Based on the existing seismic data (more than 5 seismic profiles), the study area can be segmented by identifying the continuity and morphology of deep faults. Each segment has its own master fault. On this basis, the study area can be divided into three segments—the southern, middle, and northern segments. In order to identify the structure of each segment, 5 inlines and 1 crossline were selected for structural analysis, based on surface data obtained from the 1 : 200 000 geological map and the digital elevation model data (Fig. 1).
Figures S1 and S2 show the original morphology of the 5 inlines, in which seismic lines AA' and BB' are located in the northern segment of the study area, while lines CC' and DD' are located in the middle of the study area, and seismic line EE' is located in the south of the study area. In the original seismic profile, some strong reflection wave groups were identified, which could be used as the basis for seismic horizon identification. According to surface geological data, and to layer tracing of the whole area, the characteristics of the lower boundary of the Cambrian strata are similar to those of the Silurian Longmaxi Formation. They are all a set of strong amplitude wave groups, which are filled with blank reflections of > 1 s two-way travel time. As well, in some areas, a clear medium-strong reflection wave group, which refers to the lower boundary of the Middle Cambrian, can be distinguished inside the blank reflection. These two sets of seismic waves, corresponding to the lower boundary of the Cambrian strata and Silurian Longmaxi Formation, are visible in all seismic profiles. Furthermore, they are similar in morphology, and can be classified into the same set of tectonic levels, that is, the middle tectonic level.
Due to the existence of the Silurian shale, the upper and middle tectonic levels were separated and deformed independently. Relatively independent deformation refers to the fact that the deformation is similar in general, but not in all details, which can be seen from many seismic profiles. The reason of deformation seen in the study area being similar is that the vertical deformation originated from the deep basement fault, while the shallow structure was inherited from development of the deep structure.
Not far below the lower boundary of the Cambrian strata, there is a regional angular unconformity. This set of unconformities is difficult to identify, because the wave group below the lower boundary of the Cambrian is weaker and more cluttered, but in some areas, blank reflection beneath the lower boundary of the Cambrian strata has intersected wave groups in the basement, at an angle of about 15°–30°, which can still be observed. The strata under the unconformity are named as the lower tectonic level, and consist of Sinian and basement strata. Its deformation is different from the tectonic levels mentioned above, which often develop large, high angle thrust faults and recoil faults originated from the basement detachment. These basement faults, which develop in a SE-NW extrusion setting, promote the whole, passive uplifting deformation of the overlying strata. At the same time, because of the existence of multiple sets of detachments, the upper and middle tectonic levels developed small structures independently, making the entire structural form more complicated.
As shown in Fig. S1, the SE segment of the seismic line in the northern part of the study area is similar to that in the middle part of the study area, and both developed large recoil faults in the basement layer (Fig. S1c), indicating that the transmission of extrusion stress has been hindered here, for the first time. To the NW, in seismic lines BB' and CC', we can see that the seismic wave group above the lower tectonic level has uplifted, turning into an arc. Meanwhile, the wave group in the lower tectonic level is uplifted to the NW, and intersects with the overlying wave group at high angles. This indicates that there have been several thrust faults, with top-to-the NW thrusting, developing below the wave group, which leads to the dip and uplift of the basement layer, and to the passive uplift of the strata above it. Furthermore, the wave groups above the lower tectonic level, which are in seismic line AA', are also bent, turning into the shape of the overturned anticline (Fig. S1a). This indicates that this anticline is different from the corresponding anticlines in seismic lines BB' and CC', and should be the result of the complex uplifting deformation of the basement thrust fault and the shallow recoil fault.
Further NW, a fault wave can be found at the bottom of seismic line AA', and its morphology is similar to that of folds in the overlying strata, indicating that the formation of the overlying fold was directly controlled by the fault. Signs of this recoil fault can also be found in seismic line BB', which can be identified from the dislocation of the seismic wave group (Fig. S1b), indicating that a reverse structure existed in the bottom of the NW limb of the Sangzhi-Shimen tectonic belt. This also meant that the transmission of extrusion stress was blocked for the second time, near the Xiangexi tectonic belt, rather than inside the Sangzhi-Shimen tectonic belt.
As for the southern segment of the study area, two large recoil faults, and folds sandwiched between them, can also be seen (Fig. S2). As shown in the figure, the forms of the trailing edges in seismic lines DD' and EE' are very similar, being large-scale folds filled with a large set of Ordovician– Proterozoic strata, which have been limited by large recoil faults, and affected by a plurality of reverse faults. Although the overall structure is similar, detailed review reveals differences. The uplift amplitude and width of the deep buried body (large-scale folds), whose existence has been confirmed in published studies (Dong et al., 2015), are significantly different. The uplift amplitude of the deep buried body in seismic line EE' is higher and its width is narrower, while the morphology of the deep buried body in seismic line DD' is basically flat.
Figures S3 and S4 show the final interpretation results for the 5 main inlines in the whole district, in which the important faults and layers are thickened by lines to highlight their variation tendency from the overall structural style. As can be seen, from south to north, the overall pattern of the study area is basically consistent, with gradual change. In the interpretation scheme of the five seismic lines, the common point is that recoil faults developed along the leading and trailing edges of the deformation. What's more, the recoil fault at the trailing edge constitutes the left boundary of the deep buried body beneath the Xuefeng thrust belt. Along with the similarities, there are four main differences between them: (1) the morphologies of the deep burial bodies beneath the Xuefeng thrust belt, (2) patterns and genesis of the folds between the two large recoil faults, (3) the size, number and deformation pattern of the recoil faults at the deformation leading edge, and (4) the depth of the basement detachment, which gradually decreases from SE to NW.
To sum up, large structures in the study area are formed based on growth of the basement faults developed beneath them. Thus, the kind of structural style developed in each study area segment has depended on the number, morphology, and development position of the basement faults. Whether these basement faults developed, where they developed, and how they grew, was closely related to the morphology of the basement detachment (the place where the morphology of the basement detachment changes is easier to develop faults). Therefore, the fact that the tectonic style changes from the south to the north is the result of gradual deformation of the basement detachment, under the action of compressive stress at different stages (He Z L et al., 2011; Li et al., 2011; Liu et al., 2010; Yan et al., 2003). It is not difficult to see, from the existing seismic interpretation scheme, that the basement detachment has experienced two uplifts, from SE to NW. As well, the depth of the basement detachment in the south is generally greater than that in the north (Figs. S3, S4), which corresponds well with the known major tectonic movements.
Similarly, Fig. S5 shows an interpretation scheme for a NE-SW seismic line. Unlike Fig. S3 and Fig. S4, the deformation illustrated in Fig. S5 is relatively weak, rendering the geological structure more stable, as the shallow strata are relatively flat, with only a sizable anticline. The deformation of the deep part is stronger than the shallow part, and is mainly concentrated on the NE side—with many large recoil faults developing in this area. The formation of these recoil faults should be related to the development of the Qinling-Dabie Mountains, which are located on the north side. At the same time, it is worth mentioning that, as shown in Fig. S5, the depth of the basement detachment gradually deepens from the SW to the NE, which is the reverse of the main tectonic movement direction. This occurrence of the phenomenon could also be attributed to the hindrance provided by the Qinling-Dabie Mountains.
Deformation is complicated in the study area, as, since the Paleozoic, it has undergone reformations by the Caledonian, Indosinian, Yanshan, and Himalayan movements (Zhang et al., 2013; Shu, 2012, 2006; Chen et al., 2011; He D F et al., 2011; Li et al., 2011; Wang et al., 2010; Yan et al., 2000), with the Yanshan movement being one of the most influential (Li et al., 2014; He Z L et al., 2011; Liu et al., 2010). By collecting thermal chronology data for the various predecessors, and combining this with apatite fission track data, we can confirm that there have been five key structural changes over time in the study area, at 251, 201, 163, 103 and 51 Ma (Zou et al., 2018; Mei et al., 2010; Yuan et al., 2010; Li et al., 2008). On this basis, under the premises that the thickness of the strata was constant (The study of He D F et al. (2011) showed that the tectonic belt had experienced many times of extension and convergence before the main deformation period, namely Yanshanian. Thus it's obvious that the tectonic belt does have several sets of stable stratigraphic deposits. At the same time, the tectonic movement before Yanshanian had little influence on the evolution of this tectonic belt, which also ensured there would be no large-scale stratigraphic loss), that the stress intensity in the northern segment was less than that of the middle-south segment (the tectonic stress is mainly from the southeastern side of the tectonic belt, resulting in the deformation of the northern segment of the study area is significantly weaker than that of the middle and south segments), and that the impact of the small structure on the whole structure was ignored (the impact of small structures will be discussed in the section "deformation-controlling factors"), tectonic evolution models for the south-middle segment (Fig. S6B1–Fig. S6B7), and north segment (Fig. S6C1–Fig. S6C6) were obtained, and the tectonic evolution of the study area could be divided into six stages (Fig. S6).
(1) The original sedimentary stage (Fig. S6A) was before the Early Triassic (~251 Ma). Previous studies have shown that in the Late Ordovician–Silurian, under the influence of compressive stress, the nature of the original rift basin gradually changed, and that the rudiment of the Xuefeng orogenic belt gradually formed (He D F et al., 2011). Then, during the period of 416–251 Ma (D–T), the area underwent another extensional rifting. This tectonism offset most of the influence brought about by early compressive stress, resulting in a southeast dipping normal fault, with a dip angle of 20°, developed in the southeast side of the buried body. Later, a large nappe developed along this normal fault. The pre-existing normal fault cannot be directly observed, but evidence of its existence is apparent, as in the seismic line, the basement strata in the nappe is thinner than that of the corresponding strata in the leading edge of the deformation. As the compressive stress was gradually transmitted from the SE to the NW, however, the strata involved in deformation in the nappe are considered to be have been equal to, or greater than, those in the leading edge. One reasonable explanation for this phenomenon is that, under the effect of compressive stress in the early stage, the Proterozoic strata in the nappe had experienced an uplift and denudation, and then they dropped, and were preserved under the subsequent extension, resulting in unequal thickness of the strata, between the two sides. In addition, if there was no extension which turned the reverse fault into a normal fault, it would be impossible for the thickness of the strata involved in the nappe deformation to either remain stable, or be less than that of the leading edge.
After 251 Ma, structural inversion occurred in the study area again. Under the action of compressive stress, the strata on the upper wall of the normal fault began to thrust upward, forming a large nappe in the trailing edge. Therefore, the main deformation time for the study area is considered to be 251 Ma or later, and the time before this stage has been regarded as approximately the original sedimentary stage.
(2) The formation stage of the large thrust nappe and deep buried body beneath it occurred from the Early to Late Triassic (251–201 Ma) (Fig. S6B1, Fig. S6B2, Fig. S6B3, Fig. S6C1, Fig. S6C2 and Fig. S6C3). As mentioned above, the hanging wall of the pre-existing normal fault gradually uplifted, connecting the strata above the Silurian strata with the corresponding strata of the footwall (Fig. S6B1 and Fig. S6C1). Later, they experienced the deformation stage together, gradually forming the large-scale nappe. Under the sustained action of compressive stress after the formation of the large thrust nappe, two SE-dipping thrust faults developed, in the bottom of the basement, in front of the thrust fault formed by structural inversion of the normal faults (Fig. S6B2, Fig. S6B3, Fig. S6C2 and Fig. S6C3). The last developed basement fault in this period connected two sets of basement detachments, at 22 and 15 km, and led to the uplift deformation of the overlying strata, which laid the foundation for development of later deep buried bodies (Fig. S6B4 and Fig. S6C4). In this stage, the evolution process and deformation results of the middle-south segment and north segment had few differences, and the strata shortening rate was similar.
(3) The formation stage of large recoil structures (Fig. S6B4 and Fig. S6C4) occurred from the late Late Triassic to the late Middle Jurassic (201–163 Ma). In the process of advancing NW, a recoil fault formed at the leading edge of deformation, resulting in a large suite of strata reverse thrusting on the buried bodies (and nappe). The scale of this recoil fault was not inferior to that of the pre-existing nappe fault. In addition, the lithology of the hanging wall was different to that of the contiguous footwall, which meant that conditions were in place for forming a lithologic hydrocarbon reservoir. Therefore, the fault played a role in blocking rather than intensifying oil and gas dissipation.
The boundary faults, with their deep burial and good sealing properties, and the shape of the giant bulge, indicated that the buried body was suitable for oil and gas storage. In this stage, the evolutionary processes of the middle-south segment (Fig. S6B4), and the northern segment (Fig. S6C4) were different, mainly reflected in whether or not the hanging wall strata overlay the nappe. Because the compressive stress of the northern segment was relatively small, the displacement of the reverse structure formed after the obstruction was smaller than that of the middle-south segment under the same conditions, meaning that the strata on the hanging wall were not active enough to move on the nappe. This differential evolution was also reflected in the shortening rate of the strata: the shortening rate of the northern segment was 18.7%, while the shortening rate of the middle-south segment was higher, at 27.3%.
(4) The second development stage of the nappe fault (Fig. S6B5) took place from latest Middle Jurassic to latest Early Cretaceous (163–103 Ma). During this period, the intensity of the tectonic compressional movement weakened, so the deformation pattern of the study area was not greatly changed, simply becoming more complicated in local areas. In the middle-south segment of the study area, the nappe continued to move forward under the action of compressive stress. Because the hanging wall of the recoil fault overlay the nappe, however, they were in direct contact, without any buffer zone between them. This meant that the movement of the nappe would be blocked, and that the nappe fault would develop again, cutting through the recoil fault ahead, to change it into two, small-scale recoil faults. In contrast, in the northern part of the study area, because the reverse structure and the hanging wall of the nappe were unconnected, there was a buffer space between the two structures, deforming the strata slightly.
(5) The basement thrust fault, located on the southeast side of the large recoil fault, developed again during the latest Early Cretaceous to Early Paleogene (103–51 Ma). Similarly, under the influence of compressive stress, in order to break through the recoil fault ahead, the thrust fault, like the nappe fault, cut through the large recoil fault, and acted as a bridge connecting the two sets of detachments, at 15 and 11 km. At the same time, it also caused the passive deformation and uplift of the overlying strata, complicating the structural style. In this period, the tectonic evolution processes of the middle-south and north segment were virtually the same. The shortening rates of the strata were very small for them both, at less than 1%.
(6) Since the Early Paleogene (51 Ma), we have mainly seen the vertical uplift stage (Fig. S6B7 and Fig. S6C6). After 51 Ma, the overall structural style has been basically stable, and the structural deformation has been mainly reflected in vertical uplift. After this vertical uplift, the earth's surface has been extensively eroded, as a result, most of the strata on the thrust fault hanging wall, and the strata above the Sinian strata on the nappe, disappear, destroying the original tectonic form, which made the tectonic restoration process much harder.