The Lanliao Fault is the eastern boundary of the Dongpu Sag. To the east of the fault’s footwall lies the Luxi Uplift, while the hanging wall consists of syn-rift Mesozoic–Cenozoic sediments (Qi et al., 2006). The Lanliao fault has a winding shape in plain view with three major strikes (Figure 1). From north to south, these strikes are near NNE, NE, and NS, respectively. Seismic profiles and balanced cross-sections are used to investigate the overall characteristics of faults in the Dongpu Sag (Figures 3–10). These dip cross sections provide geometry controls of the Lanliao Fault, indicating the maximum fault throw approximately 6 km or more.

The Changyuan fault (fault F2) is a major intra-basin fault in the study zone over 110 km long (Figure 1). F2 locates in the middle–southern sag. It serves as the western boundary controlling syn-rift sedimentary sequences (H₆–H₁). The strike orientation of F2 is roughly parallel to the Lanliao fault, and the fault dips SE. Maximal vertical displacement of F2 is over 6 km, which partially excesses the Lanliao fault. The Huanghe Fault (fault F3) is also important. Unlike F2, fault F3 only controlled the Oligocene sedimentary sequence (H₃–H₁). The strike of F3 is similar to the Lanliao Fault. F2 and F3 together form a graben structure, controlling the Menggangji subsag. F4 and Lanliao Fault in the southern sag suggested a two-stage fault development; the younger F4 (Machang Fault) cross-cut the older Lanliao Fault (Figure 9). Balanced restoration of the Dongpu Sag also suggested two stages of fault development (Liu et al., 2004; Liu, 2009).

In addition to these large-scale faults, a series of secondary faults are developed throughout the sag. These faults are widely distributed in the northern sag (Figure 1). Sparse seismic reflection data demonstrate that these secondary faults have minor displacement (<1,400 m) and control much of the basin geometry (Figures 3–5). Striking obliquely or diagonally to the Lanliao fault, these faults developed during the Late Eocene to the Early Oligocene, relatively late in the basin’s evolution (Qi and Yang, 2010). The localization of faults in the southern sag is significant; only two faults developed with large fault displacement over 1,200 m, intersecting the Lanliao Fault obliquely.

Rifting in the central sag concentrates along several secondary faults (e.g., fault F4 and F5). These faults have larger displacements and lengths compared to their northern counterparts, with displacements of up to 1,200 m. Faults F4 and F5 generated vertically near the central boundary fault, extending southwest (F4) and northwest (F5) into the hanging-wall basin, respectively (Figures 7, 8). The sediment characteristics of F5 in both the hanging wall and footwall indicate that F5 is originally formed during the Mesozoic. F5 was reactivated intensively in the Cenozoic along the original fault plane, crosscutting the Paleogene sedimentary sequences (H₁–H₆) toward the northwest.

The kinematics of the Lanliao Fault is reflected in the deformation and sedimentary records of the hanging wall and footwall. The Paleogene sediment in the hanging wall generally dip toward the Lanliao Fault plane, reflecting the characteristics of related fold deformation constrained by normal fault morphology. Due to significant differences in the fault plane morphology along its strike, the deformation of the overlying strata also shows variability. The hanging wall deformation is particularly complex within the study zone. The dominant deformation style is characterized by normal fault activities, which are investigated in detail (Allen et al., 1997; Yang et al., 2006; Cheng, 2009; Cheng et al., 2010; Zuo et al., 2017; Xu et al., 2018). Contemporaneous and subsequent folding in the hanging wall is also noteworthy and warrants further attention.

In cross sections, syn-rift sequences (H_r–H₁) in the northern sag exhibit broad wedge-shaped geometry and form a typical half graben. In the southern sag, syn-rift sequences are divided by the intra-basin rise, resulting in a graben–horst–graben pattern. The hanging wall strata thicken toward the boundary fault but thin and onlap onto the dip slope toward the west. Syn-rift strata close to the fault zone generally share similar dip angles with the boundary fault, defining the axis of an asymmetric syncline (Liu et a., 2001). The western limb of the syncline extends into the hanging wall, with the hinge zone forming in associated with the nearby depocenter. The syncline involves different syn-rift strata in various seismic profiles. In Figures 3, 4, sequences H₆–H₁ are all part of the syncline limb, whereas the Dongying Formation (H₂–H₁) is not involved in the syncline, as shown in Figures 6, 7. The syncline near the fault zone is unnoticeable, as shown in Figures 3, 8. The axial trace of the hanging wall asymmetric syncline is parallel to the strike of the Lanliao Fault.

Next to the asymmetric syncline is a W-trending intra-basin high (Figures 6–8). The intra-basin high exhibited neutral fold geometry with symmetric limbs and axial traces perpendicular to the Lanliao Fault. Previous studies have defined this intra-basin high as the transverse anticline, a type of synthetic transfer zone (Liu et al., 2004). Transverse anticline developed parallel to and in the hanging wall of the strike of faults. Fault displacement remained minimal on the anticline axis and increases rapidly toward both fold limbs along the strike into the nearby depocenters (Chen et al., 1998; Liu et al., 2004; Sun, 2007; Sun et al., 2006). Those anticlines are termed ‘fault nose’ structure or ‘buried hill’ by Chinese petroleum geologists; and the ‘nose’ or ‘hill’ commonly indicates the linkage site of individual fault segments (Su et al., 2011).

Two other types of anticlines are identified in the cross sections. One type abuts the top of the hanging wall with an axial trace parallel to the Lanliao Fault. The anticline consists of Es₂ and Es₁ sediments. Tectonic stress reversal caused the hanging wall of the fault to move upward, forming a series of reverse anticlines (Figure 3). The reverse anticline is related to the widespread Dongying tectonic event that separated the Paleogene and Neogene period in the Bohai Bay Basin (Su et al., 2010; Liang et al., 2016; Xu et al., 2018). The hinge zone is commonly truncated by the H₁ unconformity, denuding Ed and partial Es₁ layers of the anticline. Reverse anticline are commonly observed in the northern sag, indicating intense tectonic movement in the northern sag (Shi et al., 1999).

In addition to reverse anticlines, a set of broad anticlines are recognized in the cross sections. The eastern limb of these anticlines also comprises the western limb of the aforementioned syncline (Figures 5–8). Both limbs of the anticlines are cut off by small faults, developing the asymmetric anticline geometry. In particular, intense faulting of fault F3 in Figures 7, 8 truncated the western limb of the anticline, forming an eastern dip monocline. These anticlines have developed to form the central rise in the middle and southern sag (Liu et al., 2001). The center rise serves as the horst, creating a graben–horst–graben structure between the west dipping fault F3 and east dipping fault F2 (Figures 7, 8). The reverse anticlines are only observed in the northern sag and, the central rise exists in the middle–southern sag (Figures 3–6).

Differences in kinematic characteristics are also reflected in the displacement and rifting period of the Lanliao Fault. This article uses seismic profiles and balanced cross section to demonstrate the extension of the Lanliao Fault in the Paleogene. The extension of the Lanliao Fault shows significant changes along its direction, with very obvious segmentation (Qi et al., 2006). During the Es₄ period, the southern fault was significantly larger than the middle part, while the sedimentary thickness in the north was extremely thin and showed little correlation with fault activity. The Es₃ period was the peak period of faulting. During this time, the extension displacement of the southern and middle segments was relatively large, while the extension displacement of the northern segment was relatively small.

The largest extension displacement migrates northward compared to the Es₄ period. In the Es₂ period, the displacement of the southern Lanliao Fault was significantly smaller than that of the middle segment. Extension displacement moved to other secondary faults in the southern Dongpu Sag, such as the Huanghe Fault and Changyuan Fault. Shortening rates from balanced cross section also confirm this point of view (Figure 10). The extension rate of the Lanliao Fault was the highest in the Es₃ period, at 36%, followed by the Es₂ period at 25%, while the Es₄ period had a relatively small extension rate of 20% (Liu, 2009).

When fault segments converge, a transfer zone formed to regulate strain difference (Liu et al., 2000). Five segments of the Lanliao Fault shared approximate northwest dip; hence, only the synthetic transfer zone developed during the linkage of segments. As discussed above, the most commonly observed synthetic transfer zone comprises transverse anticlines (Liu et al., 2004). Transverse anticlines associated with boundary fault segments are common features, representing observable tips of fault segments correspond to the displacement minima (Morley, 2002). The displacement minima tend to remain constant during the linkage of fault segments. Five segments on the tip flank exhibit apparent differences in strike orientations and dip angles that constrain the overall slip within the fault zone. Although linked, displacement of each segment remains partially independent. Consequently, fragmented fault activities lead to the formation of the transfer zone in the tip zone.

MFS and QFS are the two major fault segments of the Lanliao Fault. These segments developed independently to form the Gegangji subsag and Qianliyuan subsag during the Es₄ and Es₃ ³ periods (Figures 12A, B). During their linkage in Es₃ ², the need to adjust the significant strain difference between two segments resulted in the formation of a large transverse anticline, known as the BGFS. The formation of the BGFS adheres to the typical fault evolution geometry at synthetic transfer zones as detailed by Morley (2002). Throughout the basin’s history, the development of a transverse anticline remains consistent due to significant differences in fault strike, dip angle, and activity strength (Figure 13A). Differences between the segments of the Lanliao fault are common. Transverse anticlines frequently occur between the linkage of NFS and QFS during Es₂ and the linkage of MFS and SFS during Es₃ ¹ (Figure 14).

Morley (2002) also proposed a special case of fault linkage. Once segments amalgamated, the new depocenter or displacement maximum on the fault zone may alter to fault joint where no significant geometry and strength difference between linking segments occur (Figure 13B). This scenario is exemplified by the linkage of two sub-segments to form the QFS on the Lanliao Fault. After the completion of the linkage during the Es₃ ² period, the QFS experienced slip along its entire length, dominating the Qianliyuan subsag (Figure 12D–G). The subsag developed overlaying the early transverse anticline in the control of the fault slip (Figure 15B).

The segmentation of the boundary fault has been investigated in detail; the next issue is the growth and linkage process of the boundary fault, which specifically includes two major problems: when did each fault segment link and the boundary fault finally join up? Moreover, how would the basin sediments respond during this process?

We further look forward to investigating the geometric pattern of the boundary fault linkage. A key issue is what the typical linkage geometry of the Lanliao fault is. Three models detailing the growth and linkage of boundary faults have been proposed: 1) small faults develop independently and create extensive areas of subsidence, respectively. The linkage of small faults occur at the later syn-rift stage, and the boundary fault develops relatively late in the basin evolution, lacking general controlling on the basin geometry and sedimentation. A representative of this linkage pattern is Usangu Flats in Tanzania (Figure 15A) (Ebinger et al., 1989; Anders and Schlische, 1994; Fontijn et al., 2010). 2) During the early syn-rift stage, the rapid fault linkage forms a boundary fault prior to significant basin development. Once linked, the boundary fault maintains the strike length and only increases fault displacement to dominate the development of basin deposits, which is represented by East Kigoma fault, Lake Tanganyika, North Africa (Figure 15B) (Morley, 1999). 3) The length of the boundary fault increases by the lateral propagation of small faults accompanied with simultaneous material filling and deposition. Boundary fault propagation occurs during the development of a basin, such as the Lokichar Fault in Kenya (Figure 15C) (Schlische, 1991; Morley, 2002; Macgregor, 2014).

The Lokichar Fault is a large normal fault developed in the East African rift system (Morley, 1999; Morley, 2002). During the Paleogene and early Miocene, the southern Lokichar Fault formed through the connection of two initially independent faults. The southern fault was active relatively early in the basin history and deactivated later in favor of northern fault segments activity during the late Miocene and Pliocene. The northern and southern faults were separated by a synthetic transfer zone. Later, the two faults became linked across the transfer zone. Morley suggested that the Lokichar Fault is a representative of a pattern (3).

Reviewing the linkage history of the Lanliao Fault, we observed a similarity with the Lokichar Fault. During the early Eocene, the early independent MFS and SFS linked to form the southern part of the Lanliao Fault, leading to the rapid development of the depocenter known as the Gegangji subsag. In the late Eocene, the southern MFS and SFS deactivated, while the independent QFS formed to control the Qianliyuan subsag. By the end of the Eocene, the SFS and QFS became linked across the synthetic transfer zone (BGFS), showing a similar evolution pattern to that of the Lokichar Fault.

There are two branches on the post-linkage development of boundary faults: 1) maximum displacement maintains around the center of the boundary fault (Morley, 2002); and 2) the post linkage development of the boundary fault could also induce the obvious relocation of the depocenter (Allen and Allen, 2013). The initiation of F2 was later than the Lanliao Fault. Once formed during Es₃, subsidence controlled by F2 was equal to that controlled by the MFS of the Lanliao Fault (Figures 12B–D). During the Es₃ ²–Es₂ period, fault displacement and subsidence of F2 literally exceeded the boundary fault, developing into the depocenter and controlling the Mengmangji subsag.

Large normal fault activity may end abruptly due to a major external event, such as tectonic inversion, or gradually weaken as other faults nearby increase the rifting intensity of the boundary fault (Twiss and Moores, 2007; Allen and Allen, 2013). According to the faulting history of the Dongpu Sag, two possible candidates of activity succession to induce the boundary fault to weaken are faults F2 and F3. The faulting maximum of the Lanliao fault was during Es₄–Es₂ and that of F2 was during Es₃–Es₂. The QFS fault segment and fault F2 both maintained prosperity during Es₂; subsidence declined swiftly, and the depocenter shrunk during the Es₁ (Figure 12F). This indicates that the extension of F2 and that the boundary fault shows a simultaneous feature. Generally, the formation of F2 is more likely an associated antithetic fault accommodating the huge shovel extension of the Lanliao Fault, rather than a fault succeeding the displacement. Shovel fault patterns of the Lanliao Fault lead to an issue that the rollover of the hanging wall could not bear such a significant extension in Es₃, thereby forming antithetic adjusting faults to the west or possibly forming a ramp-flat extensional duplex in depth (Qi et al., 2006; Cheng et al., 2010; Xu et al., 2018).

Stretching of the Lanliao Fault and fault F2 died out during Es₁, followed by shrank of subsidence. Therefore, the Es₁ deposition is believed to be an intermittent of episodic rifting in the Bohai Bay Basin (Qi, 2004; Luo et al., 2015; Zuo et al., 2017). During Es₁–Ed, the major active fault was fault F3. F3 formed during Es₃ ¹ and was strongly activated during Es₁–Ed later than F2. Rifting of F3 was very intense in Oligocene, subsiding over 2,000 m in the Menggangji subsag; and the footwall uplifted and eroded heavily (Figure 12G). Considering the strike and dip similarity of the boundary fault and F3, as well as the continuity in geological time, it is possible that F3 was the successive fault that occupied the displacement of the Lanliao Fault during Es₁–Ed.