The Doseo depression is a complex faulted rift basin affected and formed by extension (Figure 1B), strike slip, and compressional tectonic environments (Fairhead, 2020). The main structure of the basin is a long and narrow dustpan-like faulted depression with a north-south zonation and east-west block structure. The basin is broadly characterized by one depression between two uplifts to form the northern uplift, the central Doseo depression, and the southern uplift. The Doseo depression in the middle is the main body, which can be divided into five structural units: the northern steep slope zone, the northern depression, the central low uplift zone, the southern depression, and the southern gentle slope fault terrace (Figure 3). The northern steep slope zone mainly develops reverse compression anticline structures, and the southern gentle slope fault terrace mainly consists of fault block structures (Kong et al., 2019). The Doseo depression is controlled by the Central African Shear Zone and distributed in a NE-SW narrow strip.

Multistage active faults are developed in Southern Chad Basin and Doseo depression (Figure 1B), and the boundary faults are different in different tectonic periods (Dou et al., 2022). The early basement normal fault in the Doseo depression was superimposed with strike slip activity in the late stage (Fairhead, 2020). The boundary faults and some internal faults were mainly active in the Mangara Formation sedimentary period, which controlled the deep rift structure of the Doseo depression. The faults in the depression are mainly active in Kedeni Formation -K₂ Formation sedimentary period (Figure 3). The fault displacement of these internal faults is relatively small, the dip angle is relatively steep, and the secondary faults are less developed. The whole set of strata are uniformly compressed and deformed from top to bottom, and the main body of the depression has an obvious double-layer structure, that is, the deep North Fault (F1 and F2) and South super dustpan half graben fault depression structure superimposes the shallow depression structure (Figure 3).

By selecting three sections of the Doseo depression perpendicular to the Central African shear zone from west to east, the faults in the depression have the characteristics of multi-stage activity in the vertical direction (Figure 3). In the early stage, the depression was extensional and developed normal faults (Fairhead, 2020). In the later stage, affected by strike slip and undergoing structural inversion, the strata were uplifted and denuded, especially in the northern and western depression zones of the central depression zone. The northern depression zone has a North-South double fault structural pattern, and angular unconformity and strata denudation can be seen at the top of the seismic profile (Figure 3). During the Kedeni Formation- Koumra Formation sedimentary period, a large number of normal faults and normal strike slip faults were developed in the depression, some of which controlled sedimentation, and the fault displacement was generally small (Figure 3). In the late Cretaceous K₂a Formation sedimentary period, the fault activity weakened. At the end of the K₂a Formation sedimentary period, the northern boundary fault of the Doseo depression was reversed, forming a fault anticline, and the southern part of the depression was greatly uplifted and denuded, forming an angular unconformity (Figure 3). At the end of the Cretaceous, strike slip and tectonic activities occurred in the boundary fault of the Doseo depression and the fault zone in the middle of the depression (Fairhead et al., 2013), which induced the development of a large number of echelon oblique secondary faults intersecting with base faults in the depression (Figure 7E), forming flower structures and arch tension anticlines.

Doseo depression experienced compressional, strike slip, and extensional stages (Kong et al., 2019; Fairhead, 2020). Under the influence of these three stages, the Doseo depression mainly developed five structural styles: inversion structure, strike slip structure, fault terrace, graben-horst system, and domino structure (Figure 4). Based on the characteristics of the depression structure and planar fault distribution, combined with the planar distribution characteristics of the regional fault system map, it can be concluded that the overall structural characteristics of the Doseo depression have the rule of planar Zoning: from north to south, it can be divided into northern fault fold structural belt, central strike slip structural belt and southern extension structural belt. A nearly E-W trending main fault developed in the north of the Doseo depression (Figure 1A). Due to the late compressive and torsional tectonic stress, the hanging wall strata of some large high angle normal faults are folded and deformed (Section A F2 in Figure 3). The structural style developed near the main fault is an inversion structure (Figure 4). As the Doseo depression experienced strike slip tectonic activities in a later stage, a series of strike slip structures were inherited and developed on the pre-existing normal fault in the middle of the depression. Of these strike slip structures, flower structures are the most prominent. These faults are arranged en-echelon in map view (Figures 1A, 4) and there is little evidence for syn-depositional faulting. The structural styles mainly developed in the south of the Doseo depression are half graben, fault terrace, and domino structures. The difference between them lies in whether the fault blocks between faults rotate. Some graben-horst systems are also developed locally, and the styles are arranged in parallel in map view (Figures 1A, 4).

Under the influence of multi-stage tectonic activities, Doseo depression formed its unique structural characteristics. To dissect the active characteristics of the faults, be familiar with the active periods of faulting, and interpret fault and basin evolution, we selected a total of 17 scattered marker faults on the three interpreted sections from west to east of the Doseo depression (Figure 3), calculate their paleo-fault throw in different sedimentary periods, and compare and analyze the overall activity of the faults (Figure 5).

As shown in Section B (Figure 3), the depression presents an asymmetric double fault structure on the profile, that is, large boundary faults F1 and F6 are developed on the north and south sides of the depression. In addition to these two boundary faults, there is also a deep and large fault F2 penetrating the basement in the central part of the depression. Its occurrence is similar to the left boundary fault F1 of the depression (Figure 3). In terms of the number of faults, the fault density developed between the two large faults F1 and F2 is less than that between the faults F2 and F6. The paleo-fault throw of faults F1 and F2 was large during the deposition of the Mangara Formation, which has a thickness of more than 500 m (Figure 5). The paleo-fault throw during other depositional periods was relatively small, indicating that the two major faults were strongly active during the deposition of the Mangra Formation, and the paleo-fault throw of fault F2 was negative during the deposition of the K₂a Formation. It is speculated that the fault properties of F2 changed during this period, and the fault had a strong reversal. The activity of boundary fault F6 was relatively strong during the deposition of the Mangara Formation and the U. Kedeni Formation (Figure 5). During the deposition of the K₂b Formation, the paleo-fault throw of fault F6 became negative, and therefore we suggest that fault F6 also reversed during this period (Figure 5). The paleo-fault throw of faults F3, F4, and F5 was relatively small, and F5 reversed on a small scale during the deposition of the K₂a Formation (Figure 5).

The fracture characteristics of the profile in Section A are roughly similar to those in Section B (Figure 3). The boundary fault controls the development of the depression, and the strike slip fault is developed in the middle. The profile combined with the location of the survey line also shows that the faults on the south side of the Doseo depression are more developed (Figures 1A, 3). While the large faults in the boundary and central part of the depression were strongly active during the deposition of the Mangara Formation sedimentary period, other faults were all late active (Figure 5), and their activity was relatively stable and gradually weakened. Fault F3 and F4 have negative paleo-fault throw values in the K₂a Formation sedimentary period, and F6 has negative values in the K₂b Formation sedimentary period (Figure 5), which also indicates that faulting in the depression during deposition of the K2a and K2b Formations reversed.

The fault activity characteristics of Section A and Section B are similar: the boundary fault of the depression and the deep and large fault in the depression are strongly active during the deposition of the Mangara Formation (Figure 5) and control the deep rift structure of Doseo depression. Most of the other faults in the depression are active during the deposition of the Kedeni and Doba Formation, and the fault displacement is relatively small. In the late stage, some faults in the middle and south of the depression reversed.

Section C survey line is located in the west of Section A and Section B (Figure 1B), which is relatively far away, so its section characteristics are also quite different. The left boundary fault F1 controls the spatial distribution of the depression. On the whole, the profile still shows that the fault density of the south side is greater than that of the north side (Figure 3), indicating that the number of faults in the map view increases from north to south (Figure 1A). The activities of F1, F2, and F3 in the early Mangara Formation sedimentary period and the L. Kedeni Formation sedimentary period are relatively strong, and the later activity decreases until fault activity on F1 and F3 increases during deposition of the K2 Formation, and the F2 fault reverses (Figure 5). Faults F4 and F5 are produced during the development of the depression, with small activity and scale. In general, the overall activity of faults in Section C is weaker than that in Section A and Section B (Figure 5).

In general, the boundary fault (Section B F1 in Figure 3) and the central deep fault (Section B F2 in Figure 3) in the Doseo depression were active early, with high activity intensity, and other internal faults inherited or later formed. The overall activity of the faults east of the depression has been relatively stronger than that in the west. The number of faults in the south of the depression is more than that in the north, and most of them are small faults with weak activity. During the K₂a and K₂b Formation depositional periods, the paleo-fault throw of some faults was negative. It is inferred that the inversion occurred in the later stage of the development of the depression, and the inversion mainly affected the faults in the middle and south of the depression.

The formation and evolution of the Doseo depression are closely related to the activities of the Central African Shear Zone. The Doseo depression is located in the Midwest of the Central African Shear Zone. The Central African Shear Zone experienced multi-stage tectonic activities, including strong and weak strike slip. Controlled by the intensity of the Central African Shear Zone, the Doseo depression also undergoes multistage tectonic evolution (Fairhead, 2009; Huang et al., 2021).

Based on the analysis of regional unconformities and the recovery of the regional tectonic stress field, this paper divides the tectonic evolution stages of Doseo depression using the method of balanced cross-section restorations (Figure 6). When considering these results in the context of the past and present regional tectonic setting, this approach unravels the evolution of the Doseo Depression, which evolves between stages of extension, strike slip, and structural inversion. The tectonic evolution of Doseo depression can be divided into five stages (Figure 6):

(1) Early Cretaceous rifting period (Mangara Formation sedimentary period). During the sedimentary period of the Mangara Formation in the Early Cretaceous, the South Chad Basin was stretched in an NNW-SSE direction and developed a fault depression. The balanced cross-section restoration (Figure 6) demonstrates that during this period, the north side of the stratum was deeply concave under the control of the boundary fault, and typical syndepositional phenomena occurred. The rest of the faults were sporadically developed, with low fault density but the large scale and branch faults were not developed. Faults jointly control the deposition of strata. The depression was mainly controlled by three major faults, which are the boundary faults on the north and south sides and the deep and large faults in the middle of the depression, controlling the overall structure of the depression. At this time, the sedimentary center of the depression was located in the east of the depression, with a thickness of up to 4,000 m, thinning to the west, mostly 1,000–2,000 m (Kong et al., 2019). With the continuation of north-south extension, sedimentation expanded, gradually expanding to the west, with a sedimentary thickness of 500–1,000 m.

The depression has well-developed faults. Drilling in the depression reveals that thick lower Cretaceous strata were deposited in the depression during this period, with a thickness of 5,000–6,500 m. This period is an important period for the formation of the depression. Sedimentary fillings account for 80% of the total sediment thickness in the depression. The study of sedimentary facies shows that lacustrine sediments are widely developed during this period, and the lithology is mainly shale (Kong et al., 2019). It is also an important period for the development of high-quality source rocks in the depression.

(2) Early Cretaceous fault depression period (Kedeni Formation- Doba Formation sedimentary period). During this period, due to the expansion of the South Atlantic and the superposition of transform faults, the Central African Shear Zone was activated (Binks and Fairhead, 1992). Southern Chad Basin as a whole is located in a dextral strike slip shear zone (Fairhead, 2020). During this period, the Doseo depression is a dextral strike slip pull apart depression, and the depression is controlled by boundary extensional strike slip faults. Our cross-section restoration (Figure 6) shows that three groups of strata were deposited in this period: L. Kedeni Formation, U. Kedeni Formation, and Doba Formation. The faults in this period mainly developed in the middle and south of the depression, and the density of the faults increased and the scale decreased. The faults in the middle part are inherited and developed, forming a strike slip structural style (Figure 6). Except for the boundary faults on the north and south sides, there are no obvious syndepositional features on other faults. During the continuous evolution of the Deseo depression, the shape of the boundary normal fault changed from planer to curved.

With the rapid separation of the African plate and the South American plate (Guiraud et al., 2005), we interpret that the regional dynamics of tectonic deformation have changed. The North-South stretching of the Central South Africa block and the northeast Africa block switched to a dextral strike slip motion, and the depression began to form two sedimentary centers in the East and West. The sedimentary thickness in the East is large, up to 1,000–1,500 m, and that in the west is thin, mostly 500–1,000 m (Kong et al., 2019).

(3) Late Cretaceous thermal subsidence period (Koumra Formation sedimentary period). In the early Late Cretaceous, the strike slip motion of the Central African Shear Zone became weaker, and the torsion tension of the Doseo depression became weaker. In response, thermal subsidence occurred in the study area (Zhang et al., 2015; Fairhead, 2020), with a slow sedimentation rate and sedimentation occurring over a wide area, and the sedimentation center gradually moved southward to the Midwest of the depression (Lv and Zhao, 2018). The depression began to enter the stage of slow subsidence, producing gently sloping paleo-topography, showing a “horn shape” with a high in the South and low in the East, with a thickness of 1,000–1,500 m. During this period, the fault succession developed and the activity intensity weakened, which had a slight impact on the sedimentation, and therefore the thickness of the Doba Formation is stable.

The two stages of strike slip movement explored in this paper are located in the second and fifth stages of the structural evolution of Doseo depression respectively. For a more detailed analysis of the fault activity characteristics and more precise division of fault activity periods, the fault spatial distribution in Doseo depression, relevant fault growth index, and paleo-fault throw in each sedimentary period are superimposed to analyze fault activity.

The two strike slip movements experienced by Dosoe depression are the first stage of strike slip tectonic movement under tilting extension and the second stage of strike slip tectonic movement under transform compression. The genetic mechanisms of the two strike slip movements were different, and faults with different properties were developed, so the impact of the two strike slip movements on the depression were completely different. The first stage of the strike slip movement mainly controlled the overall structure of the depression, especially the development of the sedimentary center, which made the depression form two sedimentary centers, the East and the West. The second stage of strike slip tectonic movement mainly controlled the formation of some inversion structures and southwest northeast trending anticlines and fault anticlines. The superposition of strike slip structures in the two stages led to the extensive development of composite flower structures in the sag.

The development of source rocks and reservoirs was controlled by different faulting periods in the basin. The early depression-controlled normal fault underwent strike slip inversion during the depression period of the basin and became the main oil source fault. The strike slip inversion structural belt controlled the development of structural traps, and then controlled the enrichment of oil and gas (Hu et al., 2007).