension were revealed by structural physical simulation experiments which can superimpose different stresses of different properties at different proportions in the same spatial domain. In the experiment, dry quartz sand was selected as the simulated material, rubber was laid at the bottom as the medium for stress transmission, and the quartz sand was dyed and laid as the marker bed. During the experiment, the driving simulation was carried out according to the superimposition sequence and the intensity ratio of strike-slip and extension. The driving unit moved at a uniform speed, and the driving units on both sides shared the same speed. The extension displacement and the strike-slip displacement respectively represented the intensity of extension and strike- slip. After the end of the experiment, the sandbox was consolidated with gelatin supersaturated solution to obtain the profile of experimental result (Fig. 1). The results of the physical simulation experiment reveal that: (1) Under the condition of strike-slip being followed by extension stress, the early nearly vertical strike-slip fault is reformed by the late extensional fault, and the secondary synthetic strike-slip fault (P shear fault) or the main strike-slip fault (PDZ fault) generated by the early strike-slip is likely to reactivate in the late extension; the fault zone is wider and smaller in dip angle, and shows a negative flower shaped structure or a multistage Y-shaped structure on the section (Fig. 1a). (2) Under the condition of extension being followed by strike-slip stress, the early extensional fault is difficult to reactivate, and the late strike-slip fault zone has a narrow development plane. The head-to-tail connection of the P shear fault forms the PDZ fault, cutting the en echelon synthetic strike-slip fault, a negative flower shaped structure develops on the section, and the early low-angle extensional fault is cut by the strike-slip fault (Fig. 1b). (3) The superimposition of extension and strike-slip stresses with different intensity ratios results in the differences of tectonic styles. For the superimposition condition that the extension is more intense than the strike-slip stress, that is, when the intensity ratio of the extension and the strike-slip stress is 4:1, the comb-like style develops on the plane, the early en echelon fault will be cut by the normal fault in the late stage, the negative flower shaped structure develops on the section, and the main fault is featured by shovel shape. When they are similar in intensity, no main extensional fault is formed, and the en echelon fault on the plane is cut by the discontinuous extensional fault, and a series of normal faults are combined to form a negative flower shaped or multistage Y-shaped tectonic styles (Fig. 1c). (4) For the superimposition condition that the extension is less intense than the strike-slip stress, that is, when the intensity ratio of the extension and the strike-slip stress is 1:4, the R shear fault intersects with the extensional fault on the plane, and the negative flower shaped structure is dominant on the section (Fig. 1d).

According to the above physical simulation experiment, under the superimposition of different time domains, the extension reformation is stronger than the strike-slip reformation in the inheritance of the pre-existing fault, and the strike-slip reformation is stronger than the extension reformation in the rebirth quality of the fault. The recombination of stresses of different properties and different intensities in the same spatial domain will produce distinct tectonic styles. The faults in the Bohai Sea were dominated by extension in the Paleogene, and a certain strike-slip effect was superimposed; the superimposed strike-slip effect in the eastern sea area was obviously stronger than that in the western sea area. In the Neogene, different areas of the Bohai Sea were generally subjected to different degrees of strike-slip reformation; it had the inheritance of the early Paleogene faults, and a large number of Neogene faults emerged, leading to the obvious superimposition characteristics of the faults in the Bohai Sea. This agrees well with the above physical simulation results.

The tectonic style is the summation of the tectonic traces produced by the same-stage tectonic deformation or the same stress, and it is a comprehensive reflection of the spatial geometric character of the geological structure in the profile morphology, the plane distribution, permutation and combination, etc. According to the stress mechanism and tectonic characteristics, the superimposed faults in the Bohai Sea mainly develop 15 types of tectonic styles because of extension and strike-slip superimposition, extension and extrusion superimposition, extrusion and strike-slip superimposition.

1.2.1. Tectonic styles from extension and strike-slip superimposition

The superimposed fault structures of the Bohai Sea are most developed in the tectonic styles from extension and strike- slip superimposition. According to the relative intensity of the extension and strike-slip stress, it can be subdivided into four sub-classes of the strong strike-slip and weak extension superimposition, the medium strike-slip and medium extension superimposition, the weak strike-slip and strong extension superimposition, and the two-way strike-slip superimposition.

The strong strike-slip and weak extension superimposition subclass has three tectonic styles: the vertical negative flower shaped type, the superimposed type and the strike-slip duplex type. The vertical negative flower shaped tectonic type is closest to the classic strike-slip style in the superimposed fault mode. It is the result of the superimposition of the strong strike-slip and the weak extension stress. The profile presents a negative flower shaped structure with a nearly vertical main fault and the strike-slip fault zone on the plane has good stability and continuity (Fig. 2a). The superimposed tectonic style is formed by the superimposition of two or more faults plus the strong strike-slip effect. The profile presents a negative flower shaped structure or a negative flower-like structure. The faults at the superimposed zone are mainly developed near the end on both sides of the main strike-slip fault. The profile presents a broom shaped structure and the form resembles the strike-slip pull-apart basin (Fig. 2b). The strike-slip duplex tectonic style is similar to the superimposed type, and is also developed in the superimposed position of the strike-slip fault. It is the advanced stage of the superimposed development (Fig. 2c). The tectonic style from strong strike-slip and weak extension superimposition is similar to the situation where the extension is weaker than the strike-slip stress under the superimposition condition. The fault has good continuity on the plane, and upright occurrence on the section, indicating that the shear stress dominates.

The medium strike-slip and medium extension superimposition subclass includes three tectonic types: the imbricate fan type, the “H” type and the broom shaped type. The imbricate fan type tectonic style is mostly formed at the end of the strike-slip fault, and the secondary fault and the main strike- slip fault overlap on the plane to form a fan shape, or horsetail faults, and appears as a complex “Y” shaped structure on the section (Fig. 2d). The “H” type tectonic style is the result of the superimposition of the extension and the strike-slip effect on the time scale. The early extension structure is cut and reformed by the late strike-slip fault, and they are orthogonal, and the late strike-slip faults are mostly arranged in parallel or side by side, resulting in fault arrangement in a “H” type on the plane, and the tectonic style of the strike-slip fault and the extensional fault is maintained respectively on the section (Fig. 2g). The broom shaped type is the result of the simultaneous effect of strike-slip and extension. One end of the extensional fault on the plane is divergent, and the other end is convergent towards and overlaps with the strike-slip fault. It often appears as a complex “Y” shaped structure on the section (Fig. 2l).

The weak strike-slip and strong extension superimposition subclass includes four tectonic styles: the en echelon type, “S” type, laterally adjustable hidden strike-slip, and vertically conductive hidden strike-slip. The en echelon tectonic style is formed because during the weak strike-slip period, the going-through main fault zone does not form under the strike- slip effect, and a series of strike-slip faults of the same strike combine into en echelon structure, which takes on a negative flower shape on the section (Fig. 2h). The “S” type tectonic style develops in the bend position of the strike-slip fault strike, with “S” shaped plane. The depression section is mainly developed with the pressurized trap, while the uplift section is characterized by the depressurized type, often accompanied by the normal fault (Fig. 2f). The laterally adjustable hidden strike-slip tectonic style is the result of differential extension. The heterogeneity of geologic body leads to the uneven local extension rate. In order to adjust the difference of the extension rate, such a strike-slip fault is formed, and the extensional faults on both sides of the main strike-slip fault are usually orthogonal to the strike-slip fault on the plane, the strike-slip fault has short extension and small vertical cut range, and few strike-slip derived faults (Fig. 2e). The vertically conductive hidden strike-slip tectonic style is the result of the superimposition of the strike-slip effect in different periods. It is usually developed in the sedimentary caprock with the ancient strike-slip fault on the basement. The Neogene strike-slip activity revives the strike-slip fault of the basement, but the late strike-slip effect is weak, and the overlying sedimentary caprock is usually developed with small faults arranged in en echelon. The small scale of the strike-slip fault on the section is often difficult to identify (Fig. 2j).

The two-way strike-slip superimposition subclass includes two tectonic styles: the “X” type and the “L” type. The “X” type tectonic style refers to the contemporaneous activity of strike-slip faults in two directions, which are inversely and mutually influential, thus forming a set of conjugated pure shear strike-slip faults on the plane, and the negative flower shaped or negative flower-like structure on the section (Fig. 2k). Similarly, the “L” type tectonic style also shows the mutual intersection of two groups of strike-slip faults in the nearly orthogonal direction. However, unlike the “X” type, the two groups of faults are not generated simultaneously, they are the result of the multistage superimposition of strike-slip faults in different directions on the time scale. The early strike- slip fault is cut by the late strike-slip fault and a negative flower shaped structure is mostly presented on the section (Fig. 2i).

1.2.2. Tectonic styles from extension and extrusion superimposition, extrusion and strike-slip superimposition

The extension and extrusion superimposition and extrusion and strike-slip superimposition are the result of the superimposition of different stresses on the time scale, and they are far less than the extension and strike-slip superimposition in the Bohai Sea. The extension and extrusion superimposition includes the upper normal and lower reversed type and the conformal negative negative flower shaped tectonic style. The upper normal and lower reversed type tectonic style is similar to the traditional negative inversion structure. It is the result of the superimposition of the extensional structure and the compressional structure on the time scale, and is formed by the selective resurrection of the early compression structure under the Cenozoic extension stress. Different from the negative inversion structure, due to the superimposition of a certain degree of strike-slip factor in the later resurrection process, the early section morphology is partially inherited (Fig. 3a). The conformal negative negative flower shaped tectonic style is the product of the superimposition of the extension and strike-slip effect and the extrusion effect on the time scale. The early extension and strike-slip effect produces a negative negative flower shaped structure, and the weak extrusion effect is experienced in the late period, which, however, does not offset the normal fault throw, and only small low-amplitude anticlines are developed (Fig. 3b).

The extrusion and strike-slip superimposition is mainly manifested by the inherited resurgent tectonic style, which is the result of the superimposition of the early extrusion effect and the late strike-slip effect. The large-scale reverse fault develops under the Mesozoic regional extrusion stress, and the Cenozoic strike-slip effect leads to the recurrence of the reverse fault. A “thin bottom” structure develops at the upper wall, the deep main fault is gentle, and the shallow part is steep and has a negative flower shaped structure (Fig. 3c).

On the whole, the tectonic styles of the two sub-classes of the weak strike-slip and strong extension and the medium strike-slip and medium extension are well developed in the Liaodong Bay area, the weak strike-slip and strong extension develops in the eastern Bohai Sea, and the two-way strike-slip tectonic style is well developed in the southern and western Bohai Sea. The extension and extension superimposition is locally developed in the steep slope zone, and the extrusion and strike-slip superimposition is mainly developed in the slope zone adjacent to the large uplift.

The Bohai Sea is located at the junction of the Tanlu fault zone and the Zhangjiakou-Penglai fault zone. There are two mainstream theories about the origin of the Tanlu fault zone: one is it has existed in the Precambrian period as the block stylolite inside the Sino-Korean plate^[29]; the other is that it was originated from the collision orogeney of the Mesozoic North China plate and the Yangtze plate^{[30,31,32,33]}. Regardless of the theories, researchers generally believe that the Tanlu fault zone has experienced multistage activities since its formation, and completed the transition from the left-lateral strike-slip to the right-lateral strike-slip in the Cenozoic.

2.1.1. Tectonic responses of multistage strike-slip activities

Strike-slip faulting will generate strike-slip derived extensional faults, which often come in broom shape or en echelon arrangement on the plane. In addition, due to the variations of occurrence or combination of the strike-slip faults, local extrusion structures can also be formed, and multistage strike-slip activities would result in the superimposition of multistage tectonic styles. The Tanlu fault zone is divided into three branches in the eastern part of the Laizhouwan Sag. From the east-west typical seismic section of the Laizhouwan Sag, three stages of obvious strike-slip activities and its superimposed tectonic deformations can be clearly identified. Through the recovery of the strike-slip derived faults, the formations involved in fold deformation and tectonic evolution, it is found that the strike-slip faults in this area had obvious activities in the Eocene, Oligocene and Neogene, respectively, forming strike-slip faults in different strikes and associated structures. According to the recovery of the formations involved in fold deformation and tectonic evolution, it can also be seen that the area had three stages of structural compression superimposition deformations at the end of the Eocene (the end of the sedimentary period of Sha 1 Member -Sha 2 Member), the end of the Oligocene (the end of the sedimentary period of Dongying Formation) and the Neogene (the sedimentary period of Mingshang Member) (Fig. 4).

2.1.2. Apatite fission track analysis

The fission track analysis method is to judge the relative uplift and settlement of the stratum through the sensitivity of the mineral fission track to temperature, and then to judge the tectonic activity experienced by the geologic body^[34,35]. In this study, 12 wells in the uplift and slope zone near the Tanlu fault zone were sampled, including 10 core samples and 2 cuttings samples. Among the strata sampled, except Mesozoic granite layer in Well PL9-1-2, the remaining strata are all sandstone of the Dongying Formation and Shahejie Formation.

The simulation results show that the test results of some samples are not ideal. The reasons are as follows: (1) Some samples are newer and have few tracks. (2) Some samples have track age smaller than the formation age, indicating that these samples had reached the partial annealing zone temperature, and even entered the full annealing zone, so the track age represents the mixed age constituted by the formation cooling age or the track age formed by different degrees of annealing. (3) The track age of some samples are larger than the age of stratigraphic deposition, indicating that these samples have not completely annealed, the track age represents the combination of source rock age and partial annealing age in the provenance area. Except the above unsatisfactory samples, the thermal history recovery of the remaining samples has an uniform regularity, revealing that the uplift area and slope zone adjacent to the Tanlu fault zone experienced three major uplift cooling periods, 54 Ma ago, 23-27 Ma ago and 5.3 Ma ago, and reflecting three major periods of strike-slip movements (Fig. 5).

2.1.3. Polarity reversal of the strike-slip fault revealed by prototype basin recovery results

The strike-slip faults and their derivative faults show an obvious depression control effect in local areas, which affects the migration of the deposition and subsidence center of the sag. Therefore, the active process of the superimposed strike-slip faults can be reckoned by restoring the evolution of prototype basins in different periods.

The Qingdong sag is located in the southern Bohai Sea. The strike-slip fault is connected with the Weibei uplift in the east, and the slope zone experiences a transition to the Qingdong-Kentuozi uplift in the west. The early strike-slip and its associated faults were strongly active, and the north-north-east superimposed fault and its derived faults are closely related to the formation and evolution of the sag. The results of the basin prototype recovery show that during the sedimentary period of Kongdian Formation-Sha 4 Member, the Qingdong sag was north-west-west trend axially, which is characterized by that the sag sedimentation and subsidence center is controlled by the north-west-west fault derived from the left- lateral strike-slip activity of the Tanlu strike-slip fault. During the sedimentary period of Sha 3 Member- Sha 1+2 Member, the sag rotates counterclockwise to north-east trending axially, and the combined style of the north-east sag control fault and the Tanlu strike-slip fault indicates that the Tanlu fault is featured by the right-lateral strike-slip during this period (Fig. 6). The rotation migration of the Qingdong sag from the north-west to the north-east verifies that the Tanlu fault completed the polarity reversal from left-lateral to right-lateral in the late depositional stage of Sha 4 Member - early stage of Sha 3 Member.

The strike-slip fault is the active boundary and stress subduction zone between plates, and its evolution is usually closely related to the subduction rate and direction of the plates. Since the Cenozoic, the Bohai Sea has entered the development and evolution stage dominated by the Pacific tectonic domain. The multistage changes in the subduction rate and direction of the Pacific plate have led to frequent tectonic movements and constant regional stress adjustments in the Bohai Sea. According to the distribution characteristics, expansion rate and chronological data of the submarine volcanic island chain in the Pacific plate, Engbretson, et al.^[36,37,38] concluded that the motion direction and velocity of the western Pacific plate has changed three times since the Cenozoic, triggering obvious tectonic responses in the Bohai Sea. In the middle Eocene (42.5 Ma ago), the subduction direction of the Pacific plate changed from north-north-west to north-west- west trending. The subduction angle changed from oblique to orthogonal, and the subduction rate dropped sharply from 30.0 cm/Ma to 5.5 cm/Ma. The Bohai Sea section of the Tanlu fault changed completely from left-lateral to the right-lateral. At the end of the Oligocene (23.2 Ma ago), the subduction direction of the Pacific plate changed to north-west trending, and the subduction rate increased to 9.4 cm/Ma, resulting in the compression and reversal of the Bohai Sea section of the Tanlu fault, forming numerous compressional traps, such as the compressional trap of Penglai 19-3 oilfield. Since the Pliocene (5.0 Ma ago), the subduction direction of the Pacific plate changed to the north-west-west trending again, and the sub- duction rate decreased slightly. At the same time, the remote response of the northeastward collision of the Indian plate made the Bohai Sea enter the neotectonic period, when north-east and north-west conjugated strike-slips developed, and the shallow secondary faults came about densely. Since the Cenozoic, the continuous changes of the subduction direction and rate of the Pacific plate have caused changes in the deep mantle in eastern China and adjacent sea areas. The extension caused by the deep mantle material uplift and the oblique compression component of the plate margin were the main driving forces of the multistage activities of the superimposed strike-slip faults in the Bohai Sea. The Bohai Sea section of the Tanlu fault turned from left-lateral to right-lateral in the late depositional stage of Sha 4 Member - the early stage of Sha 3 Member (45-42 Ma ago).

As the most important tectonic element of the Bohai Sea in the Cenozoic, the fault has strong control on the basin structure. Because of the different boundary conditions defined by the early pre-existing basement faults in different regions, and the obvious differences in the space-time superimposition of extension and strike-slip stresses in the late stage, the fault distribution pattern and sag structure in different regions also differ widely, with obvious zoning feature. According to the distribution law of the main faults and the superimposition effect of the superimposed strike-slip structure, the Bohai Sea area contains the Liaoxi S-type weak strike-slip zone, the Liaodong braided strong strike-slip zone, the Boxi conjugated medium strike-slip zone, the Bodong broom shaped medium strike zone, and the Bonan parallel strong strike-slip zone.

Located in the northern Bohai Sea, the Liaodong Bay Depression is a natural extension of the Lower Liaohe Depression to the sea. It contains a series of graben-horst structures controlled by the north-north-east fault. However, due to the obvious different superimposition and strike-slip effect of the main depression control faults, the Liaodong Bay Depression is divided into the Liaoxi S-type weak strike-slip zone and the Liaodong braided strong strike-slip zone in the tectonic style and depression structure (Figs. 7 and 8a).

3.1.1. Geometry characteristics of faults

The main depression control faults of the Liaoxi S-type weak strike-slip zone present shovel shape steep in upper part and gentle in lower part on the profile. Affected by the weak strike-slip on the plane, the fault zone has wide section and poor strike stability, and shows different degrees of S-type bending distribution. The zone mainly develops strong extension and weak strike-slip tectonic styles represented by the S type, the imbricate fan type and the broom shaped type. The S type is mainly developed in the middle section or intersection of the main depression control fault; the imbricate fan type is widely developed at the pitching end of the strike-slip fault, and the Jinzhou 20-2 north structure in the western Liaoning zone is a typical development area; the broom shaped type is related to the strike-slip activity and is common in the Lüda 5-2 Area.

3.1.2. Evolution characteristics of superimposed strike-slip

The evolution of the western Liaoning area is controlled by the superimposition effect of strong extension and weak strike-slip in different geological periods, which are manifested in two aspects: (1) During the Paleocene-Eocene, Liaodong Bay and the Bohai Sea were in the extension rifting period on the whole. The western Liaoning experienced the northwest-southeast extensional deformation. The early strong tensile tectonic stress background led to the activity of the north-north-east pre-existing faults, which evolved into the boundary faults that controlled the development of the sub-sags. Since the Oligocene, the right-lateral strike-slip fault activity has become the main fault activity in the Liaodong Bay area, superimposing on the extensional structural deformation. (2) As the western Liaoning area is not in the area long affected by the Tanlu right-lateral strike-slip fault, the strike-slip movement has limited influence on the early extensional structure. The difference in the activity intensity of different tectonic stresses results in the relatively well preserved extensional structural deformation in the western Liaoning. The reformation of the strike-slip activity led to the connection and S-type bending of the early extensional main faults, and finally came the Liaoxi S-type weak strike-slip zone (Fig. 8b - 8d). The western Liaoning was subjected to weak strike-slip reformation in the Neogene, so few faults came about in the late stage, which is not conducive to the further migration of oil and gas to the shallow formations.

Therefore, the oil and gas mainly enrich in the buried hill and the Paleogene there.

3.2.1. Geometric characteristics of the faults

The Liaodong braided strong strike-slip zone consists of three secondary tectonic units, Liaozhong Depression, Liaodong Uplift and Liaodong Depression. Compared with the shovel-type faults in the western Liaoning, the faults in eastern Liaoning have more obvious strike-slip characteristics. The main depression control faults are more upright, and strike-slip negative flower shaped structures are seen locally, with obvious ribbon effect (Fig. 8a). On the plane, the faults have narrower fractured surface and flat and stable strike, and are in braided distribution in general, indicating that the faults have undergone strong reformation of strike-slip tectonic stress. The area mainly has vertical negative flower shaped, superimposed, duplex and other strong strike-slip and weak extension tectonic styles.

3.2.2. Evolution of superimposed strike-slip

The eastern Liaoning area is located in the braided strong strike-slip zone formed by the direct and strong reformation of the Tanlu right-lateral strike-slip fault. Similar to the western Liaoning area, it was dominated by extensional tectonic movements in the early Paleogene, forming deep basins at the south and north ends. In the sedimentary period of the Dongying Formation, especially in the late sedimentary period of the Dongying Formation, the eastern and western Liaoning areas experienced differential evolution. As an area directly penetrated by the eastern branch of the Tanlu strike-slip fault, the eastern Liaoning area was strongly reformed by the right-lateral strike-slip activity of the Tanlu strike-slip fault. The late strike-slip activity reformed the early extensional structures successively, which is embodied in two aspects: (1) Strike-slip inversion structure came about in the south sub-sag of Liaozhong Depression. The strike-slip effect during the depositional stage of Dongying Formation led to the inversion of the early extensional fault, forming the compressional tectonic deformation under the strike-slip effect. The Shahejie Formation deposited early was reversed and uplifted, and eroded in local part; (2) According to the fault activity and stratigraphic development characteristics, the Liaodong No. 1 fault began right-lateral strike-slip activity in the sedimentary period of Dongying Formation. In Mesozoic- Cenozoic, the rigid block of the buried hill gradually separated from the Jiaoliao uplift, forming the independent Liaodong uplift (Fig. 8b-8d). The late strike-slip reformation in the eastern Liaoning area was stronger than that in the western Liaoning, which resulted in the birth of high-abundance reservoirs such as Jinxian 1-1 Oilfield and Lüda 6-2 Oilfield; besides, the oil and gas could migrate to the shallower formations to form Neogene reservoirs represented by Jinzhou 23-2 Oilfield.

3.3.1. Geometric characteristics of the faults

In the western Bohai Sea, due to the interweaving and inter-cutting of the north-south-east Tanlu right-lateral strike- slip fault zone and the north-west Zhangjiakou-Penglai left-lateral strike-slip fault zone (referred to as the Zhang-Peng fault zone), the basin presents a bucket-like structure as a whole, forming a structure pattern of double strike-slip faults with conjugate overlap. The positive tectonic units such as the Chengbei low uplift, the Shaleitian uplift and the Shijiutuo uplift in the area are in the north-west trending axially, forming an “X” or “L” type alternating uplift-sag tectonic pattern (Fig. 9). On the section, the north-west main fault is characterized by shovel-like feature steep in the upper part and gentle in the lower part, and the north-east faults are mostly plate type normal or nearly vertical faults (Fig. 10a).

3.3.2. Evolution of superimposed strike-slip

The evolution of the western Bohai Sea is controlled by the superimposition of strike-slip faults in north-north-east and north-west strikes. During the Paleocene-Eocene, the western Bohai Sea experienced strong extensional fault depression. Under the strong tensile effect, the early pre-existing north- north-east and north-west faults began to activate, and the north-north-east fault in the Qikou area had a strong depression control effect, while north-west faults dominated in the areas such as the south of the Shijiutuo uplift, the south of the Chengbei low uplift and the Shaleitian uplift, and became boundary faults that controlled the boundary between the uplift and the depression. Since the Oligocene, the strike-slip activities of the north-north-east and north-west faults have increased to different degrees, resulting in the development of a large number of late active faults (Fig. 10b-10d). The new late active faults lapped over the north-north-east and north- west long-term active faults, forming complex “Y” shaped or semi-negative flowered structure. On the whole, the north- north-east Tanlu fault zone and the north-west Zhang-Peng fault zone are two regional strike-slip fault systems in eastern China, and intertwining in the western Bohai Sea to form a group of conjugated shear zones. In the late Paleogene, the two groups of strike-slip faults were opposite in polarity and offset in active intensity. The shallow associated faults were very dense and arranged in en echelon. Due to the differential superimposition of the two groups of conjugated faults, the uplift area in the western Bohai Sea is dominated by Neogene reservoirs such as Kenli 11-1 Oilfield and Qinhuangdao 32-6 Oilfield, and the conjugated fault zone is characterized by obvious deep and shallow complementary accumulations, such as Caofeidian 6-4 Oilfield.

3.4.1. Geometric characteristics of the faults

Located in the eastern part of the Bohai Sea, the eastern Bohai area mainly includes the Bodong low uplift, the Miaoxi north uplift, the Miaoxi south uplift, the Bozhong sag, the Miaoxi sag and other tectonic units. On the plane, the fault is relatively flat and has poor continuity and great change in trending. It is represented by the broom shaped fault system interconnected and combined by a plurality of north-north- east faults and north-east faults (Fig. 11). On the section, the fault occurrence is steep dipping, and it is mostly characterized by the plate-type normal fault or vertical fault. The local negative flower shaped structure is well developed with obvious strike-slip characteristics. The intensity of the strike-slip activity is between western Liaoning and eastern Liaoning. The eastern Bohai is generally characterized by the graben- horst combination under the control of the north-north-east strike-slip faults. The axial depression is in good agreement with the main fault in trending, with the deep and steep depression structure as the main feature (Fig. 12a).

The area mainly develops three tectonic styles of the en echelon type, the laterally adjustable hidden strike-slip type, and the conformal negative negative flower shaped type. Among them, the en echelon type is mainly developed in the interior of the Bodong sag; the laterally adjustable hidden strike-slip type is mainly affected by the differential extension, and mainly developed in the Miaoxi area; the conformal negative negative flower shaped type is mainly developed in the Bodong low uplift and Bonan low uplift, such as the Penglai 19-3 Structure.

3.4.2. Evolution characteristics of superimposed strike-slip

The east branch of the Tanlu strike-slip fault passes through both the eastern Bohai Sea and the eastern Liaoning directly. Compared with the eastern Liaoning, the strike-slip activity intensity of the eastern Bohai Sea is relatively weaker, and its developmental evolution is characterized by the differential superimposition of the extension and strike-slip activities in different periods. During the Paleocene-Eocene, the north- north-east main depression control fault with the strike-slip and extension properties started with activity, but its strike- slip nature was relatively weak, it was still dominated by strong extensional fault depression, forming a series of graben-horst structure combination controlled and formed by the north-north-east fault. At the end of the Oligocene, the strike- slip activity was intensified, and the extension activity began to weaken. It was still characterized by weak strike-slip and strong extension on the whole, and was mainly developed with the en echelon tectonic style. After entering the Neogene neotectonic period, the strike-slip activity was intensified, the associated faults were densely developed at the shallow layer, the number of faults was significantly increased, and the characteristics of the broom shaped fault system formed by the combination of the north-north-east main faults were basically finalized (Fig. 12b-12d). The local squeezing action caused by the strong activity of Neogene strike-slip reversed the normal fault in the negative negative flower shaped structure, resulting in the development of the conformal negative negative flower shaped structure in this area. On the other hand, the Neogene strike-slip activity gradually strengthened, promoting the migration and accumulation of oil and gas in the shallow layer, forming a series of Neogene oilfields represented by Penglai 15-2 Oilfield, Penglai 7-6 Oilfield and Penglai 20-2 Oilfield.

3.5.1. Geometric characteristics of the faults

Bonan parallel strong strike-slip zone, located in the southern part of the Bohai Sea, belongs to the sea area of the Jiyang Depression. The Tanlu strike-slip fault divided into three branches nearly parallelly passes through the zone, and the east and centural branches have apparent strike-slip. On the plane, the north-north-east fault and the nearly east-west fault are nearly vertically intersected to form an “H” type pattern. The north-north-east strike-slip fault has stable trending and good continuity. It is characterized by the superimposition and connection of multiple faults, and cuts the nearly east-west extensional fault (Fig. 13). On the section, the north-north- east strike-slip fault is featured by steep and straight occurrence and apparent strike-slip, which reflects the strong reformation characteristics for the early nearly east-west fault (Fig. 14a). The nearly east-west faults are mostly shovel-like normal faults steep in the upper part and gentle in the lower part, with obvious control on the sedimentation and settlement of the depression. The main axial direction of the depression is consistent with the distribution direction of nearly east-west fault, showing the characteristics of deep north and light south.

Compared with the obvious dominant position of the north- north-east fault in the Liaodong Bay area and the eastern Bohai Sea, the north-north-east fault system and the nearly east-west fault system in the southern Bohai Sea have an obvious cutting relationship, showing the obvious characteristics of the depression control of the early east-west fault and the reformation of the late north-north-east strike-slip fault. The north-north-east fault and the north-west fault of the southern Bohai Sea are dominant in different geological periods, with orthogonal characteristics on the whole. Therefore, the area mainly develops three tectonic styles of the “H” type, the vertically conductive hidden strike-slip, and the “L” type. Among them, the “H” type is mainly developed in the steep slope zone of the depression clamped by the east branch and the middle branch of the Tanlu strike-slip fault; due to the strong reformation of the late strike-slip activity on the early basement strike-slip fault, the vertically conductive hidden strike-slip is mainly developed in the depression and the slope zone; the “L” type is the result of the superimposition of the strike-slip faults in different directions on the time scale, mainly developed in the western part of the Bonan low uplift.

3.5.2. Evolution characteristics of superimposed strike-slip

The southern Bohai Sea is a parallel strong strike-slip zone formed by the depression control of the early east-west fault and the regional reformation of the late north-north-east strike-slip fault. Compared with other regions, the early extensional faults were utilized and reformed by the strike-slip faults in different degrees in the late stage, resulting in various fault systems and complex tectonic styles. The nearly east- west extensional fault and the north-north-east strike-slip fault in the southern Bohai Sea have relatively independent genesis and evolution, and completely retain their early structural traces relatively. In the Paleocene-Oligocene, the southern Bohai Sea was in a period of strong fault depression, and the nearly east-west fault experienced the intense faulting, creating the depression pattern faulted in the north part and overlapped in the south part in the southern Bohai Sea. Compared with the strong fault depression activity of the nearly east- west fault, the north-north-east fault has a relatively weak strike-slip activity during this period, and weak control on the sedimentation of the depression (Fig. 14b-14d). In the Neogene, the southern Bohai Sea entered the depression period, and the fault depression activity of the nearly east-west fault became weak, while the north-north-east fault became intense in the strike-slip activity, and gradually replaced the nearly east-west extensional fault to become the dominant geologic element of the southern Bohai Sea. The strong activity of the north-north-east strike-slip fault in the Neogene not only caused the broken and complicated shallow structure, but also reformed the early pre-existing faults, resulting in some basement faults to resurrect and a large number of shallow faults to regenerate, and the shallow new faults were mostly in en echelon arrangement (Fig. 14d). The southern Bohai Sea is the most intensely reconstructed area of the Neogene strike- slip activity in the entire Bohai Sea area. The north-north-east strike-slip fault and its derived shallow new faults act as oil and gas migration channels, which facilitates the further migration and accumulation of oil and gas to the shallow layer, so the north-north-east strike-slip fault zone in the southern Bohai Sea is often a shallow hydrocarbon accumulation zone, such as the Bozhong 28-34 oilfield group, the Bozhong 36-1 oilfield, the Bozhong 25-1 south oilfield, and the Kenli 9-5/6 oilfield. For a few steep slope zones controlled by the nearly east-west fault, the strike-slip reformation is relatively weak, and the deep hydrocarbon can also be well preserved, and it is easy to form the deep and shallow complex reservoir, such as the Kenli 10-1 oilfield near the Chengbei low uplift steep slope zone.

Since the Cenozoic, due to the oblique subduction of the Pacific plate, the back-arc mantle uplift extension and oblique strike-slip pull-apart have become the two main tectonic powers for the superimposed strike-slip in the Bohai Sea. Two groups of large strike-slip faults, the north-north-east Tanlu fault and the north-west-west Zhangjiakou-Penglai fault intersect in the Bohai Sea, laying the spatial structure foundation for the development of superimposed strike-slip faults. As the final destine of the basin evolution, different from the traditional rift basin with faulting activity weakening during the depression period, the Bohai Sea had stronger strike-slip activity in the Neogene neotectonic period, forming rich strike- slip tectonic styles under the superimposed strike-slip effect. According to the superimposition forms of the strike-slip faults of different intensities and natures at different ratios, the superimposed faults can be divided into 15 types of typical tectonic styles because of extension and strike-slip superimposition, extension and extrusion superimposition, extrusion and strike-slip superimposition. Based on the strike-slip faults and their associated structural responses, reconstruction of prototype basin and apatite fission track analysis, the Bohai Sea area has experienced 3 stages of major strike-slip activities since Cenozoic, which, in good agreement with the variations of subduction direction and rate of the Pacific Plate, are the intrinsic driving force behind the development of superimposed strike-slip faults in this area. The Tanlu and Zhang- Peng two groups of large strike-slip faults not only provided active boundary and stress adjustment reduction for the movements between the plates, also their spatial intersection and temporal superimposition provided objective conditions for the development of superimposed strike-slip faults in the Bohai Sea. According to the differences of plane combination type, the section tectonic style and the superimposed strike- slip tectonic intensity of the main depression control faults, the Bohai Sea can be divided into five superimposed strike- slip zones, Liaoxi S-type weak strike-slip zone, Liaodong braided strong strike-slip zone, Boxi conjugated medium strike-slip zone, Bodong broom shaped medium strike-like zone, and Bonan parallel strong strike-slip zone.