Introduction
Deep (deeper than 3500 m) to ultra-deep (deeper than 6000 m) hydrocarbon resources are gradually becoming key targets for oil and gas exploration and scientific research [1-2]. However, such reservoirs tend to be tight and heterogeneous, the fractures are therefore becoming more important [1⇓⇓⇓-5]. Deep-ultra-deep extensional and compressional structures are rich in hydrocarbon resources, hydrocarbon reservoirs associated with strike-slip faults in intracratonic basins are gradually found in recent years [6-7]. An oil field of about 9×104 km2 controlled by strike-slip fault systems was found in the central Tarim cratonic basin, with proven reserves of more than 10×108 t and annual crude oil production of 200×104 t [3,8 -9], confirming that strike-slip faults have a significant impact on reservoir reconstruction, hydrocarbon transport and accumulation and enrichment [7⇓⇓⇓⇓⇓-13]. Similarly, the development of strike-slip faults and the associated hydrocarbon accumulation in the Sichuan cratonic basin has begun to attract attention.
In the Sichuan Basin, since the discovery of deep Sinian-Cambrian gas fields around the Deyang-Anyue trough in 2011, facies-controlled gas reservoir model has been gradually established [14⇓⇓-17], and the deep carbonate reservoirs became the key exploration direction [17-18]. For tight and deep reservoirs, the development of strike-slip faults is crucial to forming large fault-controlled gas reservoirs, so the characteristics of strike-slip faults attract scholars to explore [19⇓⇓-22]. Based on 3D seismic data of 6600 km2, Ma et al. concluded that Paleozoic dextral and sinistral transtensional strike-slip faults were developed in the Gaoshiti-Moxi (GM for short) area by analyzing the coherence and amplitude attributes, which experienced two stages of fault activities in the early Caledonian and Late Hercynian respectively [20]. Based on 3D seismic data of 2778 km2 in the central Sichuan Basin, Li et al. proposed that dextral transtensional strike-slip faults were developed in the Xingkai cycle and sinistral weakly transpressional strike-slip faults were developed in the Caledonian cycle [21]. Guan et al. analyzed the Triassic fault system in the central Sichuan Basin and concluded that the faults experienced the Late Ordovician-Silurian dextral strike-slipping period, the Late Paleozoic-Triassic fault silence period, and the Middle Jurassic to present sinistral strike-slipping period, and they have been influenced by the circumferential orogeny since the Caledonian and Middle Jurassic [22]. These results show that the strike-slip fault system in the central Sichuan Basin has a complex genetic mechanism and undergoes multiple stages of faulting activity, but few chronological evidence has been found to confirm the timing and dynamic mechanism.
The formation of intracratonic strike-slip faults is generally related to the adjustment mechanism of intraplate contraction and deformation caused by plate subduction and collision [23⇓⇓-26], such as the transpressional strike-slip fault system in the Tarim cratonic basin [27]. However, the deep transtensional faults in the central Sichuan Basin [19⇓-21] are likely to have a different genetic mechanism from the intraplate strike-slip fault under the convergent plate background. It is necessary to study the initial timing and genetic mechanism of the strike-slip faults, which influences the development of later strike-slip faults. Based on the seismic and tectonic analysis of the strike-slip faults in the central Sichuan Basin, this paper combines the U-Pb dating results of fault-related fracture cement to comprehensively determine the timing of the strike-slip fault and explore the evolution of the intracratonic strike-slip fault.
1. Geological setting
The Sichuan Basin is located in the northwest Yangzi block and has undergone multiple phases of tectono-sedimentary cycles, which are closely related to the opening and closing of the Proto-Neotethys Ocean [28] (Fig. 1 ). Spatially, it is composed of the Sinian-Silurian marine cratonic rift basin, Carboniferous-Middle Triassic marine cratonic depression basin, Upper Triassic marine-continental rift basin, Upper Triassic-Jurassic continental depression basin, Cretaceous-Cenozoic foreland basin and other prototype basins [26-27].
After experiencing multiphase extension-compression cycles, a series of thrust structures were developed on the margin and strike-slip faults in the cratonic center of the Sichuan Basin [19⇓-21,28], showing obvious zoning features in the plane (Fig. 1 a). The central Sichuan Basin experienced the late Sinian initial uplifting and late Caledonian- pre-Permian strong uplifting to form the main body, and finally formed the Leshan-Longnüsi paleo-uplift during Himalayan [29-30]. In addition, the basin underwent at least two important extensional cycles [31-32]. The first extensional stage was from the late Sinian to the Cambrian Longwangmiao Formation deposition period under the Xingkai Movement, forming the Anyue trough with a near S-N orientation. The second weak extensional stage was from the Late Permian to the Early Triassic under the Emei Tafrogecy, which induced the formation of the NNW-SSE-striking Kaijiang-Liangping trough. The study area is composed of the Sinian, Cambrian, Ordovician and Permian sedimentary strata from bottom to top, with Devonian strata generally missing and Ordovician-Carboniferous strata remained at the margin of the paleo- uplift (Fig. 1 b).
2. Strike-slip faults in the central Sichuan Basin
2.1. Distribution of strike-slip faults
Combining with the tectonics of the Sichuan Basin, a method of identifying strike-slip faults using high-quality seismic data [19-20] was used. First attributes with sensitive responses to strike-slip faults such as symmetry illumination and maximum likelihood [20] were selected to carry out seismic interpretation, and then the distribution of strike-slip faults was analyzed based on 3D seismic data of about 21 590 km2 (Fig. 2 ).
Fig. 2. Distribution of strike-slip faults at the Sinian top in a 3D seismic survey in central Sichuan Basin (see the location in |
The interpretation results show that 34 large strike-slip fault zones were developed, the length is generally 30-80 km, and the maximum (FI6) can reach 160 km and extends outside the 3D survey. The strike-slip faults are almost NWW-EW, NW and NE. Nearly 20 NWW-EW faults are longer than 50 km, which constitute the framework of the strike-slip fault system. The fault spacing is about 10 km, the minimum is close to 5 km, and the maximum can reach 15 km. Some NW-striking faults intersect with each other and diverge and spread out by taking the NWW faults as the axis. NE faults also go through the fault blocks. In the slope zone on the north of the GM area (the north side of the division line in Fig. 2 ), three groups of NWW-nearly EW faults are mainly developed with weak continuity or discontinuity characteristics. A single fault extends in a short distance, mainly including fault F1 and its surrounding secondary faults. The fault strike gradually changes from NW to nearly EW direction and spreads southeast to form a horsetail structure. The single faults are still linear with discontinuity characteristics (Fig. 2 ).
The faults are divided into three orders according to parameters such as extension length and throw (Fig. 2 ). (1) The total extension length of the first-order fault is more than 50 km, and the maximum throw is more than 60 m. (2) The total length of the second-order fault is generally greater than 20 km, and the maximum throw is greater than 40 m. (3) Other short-distance faults are at a lower order and not distinguished here. According to this division scheme, there are 12 first-order strike-slip faults with NWW-near EW trends at the bottom of the Cambrian in the central Sichuan Basin, with a total length of about 832 km. Among them, FI12 extends to the two-dimensional area with an obvious fault-related seismic response and a large throw on the profile, so it is also defined as a first-order fault. There are 22 second-order strike-slip faults with a total length of about 570 km, including 6 NE faults with a total length of about 146 km and 17 NWW-near EW strike-slip faults with a total length of about 424 km. There are nearly 700 lower-order smaller faults, of which NE faults are about 450 km long, and NWW and NW faults are about 970 km long. In the paleo-uplift zone of the central Sichuan Basin, the strike-slip faults extend long and show clear fault styles. Most of them are linear, en echelon and oblique faults are rare, but horsetail and overlapped structures are frequent.
2.2. Stratified features of strike-slip faults
The seismic profile shows that the strike-slip faults in the central Sichuan Basin are mainly distributed in the Sinian-Permian formations. Many faults terminate at the bottom of the Permian, a small number of faults extend upward into the Permian, and a very small number extends to the Triassic and above (Fig. 3 ). The faults have typical structures of strike-slip faults on the profile, such as the flower-like structure, steep and upright structures, reversal of dip direction, etc. The strike-slip fault has the characteristics of stratification in the vertical direction, with different styles in different layers. Some faults develop under the Cambrian unconformity, and others develop within the Cambrian. The seismic events in the Cambrian strata are obviously discontinuous, while those near the Permian bottom are weakly deformed. The seismic profile shows that some strike-slip faults are characterized by obvious multi-layer flower structures, and the seismic response characteristics, deformation and properties of faults passing through the upper and lower strata are different.
Fig. 3. Interpretation of seismic profiles showing typical strike-slip fault system through central Sichuan Basin (see the profile location in |
In the GM area, strike-slip faults have been interpreted at the bottom of the Dengying Formation, the bottom of the Cambrian, the bottom of the Longwangmiao Formation, the bottom of the Lower Permian, and the bottom of the Upper Permian (Figs. 2 and 4 ). Referring to the distribution and characteristics of the strike-slip faults above, they are mainly composed of NWW and NEE faults. The fault network of 5 layers remains unchanged, in addition, the structural style and scale have obvious inheritance and certain differences.
Fig. 4. Distribution of strike-slip faults of different horizons in central Sichuan Basin. (a) Bottom of the Sinian Dengying Formation; (b) Bottom of the Cambrian Longwangmiao Formation; (c) Bottom of the Lower Permian; (d) Bottom of the Upper Permian. |
(1) The Sinian Dengying Formation is mainly composed of rigid carbonate rock. At the bottom of the Dengying Formation (Fig. 4 a), the faults are characterized by long extension and good seismic continuity, and the seismic events are faulted on the seismic profile (Fig. 3 ). (2) At the Dengying Formation top (Fig. 2 ), the faults extend long and are characterized by obvious linear and en echelon structures, and the horsetail structure, overlapping structure, oblique structure and dextral en echelon structure can be seen in the local area. The faults inherit the structural styles at the bottom of the Dengying Formation, but become more complicated. (3) At the bottom of the Cambrian Longwangmiao Formation (Fig. 4 b), the en echelon structure is the most prominent. Most of the first-order and second-order fault segments show typical dextral en echelon styles, such as FI6-FI11. The horsetail structures spread out at the ends of some first-order faults are also the products of dextral movement. Huge and thick Qiongzhusi Formation mudstone was formed between the Longwangmiao Formation and Dengying Formation, therefore the displacement of the strike-slip fault is easy to offset in the plastic strata, inducing the strike-slip fault to disappear inside the Cambrian. This is one of the reasons why the strike-slip faults have the most mature characteristics, the largest scale and length at the bottom of the Cambrian (Fig. 2 ). (4) The bottom of the Lower Permian is the most denuded unconformity in the central Sichuan Basin (Fig. 4 c). Compared with the bottom of the Cambrian, the strike-slip faults at the bottom of the Lower Permian still inherited the fault system, but the scale significantly reduced. They often appear in pairs, and the principal displacement zone is more fragmented and complex. Their lateral continuity is weak along the strike. Many discontinuous fault segments are distributed in various regions. The strike-slip fault system mainly includes en echelon, obliquely oblique and horsetail structures, which is consistent with the phenomenon that the strike-slip faults spread from the bottom to the top from a single fault. There are still many en echelon structures that could be observed, but the movement direction of some faults has changed. For example, the northwest segment of FI11 is composed of dextral en echelon faults, while the southeast segment is composed of sinistral en echelon faults. FI12 is also characterized by sinistral en echelon faults. (5) At the bottom of the Upper Permian (Fig. 4 d), the fault scale decreased sharply, and the number of NWW faults also decreased significantly in the GM area. Many faults have obvious sinistral en echelon characteristics (FI9-FI12). Some discontinuous fault segments developed along the fault zone only in local areas. The fault system as a whole is commonly characterized by discontinuous and echelon structures, but the NE FII19 still shows a continuous and long fault zone.
3. Timing of strike-slip fault formation
The vertical displacement of the strike-slip fault is small, and the formation where the fault terminates may not be the youngest [33]. And after superposition and re-construction by multiple stages of fault activity, it is difficult to identify the precise timing of fault formation. The seismic-geological method combined with the analysis of tectonic background is a common method [27,34]. In addition, chronologic methods by analyzing fluid inclusions and dating fault-related fracture carbonate cement become more and more important to determine the timing of strike-slip fault formation [27,35].
3.1. Seismic-geological analysis
3.1.1. Horizon where most faults terminate
Physical simulation experiments show that although strike-slip faults do not necessarily develop to the surface[33], for more mature strike-slip fault systems, the formation period can be traced by identifying the horizon where they terminate [27,33]. The seismic profile in Fig. 3 shows that most strike-slip faults in the central Sichuan Basin terminated at the bottom of the Permian. The seismic profile in Fig. 5 indicates that a considerable number of faults disappeared within the Lower Cambrian, and fewer faults disappeared below the bottom of the Cambrian, indicating that most strike-slip faults ceased their activities before the Permian, while some faults stopped their activities in the pre-Sinian and the Early Cambrian.
Fig. 5. Typical seismic profile illustrating fault activity timing (see the profile location in |
3.1.2. Horizon where en echelon faults terminate
The disappearance of the en echelon fault reveals the termination of strike-slip fault activity. According to the fault interpretation of the major seismic horizons, en echelon structures could be observed in different positions at formation bottoms. For example, a large scale of dextral en echelon faults can be observed at the bottom of the Longwangmiao Formation (Fig. 6 a), dextral and sinistral en echelon faults co-exist at the bottom of the Lower Permian (Fig. 6 b, 6c), and sinistral en echelon faults could be observed in several areas at the bottom of the Upper Permian (Fig. 6 d, 6e). As observed on the seismic profile, the en echelon faults at the bottom of the Longwangmiao Formation terminate inside the Longwangmiao Formation, and these en echelon faults do not develop successively at the bottom of the Lower Permian (Fig. 6 f). The en echelon structure at the bottom of the Lower Permian also terminates inside the Longwangmiao Formation. The en echelon faults at the bottom of the Upper Permian terminate inside the Upper Permian (Fig. 6 g). The age of the formations where these en echelon faults terminate constrain the latest timing of the fault activity, revealing that strike-slip fault activities took place in the Early Cambrian, pre-Permian and Late Permian.
Fig. 6. Local symmetrical illumination attributes show en echelon faults in different formations and seismic profiles across en echelon faults in central Sichuan Basin (see A-E in |
3.1.3. Differences in deformation styles and properties of upper and lower faults
The Sichuan Basin has experienced multiple tectonic movements, and the properties and direction of the regional stress field have changed accordingly. The fault activation is accompanied by differential distribution of extension, compression and strike-slip components, resulting in differences in fault scale, structural style and fault properties, which can constrain the activity time of strike-slip faults, such as the multi-layer flower structure and property transformation of transtension and transpression.
The seismic profile shows that the faults within the Sinian strata (below the bottom of the Cambrian) have relatively complex deformation styles. Multiple faults developed from the basement and spread upwards, with multiple secondary faults terminated at the bottom of the Cambrian. Many faults only exist in the Lower Cambrian with relatively large throws (Figs. 3 , 5 and 7). In Fig. 5 , fault FII13 develops at least 3 layers of flower structures. (1) The first layer is under the bottom of the Cambrian and diverges from the basement upward. Its seismic events are obviously faulted, forming a fracture zone. Some secondary branches disappear upward in the Lower Cambrian. (2) The second layer exists under the bottom of the Permian, and a few faults continue to extend upward from the Sinian. The seismic events are obviously faulted, such as that at the bottom of the Longwangmiao Formation. But only a slight flexural fold could be observed below the bottom of the Permian (Fig. 7 a, 7b). (3) A layer of flower structure is also formed in the Permian and above. Due to the weak fault activity, the fault breaks the events with small vertical displacement, and many of them tend to have flexure deformation. Fig. 7 shows obvious negative flower structures in the Cambrian, while Fig. 7 c shows a positive flower structure in the Upper Permian, confirming multiphase reactivation and superposition of strike-slip faults in the Late Permian. The three layers of flower structures developed in the Lower Cambrian, below the bottom of Permian and the Upper Permian have proved at least three stages of strike-slip fault activities.
Fig. 7. Typical seismic profiles show (a) NE-striking faults, (b) NW-striking faults and (c) reverse flower structures (see the profile locations in |
The changes in fault properties are important evidence of multiphase activities of strike-slip faults. The specific characteristics are as follows. (1) The en echelon faults show distinctly opposite movement directions below and above major formations. The large-scale development of the dextral en echelon structure in the Lower Cambrian and the sinistral en echelon structure in the Upper Permian bottom represent at least two stages of strike-slip fault activities in the Early Cambrian and the Late Permian (Fig. 6 ). (2) The typical phenomenon that the strong transtension characteristics with “large drops” in the Lower Cambrian (Figs. 6f and 7), the transpression characteristics shown as fault-related deflections below the Permian bottom (Figs. 6f and 7a), and the weak transtension characteristics shown as “small drops” in the Upper Permian (Fig. 6 f, 6g) indicate that the strike-slip faults have experienced three stages of strike-slip activities, i.e., strong transtension movement in the Early Cambrian, transpression movement before the Permian, and weak transtension movement in the Late Permian. (3) The fault in the Cambrian below the Permian bottom has a large drop and strong seismic response, while the fault has a small drop and weak seismic response also represents at least two periods of activity (Fig. 8 ).
Fig. 8. Sinian Deng 4 seismic facies overlapped by strike- slip faults and typical seismic profile (see |
3.1.4. Separated by large unconformities
The deformation style and property of faults show distinct features below and above the large unconformity. Key unconformities, such as the bottom of the Cambrian, the bottom of the Permian, and the bottom of the Upper Permian, are the products of important movements such as the Tongwan II movement, the Hercynian movement and the Emei Tafrogecy. The Permian bottom and the Cambrian bottom separated the strike-slip faults in scale and style, forming a multi-layer flower structure (Figs. 3 , 5 and 7). Especially, the fault properties, rotating directions and fault throws of the en echelon faults are very different below and above the Permian bottom unconformity. These evidences suggest different fault activities before and after the Permian.
It is generally difficult to judge whether the strike-slip faults were active or not before the Cambrian by seismic profiles or fault throws because the maximum vertical displacement was usually found in the Lower Cambrian. However, the significant difference in seismic facies is obvious on both sides of the faults (Fig. 8 ). On the plan map, the boundary where seismic facies change is highly consistent with some strike-slip fault segments (Fig. 8 a, 8b), revealing that the strike-slip faults were initially shaped in the Late Sinian, and may affect the local change of sedimentary facies. The denudation at the end of the Sinian and the later fault activities made it more difficult to identify the early fault activities.
3.2. U-Pb dating of fault-related fracture cement
In recent years, U-Pb dating of carbonate cement in fault-related fractures gradually becomes an important method for determining the period of fault activity and has achieved good application in the Tarim Basin [27,35]. Various fractures were widely developed in the Sinian- Permian carbonate strata in the Sichuan Basin, and a large number of tectonic fractures were observed in cores[36]. Cores were taken from three wells 1.5 km from a fault. These carbonate samples, including Sinian and Permian fault breccias and fracture cements, were dated using a LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometer), and the precise U-Pb age of the fracture cement was obtained (Fig. 9 ). For example, Well PT1 was drilled on the FI7 fault in the GM area where many structural fractures were developed in the second member of the Dengying Formation. The logging data of Well PT1 show that the primary fracture orientation is consistent with the fault orientation, and the U-Pb ages of the fracture carbonate cement are (474 ± 24) Ma and (531 ± 24) Ma (Fig. 9 a, 9b). Well GS1 was drilled on the FI10 fault zone. The U-Pb age of the carbonate in the fault breccia in the Fourth Member of the Dengying Formation is (510 ± 26) Ma (Fig. 9 c). Well GS2 is adjacent to a NW-striking strike-slip fault, where the ages of the fracture-related cement in the Qixia Formation are (267.7 ± 1.2) Ma and (260.8 ± 1.4) Ma (Fig. 9 d, 9e). The measured precipitation periods indicate at least three stages of fault-fluid activities in the Early Cambrian, Ordovician, and Late Permian, respectively. Since the fault activity should be earlier than or at the same time as the fracture cement, the age has a good matching relationship with the strike-slip fault activity time from seismic analysis.
Fig. 9. U-Pb age concordia plot of carbonate cement (MSWD—mean standard weight deviation). (a) Sample 1, Sinian Deng 2 Member, Well PT1, 5779 m; (b) Sample 2, Sinian Deng 2 Member, Well PT1, 5779 m; (c) Breccia, Sinian Deng 4 Member, Well GS1, 4971 m; (d) Sample 1, Permian Qixia Formation, Well GS2, 4614 m; (e) Sample 2, Permian Qixia Formation, Well GS2, 4614 m. |
In addition, He et al. analyzed the temperature of the fluid inclusions in the structural fractures in the central Sichuan Basin, and revealed three stages of structural fracture activities [36], namely the Tongwan II movement, the Caledonian and the Yanshan-Himalayan periods. The results also suggest the activity periods of the strike-slip faults in the central Sichuan Basin to a certain extent.
3.3. Timing of multiphase strike-slip fault movements
Comprehensive seismo-geological analysis, fracture cement dating and fluid inclusion temperature measurement data confirmed that the strike-slip faults in the central Sichuan Basin have the characteristics of multiphase activities. The bottom of the multi-layer flower structure and a considerable part of the faults terminated at the bottom of the Sinian, and the difference of seismic facies on both sides of the faults in the fourth member of the Dengying Formation are the proofs of active strike-slip faults at the end of the Sinian period. In the Early Cambrian, the strike-slip faults went through strong dextral extension reflected by the disappearance of many faults in the Lower Cambrian, the termination of en echelon faults in the Lower Cambrian, the property difference between the strong transtensional deformation of the Lower Cambrian and the weak deformation or deflection below the bottom of the overlying Permian, the largest fault throw, the U-Pb dating results of fracture cement (Fig. 9 b, 9c) and the chronological evidence of inclusions [36]. Before the Permian, the strike-slip faults reactivated with weak transpressional features and weak strata deformation, and only deflection and deformation occurred near some faults. Large-scale faults terminated at the regional Permian unconformity bottom. In addition, U-Pb dating data (Fig. 9 a) also indicate Ordovician fault-fluid activity. The termination of the weak transtensional sinistral en echelon structure in the Upper Permian, the U-Pb dating results of the Late Permian (Fig. 9 d, 9e), and the termination of a considerable number of faults under the top of the Permian all demonstrate the activation of the strike-slip faults in the Late Permian. Only a small number of faults were activated and developed in the Triassic, and their steep characteristics and weak structural deformation make it difficult to determine the specific activity time (Fig. 7 a).
It can be seen that the strike-slip faults in the central Sichuan Basin were developed on the early basement. They went through multiple stages of evolution: Embryonic stage of strike-slip faulting in the Nanhua period, dextral transtensional strike-slip faulting stage from the Late Sinian to the Early Cambrian, weak transpressional strike-slip faulting stage from the Ordovician to the pre-Permian, Ordovician-Permian weak compressional strike-slip movement stage and weak sinistral transtensional strike-slip faulting in the Late Permian.
4. Evolution of strike-slip faults
4.1. Embryonic stage of strike-slip fault in the Nanhua period
The northwest margin of the Sichuan Basin is extensively developed with Neoproterozoic (700-1000 Ma) igneous rock [37⇓-39]. The South China block may be involved in the assembly and breakup of the Rodinia supercontinent. Recent studies believe that the South China block belongs to the active continental margin[40⇓⇓⇓-44], which is consistent with the existence of the 730-850 Ma circumferential Rodinia subduction system [45⇓⇓-48]. According to the NE-striking Nanhua magmatic belt [37⇓-39] developed in the northwest margin of the Sichuan Basin and the NE-striking high magnetic anomaly zone [32], it is believed that the northwest margin of the Sichuan Basin underwent NW-SE forward to retreat of subduction from the Proto-Tethys Ocean in the Neoproterozoic period, and the subduction direction was perpendicular to the magmatic belt. The subduction force from the plate margin was transmitted to the plate interior, and the differential contraction deformation may directly induce the formation of NW-striking intraplate strike-slip faults (Fig. 10 a). Subsequently, the plate subduction retreated to form an extensional background in the plate, and the plate began to breakup along the early magma zone, forming a Nanhuanian rift system dominated by NE-striking, NEE-striking and nearly WE-striking faults, or a rift system controlled by normal faults in the NW direction [37,47 -48]. The normal boundary faults of the rift basin were linked together by perpendicular transfer faults (Fig. 10 b), which may be the original pre-existing structures activated by late strike-slip faults. But the nature and activity of faults in this period are difficult to confirm by data. Deep geophysical data such as gravity anomalies and aeromagnetic anomalies [46] revealed that more than 10 deep NW-striking and NE-striking deep basement faults developed in the basin [32], and there were obvious NE-striking magnetic anomaly zones separated by the NW-striking tectonic structures in the central Sichuan Basin [49]. These deep NE-striking, NEE-striking and NW-striking faults are consistent with the strike-slip fault framework in the Sinian and above strata, which are the basis for the formation and development of late strike-slip faults.
Fig. 10. The formation model of Sinian-Early Cambrian strike-slip faults in Sichuan Basin. |
4.2. Dextral transtensional strike-slip faulting stage in Late Sinian-Early Cambrian
The distribution of strike-slip faults has a spatiotemporal relationship with the development and evolution of the Deyang-Anyue trough. Although many scholars have carried out a lot of research on the morphology, structure and sedimentary characteristics of the Deyang-Anyue trough [49⇓-51], its formation time and mechanism are controversial. In the final analysis, the existence of the third and fourth members of the Dengying Formation in the trough is still doubtful, resulting in a controversy over the formation timing of the trough in the Sinian or Cambrian, and the controversy over the formation mechanism of whether the trough is an extensional trough or an erosional trough. Based on the drilling data, analysis of outcrop profiles [51] and Sinian sedimentary characteristics [50-51], scholars gradually confirmed in several opinions: (1) The Deyang-Anyue trough was formed in an extensional environment, but may not be a typical rift system; (2) Deep water deposits of the Dengying Formation possibly exist in the trough, and no obvious erosion of the Dengying Formation observed in the northern part. In other words, the trough was initially formed in the Sinian. Controlled by the Tongwan Movement, the paleo-uplift in the central Sichuan Basin gradually rose, and seawater in the trough retreated, resulting in the strata being exposed and eroded, then the high-energy facies belt was developed and finally remained along the platform margin.
The Deyang-Anyue trough was like a bell mouth whose width increased from south to north during the period of Dengying Formation deposition to the Cambrian, and the strike of the trough basin was oblique at a large angle to the deep Nanhuanian rift. In addition, the basin was not shown as a typical rift. The bell mouth shape and cratonic depression structure of the trough reveal that the basin underwent uneven weak extension during that period when the extensional strength gradually decreased from north to south. Therefore, it is easy to form an oblique strike-slip fault system to adjust the uneven extension displacement. The NW-striking, NEE-striking and NWW- striking strike-slip faults were activated with dextral transtension under the nearly WE stretching stress, and made some NE-striking and NEE-striking faults activated and affected the sedimentary facies [52] (Fig. 8 ), and even some faults interacted with NW-striking faults to form crosscutting structures or linked with the NW-striking faults to form curved morphology (e.g., FI8).
From the Late Sinian to the Early Cambrian, the trough underwent an evolutionary process from its prototype to rapid development [50,53 -54]. The trough was significantly typical during the Early Cambrian, in which huge thick mud shale of the Qiongzhusi Formation was deposited, and the most obvious transtensional structure can be observed in the Lower Cambrian. Compared with the Sinian period, the significant difference in sedimentary thickness in the trough and the platform occurred in the Early Cambrian. During that period, mega thick mudstone was deposited in the trough, which was often considered the result of the strong extension, and the fault in the platform margin is also interpreted as a normal fault. However, the "normal fault" does not crosscut the Dengying Formation or the vertical fault throw is much smaller than that of the Cambrian bottom. This is not in line with the development law of normal growth faults. Therefore, large-scale and large-displacement normal faults were not developed during that period, and the basin architecture at that time was similar to a cratonic depression basin where strike-slip faults were transfer faults that accumulated the uneven extension from north to south. The analysis of seismic data showed that the largest fault system and the largest fault throw could be observed in the Lower Cambrian, and the activity intensity of the strike-slip faults was consistent with the strong extension of the trough from the perspective of forming period. In addition, the strike-slip faults are perpendicular to or obliquely intersect the platform margin at a high angle, and there is no obvious horizontal displacement proving the strike-slip faults crosscut the margin (Fig. 2 ), suggesting that the development of the trough and the formation of the strike-slip faults are consistent in time. U-Pb dating of fault cement revealed fault-fluid activities were no later than the Early Cambrian (531±24 Ma, 510±26 Ma, Fig. 9 ), which was temporally consistent with the intense trough deposition in the Early Cambrian. After the Sinian, the second episode of the Tongwan Movement induced tectonic reversal and interrupted the subsidence of the Deyang-Anyue trough. This is another cause why weak strike-slip activities were detected at the end of the Sinian.
4.3. Weak transpression from the Ordovician to the pre-Permian
In the Cambrian or even the early Paleozoic, whether the nearly WE-striking Anyue trough or the NEE-trending Leshan-Longnüsi paleo-uplift implied that the Sichuan Basin suffered from a maximum nearly NS compressional stress and a nearly WE extensional stress which forced the superimposition of the trough with the paleo-uplift in different periods in the center part. On the stress field, the nearly WE, NWW and NEE faults were easy to be renewed, so many strike-slip faults with negative flower structures were found in the Sinian- Lower Cambrian, and fault-related angular folds were developed under the bottom of the Permian, revealing the superposition of transpressional fault activity.
Most faults terminated directly under the Permian, indicating that these faults may have been renewed at some episode of the Caledonian movement. U-Pb dating result of the fracture cement also suggests Ordovician fluid activity, revealing at least one episode of fault activity before the Permian. In addition, seismic profiles show that most of the faults have great differences in tectonic deformation above and below the bottom boundary of the Permian, and some faults under the bottom of the Permian are associated with obvious angular folds, suggesting that the strike-slip faults were subjected to compressive stress and formed transpressional structures before the Permian. Because many strata are missing in the paleo-uplift, it is difficult to determine exactly how many episodes of fault recurrence occurred before the Permian through seismic analysis.
4.4. Weak sinistral transtensional strike-slip faulting in the Late Permian
In the Permian, the regional stress field changed from early long-term compression to extension. The Emei Tafrogecy was a major tectonic event affecting the Sichuan Basin and even the entire Yangtze Plate [55]. An NW-SE basin tectonic framework with uplifts and depressions was formed in the Middle and Late Permian and even Early Triassic. It implies that the NE-SW extension stress field led to the formation of the principal displacement zone at the bottom of the Upper Permian as a NW-striking sinistral transtensional en echelon structure.
During the Indo-Yanshan period and even the later Himalayan period, fault recurrence may be still going on. However, the deformation of the strike-slip faults in the Permian has become weak, and plastic intervals such as mudstone and gypsum salt rock were developed in the Triassic. Therefore, in the later tectonic movement dominated by a compression stress field, the deformation and displacement in the central Sichuan craton may be transferred and adjusted through these plastic intervals. Moreover, stratified deformation was likely to occur above and below the plastic intervals. Even if a steep linear fault was found in the upper interval, it may be a shallow strike-slip fault system after contraction and deformation above the plastic intervals. Such a fault system needs a lot of data to support and analyze.
5. Conclusions
There are 34 large strike-slip fault zones in the central Sichuan Basin with typical strike-slip characteristics such as negative flower, steep and upright, horsetail, and en echelon structures, mainly developed in the Sinian-Permian, and a small number extending to the Triassic and above strata.
Based on the seismic-geological analysis on the horizons where most faults and en echelon faults terminate, the differences in structure and property between upper and lower faults, and the latest chronological method of dating fault carbonate cement, the strike-slip fault system was initially formed in the Late Sinain, almost finalized in the end of Sinian-Early Cambrian with a large-scale dextral transtensional structure, then at least one stage of weak transpressional deformation before the Permian, sinistral transtension in the Late Permian, and multiple stages of recurrence during and after the Triassic.
The formation of the strike-slip faults is consistent with the evolution of the Deyang-Anyue trough in terms of time and intensity. It is an intraplate transfer fault that regulated the uneven extension of the bell-shaped trough in the N-S direction, in which the extension strength was strong in the south and weak in the south. In addition, the NW, NWW, NEE and NE pre-Sinian fault systems are pre-existing structures that influenced the formation of later strike-slip faults.