Geological Structure and Dynamics of the Yinggehai Active Rift Basin, South China Sea

0. INTRODUCTION

The Yinggehai Basin, as the only NNW-trending basin in the northwestern South China Sea (Fig. 1), is an important petroliferous basin (Huang et al., 2019; Wang et al., 2019; Cao et al., 2015; Jiang et al., 2015a; Yan et al., 2011). Located in the convergence zone of the Eurasian Plate, Indian Plate and Pacific Plate, the Yinggehai Basin has a complex tectonic setting (Jiang et al., 2022; Lei et al., 2011; Yan et al., 2007) that is characterized by a thin crust, deep subsidence, rapid sedimentation, a high geothermal gradient, and considerable natural gas accumulation (Fu et al., 2016; Sun et al., 2014; Zhang et al., 2013; Wang and Huang, 2008). While the geological structures and dynamic mechanisms of basin formation are still hotly debated, it is generally accepted that the Yinggehai Basin is a strike-slip extensional basin (Fan, 2018; Ren, 2018; Fyhn and Phach, 2015; Zhu et al., 2009; Sun et al., 2003). Our data indicates that the basin forming period included active rifting, rather than strike-slip motion; however, strike-slip movement occurred both before and after the formation of the Yinggehai Basin. The regional tectonic events and the dynamic mechanisms driving the sinistral and dextral strike-slip motion vary in different regions of the basin. Clarifying the complex tectonic and structural history of this typical active rift basin will provide valuable insights into the oil and gas exploration and development of the Yinggehai Basin.

Figure  1.  Regional geological map of Yinggehai Basin (modified after the International Geological Map of Asia; Ren et al., 2013). The Ailaoshan-Troung Son extruded terrane is composed of six NNW-trending secondary extruded bodies: Ⅰ. Ailaoshan; Ⅱ. Hoang Lien Son; Ⅲ. Sip Song Chau Thai; Ⅳ. Sam Neun-Nghe Tinh; Ⅴ. Truong Son; Ⅵ. Kon Tum. Fault zones: (1) Lo River; (2) Red River; (3) Ailao Mountain; (4) Da River; (5) Ma River; (6) Lam River; (7) Troung Son; (8) Pingxiang-Suzhou.

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Due to the complexity of this region, there is no widely accepted tectonic history that adequately explains the observed tectonic and structural spatial variations within the basin (Ye et al., 2020; Bai et al., 2019; Wang et al., 2016; Morley, 2013; He et al., 2002). Ru (1988) pointed out that the collision between the Indian Plate and the Eurasian Plate caused the south-eastward extrusion and clockwise rotation of the Indochina Block. The sinistral strike-slip nature of the Red River fault zone coincided with the NS-oriented extension to form the sinistral tension-shear Yinggehai Basin; when the South China Block migrated eastward faster than the Indochina Block, the Red River fault zone transformed into a dextral strike-slip fault zone. Rangin et al. (1995) examined the small NS folds and EW normal faults in the Hanoi depression and concluded that the fault zone accommodates sinistral strike-slip motion. Li et al. (1998) believed that the Yinggehai Basin is a transform-extension basin controlled by lithospheric extension caused by mantle upwelling and the dextral strike-slip. Sun et al. (1995) noted that the Yinggehai Basin had experienced two historical periods of dextral strike-slip motion. After analyzing the regional geology and geotectonic, scholars (Li et al., 2017; Sun et al., 2007; Zhong et al., 2004; Guo et al., 2001) simulated the tectonic history of the Yinggehai Basin and concluded that the Yinggehai Basin experienced deformation initially due to sinistral strike-slip motion, while dextral strike-slip motion occurred later. In this model, they found that the Paleocene–Early Miocene Indochina Block extruded toward the SE and rotated clockwise, while the Yinggehai Basin underwent sinistral strike-slip. From the Miocene to the Quaternary, the South China Block began extruding, and the Yinggehai Basin underwent dextral strike-slip. The Yinggehai Basin is generally categorized as a strike-slip extensional or pull-apart basin (Zhu et al., 2015; Ren and Lei, 2011; Yao, 1998).

To reconcile various explanations for the basin characteristics and the regional dynamic mechanism, we created a four-dimensional structural and tectonic history of the basin by closely examining the geological structures, basin formation process, crust-mantle structure, and dynamic mechanisms of the Yinggehai Basin by collecting stratigraphic, tectonic and seismic data. Firstly, the process of basin development and evolution has been clarified. In the Early Eocene, before basin formation, the Ailaoshan-Troung Son terrane in the west of the basin extruded and escaped in the SSE direction, the South China continental margin in the east of the basin extended in the SSE direction. The Yinggehai mantle ridge developed along the Red River fault zone, which was in the transform-type sinistral strike-slip state. During the main basin formation period, from the Eocene to the Miocene, the basin exhibited extensional behaviors such as rift depression, fault depression, and fault subsidence. At the end of the Miocene after the basin had formed, the northern Hanoi sub-basin began accommodating thrust sinistral strike-slip motion, while the southern Yinggehai main basin experienced dextral strike-slip motion, structural superposition, and thermal subsidence. Secondly, we refined our understanding of the basin by examining its tectonic, geological, and structural features. As a typical active rift basin caused by ridged mantle uplift (Li et al., 1998), the Yinggehai Basin is characterized by intense crustal thinning, rapidly filled and thick sedimentary depositions, a high temperature gradient, and large-scale abnormal pressures. Lastly, we identified that the Qiangtang-Simao-Yinggehai (Qiang-Ying) mantle flow channel as part of the Sanjiang belt mantle flow channel (Mo et al., 2009, 2007, 2006). The Yinggehai Basin, as a feature that lies between the Qinghai-Tibet orogenic belt and the South China Sea Basin, is not only a unique NNW-trending member of the South China Sea basin zone, but also a Cenozoic member of the Qiang-Ying basin zone. The Yinggehai Basin is a product of plate subduction, block escape, mantle extrusion, and crustal extension. Because of its unique location and features, the Yinggehai Basin has significant potential as a source of valuable natural resources such as oil and gas.

1. GEOGRAPHICAL AND GEOLOGICAL SETTING

The Yinggehai Basin (Figs. 1 and 2) is an NNW-trending, elongated spindle-shaped basin with an area of 12.17 × 10⁴ km² (Huang et al., 2020; Guangdong Geological and Mineral Bureau, 1998). Located in the southern section of the Red River fault zone, the Yinggehai Basin lies at the convergence of the Indian, Eurasian, and Pacific plates (Guo et al., 2016; Cheng, 1994). The basin is bounded by the Qinghai-Tibet orogenic belt and the South China extension zone. The north side of the basin is bordered by the Red River fault zone, the southern Yangtze Block, and the Pingxiang-Suzhou fault zone (Luan et al., 2021; China Geological Survey, 2004; Pan et al., 2002). The Ailaoshan-Truong Son extruded terrane (Fig. 1), on the west side of the basin, formed in the Early Eocene due to the subduction of the Indian Plate (Sun and Zhang, 2022; Ren et al., 2013; Wu, 2009). This terrane, with its linguloid extruded bodies, broken folds, and dense faults, is composed of a series of transform-type strike-slip fault zones such as the Da River fault zone, the Ma River fault zone, the Lam River fault zone, and the Truong Son fault zone. The Beibu Gulf Basin, Hainan Island, and the Qiongdongnan Basin lie east of the basin (Chen et al., 2020; Ma et al., 2018; Wu and Suppe, 2018; Jiang et al., 2015b). The Yinggehai Basin is also an NNW-trending thin crust zone located in the northern crustal extension-thinning area of the South China Sea, and the uplift of the mantle forms a prominent mantle ridge with a Moho depth of ~22 km (Fig. 2b) and a deposition thickness of ~17 km (Fig. 1) in the center of the basin (Qiu et al., 2021; Hainan Geological Survey Institute, 2017; Li et al., 2017). Thus, this prominent mantle ridge basin is superimposed on the Ailaoshan-Truong Son extruded terrane, the Red River fault zone, and the Hainan Island uplift.

Figure  2.  Structural units and stratigraphic column of the Yinggehai Basin. (a) Structural units in Yinggehai Basin. The Lingao uplift represents the boundary between the Hanoi sub-basin and the Yinggehai main basin, A-A1, B-B1, C-C1, D-D1, E-E1 are seismic lines across the basin (modified after Wang et al., 2019; Li et al., 2017). (b) Moho depth map (modified after Mo et al., 2007). (c) Yinggehai Basin stratigraphic column (modified after Wang et al., 2019; Jiang et al., 2015a).

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Drilling data reveal that the age and depositional environment from the Oligocene to the Quaternary in the Yinggehai Basin (Fig. 2c). The Early Oligocene Yacheng Formation consists of fluvial to lacustrine deposition facies, while the Quaternary formation consists of neritic to abyssal deposition facies. These strata form a transgressive filling sequence with a maximum sedimentary thickness of about 17–18 km (Xie and Fan, 2010). Unconformable interfaces, such as T80, T70, T60, and T30 (Xie et al., 2016; Tapponnier et al., 1986), are found in these strata.

2. BASIN STRUCTURE, DEVELOPMENT, AND EVOLUTION

Using the Lingao uplift as a boundary marker (Fig. 2a), we divided the basin into two parts: the Hanoi sub-basin in the northwestern end of the basin, and Yinggehai main basin in the southern central part of the basin. The Hanoi sub-basin is composed of the Hanoi depression and its slopes. The Yinggehai main basin is composed of the central depression, the Yingxi slope, and the Yingdong slope. Compared to Yinggehai main basin, the Hanoi sub-basin has a smaller size, a thinner sedimentary layer, and a more complex structure.

2.1 Hanoi Sub-basin

The Hanoi sub-basin has a length of ~100 km and an average width of ~25 km; its narrow profile in the northwest gradually widens in the south-east. The strata have a maximum sediment thickness of approximately 3 500 m, and are characterized by unconformities and sedimentary hiatuses at 30, 15.5, and 5.3 Ma. Based on our analysis of the geological structure and the seismic profile (Fig. 3), the basin has experienced rift depression, fault depression, fault subsidence, compressional thrust sinistral strike-slip superposition and reworking, and thermal subsidence.

Figure  3.  Hanoi sub-basin structural profile (modified after Rangin et al., 1995). (a) Geological structural profile C-C1. (b) Seismic profile B-B1. (c) Seismic profile A-A1. The locations of the sections are shown in Fig. 2a.

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(1) Eocene to Early Oligocene (56 to 30 Ma) rift depression

Seismic data is limited in this time period. According to the geological structure profile (Fig. 3a), the basin is mainly controlled by a series of NNW-trending normal faults, which contain a few graben and half-graben type fault sags, indicating the presence of strong extensional rifting.

(2) Late Oligocene to Early Miocene (30 to 15.5 Ma) fault depression

According to Fig. 3a, at this time, the number of basin-controlling normal faults was lower than that of the rift depression stage, and the basin-shaped fault depression had a relatively simple structure.

(3) Mid–Late Miocene (15.5 to 5.3 Ma) fault subsidence

At this stage, faults did not develop, and a large-scale depression with a slightly irregular dish shape formed.

(4) Pliocene to Quaternary (after 5.3 Ma) compressional thrust sinistral strike-slip tectonic superposition and reworking, and thermal subsidence

At this stage, the basin formation process was essentially complete. Because of compression caused by the extrusion bodies from the Hoang Lien Son in the Indochina Block on the southwestern side of the basin, a series of NNW-striking thrust sinistral strike-slip fault zones formed in the basin; some of the early-stage normal faults became reverse faults (Fig. 3a). Many NS-striking small folds and flower structures (Figs. 3b and 3c), as well as a dense set of EW-striking normal faults also formed at this time. This is consistent with the north-south tensile stress field derived from the sinistral strike-slip motion. These faults cut through the Oligocene–Late Miocene strata, and, at the end of Miocene, they were covered by Pliocene sediment series. The Hanoi sub-basin experienced intense reworking, to the point where its effects were observed as far away as the Lingao uplift and the northern part of the Yinggehai Basin. The thermal subsidence is characterized by the lower rate of depression and the basin depression is eventually stopped altogether, allowing a nearly horizontal layer of fast unconformable sediments above the Miocene layer with a thickness of approximately 700–1 300 m to form on top of the depression.

2.2 Yinggehai Main Basin

The Yinggehai main basin (Fig. 2a) is located in the central and southern Yinggehai Basin, between the Ailaoshan-Truong Son extruded terrane and the Hainan Island uplift. This is the widest part of the spindle-shaped Yinggehai Basin. The strata in the basin have unique structural, temperature, and pressure characteristics. According to the data from the CNOOC Zhanjiang Branch, the maximum burial depth of sediments with ages from the Oligocene to the Pliocene exceeds 15 000 m; the time of filling and burial is approximately 3 Ma, and the maximum sedimentation rate of the basin is 920, 340, 520, and 440 m/Ma for the Yinggehai Formation, Huangliu Formation, Meishan Formation, and Sanya Formation, respectively. The geothermal gradient generally ranges from 38.0 to 44.0 ℃/km, with a maximum value of 52 ℃/km and a minimum value of 36.5 ℃/km. Similarly, the pressure coefficient typically falls in the range of 1.6 to 2.4, an indication that these strata reside in an ultra-high temperature and ultra-high-pressure environment. Since the Pliocene, the thermally flowing mud diapir has penetrated the surface; the resulting structure facilitates the accumulation of natural gas. To date, fourteen mud diapir structures have been found in the main petroliferous area in the Yinggehai Basin (Lei et al., 2011; Wang and Huang, 2008).

The NWW-SSE-trending seismic profile (Briais et al., 1993) of the Yinggehai main basin shows that the deepest subsidence in the central depression occurs in the south, where the basin slope is the steepest. Based on our interpretation of the seismic and geological structural profiles from the basin (Fig. 4), we divided the Yinggehai main basin evolution and tectonic activities into four stages.

Figure  4.  Seismic profiles for the Yinggehai main basin. (a) Regional stratigraphic boundary interpretation, modified after Lei et al. (2015); (b) seismic profile for the mud diapir in the central depression, modified after Lei et al. (2011) (see the location of D-D1 and E-E1 in Fig. 2a).

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(1) Eocene–Early Oligocene (56–30 Ma) rift depression

At this stage, a large number of NNW-trending normal faults developed in the basin, creating strong graben and half graben basin structures with alternating convex-concave features (Fig. 4a). At this time, the humid climate of the Yacheng Formation created the river delta and the lacustrine mudstone layers.

(2) Late Oligocene (30–23 Ma) fault depression

At this time, the bottom of the Lingshui Formation unconformably completely covered the early rift depression basin, forming a basin-shaped deep depression with rapid sedimentation and a large layer thickness in the central depression. With the reduced fault activity, the tectonic morphology was relatively simple. As the sedimentation range expanded, seawater entered the basin, forming the neritic-fan delta sandstone layer.

(3) Miocene (23–5.3 Ma) fault subsidence

As the control on the basin exerted by the faults weakened in the Miocene, this simple structure persisted. The ongoing deposition consistently covered the original basin footprint, which is shaped like a dish. Since the Miocene, neritic to abyssal turbidite clastic rocks and carbonate rocks developed (Fig. 2c). These sedimentary layers are most likely to host hydrocarbons. The Early Miocene Sanya Formation, the Middle Miocene Meishan Formation, and the Late Miocene Huangliu Formation (Fig. 2c) had average layer thicknesses of 1 100, 1 150, 350 m and maximum thicknesses of 2 950, 1 750, and 780 m in the center of the depression, respectively. Because the sediment amplitude and volume gradually decreased (Li et al., 2017), we infer that the basin-formation process ended at the end of the Miocene.

(4) Pliocene to Quaternary (after 5.3 Ma) rapid thermal subsidence, extensional dextral strike-slip superposition, and reworking

Research on the Yingdong slope reveals that before the Pliocene, the subsidence center was located in the middle of the central depression. During the Pliocene, the center of the pull-apart basin shifted toward the southeast, to the Yingdong slope area (Li et al., 2017). As the basin began developing extensional dextral strike-slip faults, a NS tensional fault zone that was an offshoot of the Red River fault zone caused the tectonic activity in the basin to intensify. The thermal flow in the mud diapir escalated (Fig. 4b), and eventually it became a channel through which hydrocarbons could migrate or accumulate. At this stage, the basin experienced rapid thermal subsidence, as shown by the Pliocene Yinggehai Formation, which had an average thickness of 1 100 m and a maximum thickness of 2 200 m in the center of the depression.

3. RESULTS

The structural characteristics and geological history of both the Hanoi sub-basin and the Yinggehai main basin demonstrate that the Yinggehai Basin is an extensional basin that experienced rift depression, fault depression, and fault subsidence from the Eocene to the Miocene (Fig. 5). The Yinggehai main basin and the Hanoi sub-basin experienced fault subsidence in the Early Miocene and in the Middle Miocene, respectively. The termination of fault depression was delayed in the Hanoi subbasin because it is smaller, had a lower sedimentation rate, and had a thinner sediment layer. Conversely, the termination of fault depression in Yinggehai main basin occurred earlier because of its rapid filling and thick depositional layers, which buried the basin-controlling faults in a relatively short time frame. After the Yinggehai Basin formed, it entered a period of dynamic environmental transformation, tectonic superposition, and reworking. In the beginning of the Pliocene, after the basin forming tectonic stage, the Hanoi sub-basin experienced strong compressional sinistral lateral strike slip motion. Since Pliocene, the dynamics in the Yinggehai main basin have changed from extension to extensional dextral strike-slip, activating the structure and mud thermal diapir development, which promotes natural gas accumulation in the basin.

Figure  5.  Evolutionary diagram of the differences and similarities of basin forming mechanisms between the Hanoi sub-basin and the Yinggehai main-basin in the Yinggehai active rifting basin.

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From the Hanoi sub-basin to Yinggehai main basin, the basin gradually widens, and the amplitude of subsidence increases gradually from 3 500 to 17 000 m. The central depression of the Yinggehai Basin exhibits the strongest rapid sedimentation and filling. From the Pliocene to the Quaternary, the basin was characterized by thermal subsidence. With extension activity, the mantle magmatism exists in Yinggehai Basin (Sun et al., 2021). Samples from well Y32-1-1 (Fig. 2a), located at the margin of the basin, included a porphyry basalt layer with a thickness of 115 m (Li et al., 1998). This type of basalt is a product of upper mantle material residing in an extensional environment. The transform-type sinistral strike-slip motion of the Red River fault zone and the extrusion of the Ailaoshan-Truong Son extruded terrane happened before the basin formation, which will be further explained in the discussion part.

4. DISCUSSION

This study identified three developmental evolution and dynamic transformation periods in the basin's history through tectonic time series analysis (Fig. 5). And this part of the discussion will explain the mechanism behind each phase thoroughly. The first series of events occurred around the Early Eocene, with the onset of transform type sinistral strike-slip motion regime before the basin forming stage. The second period extended from the Eocene to Miocene in the basin forming stage, including mantle ridge overarching, crustal extension, rapid sedimentation, and hydrocarbon generation caused by rift depression, fault depression, and fault subsidence. Tectonic superposition, thermal subsidence, mud diapir activity, and natural gas accumulation define the third period, which began at the end of the Miocene after the basin forming stage.

4.1 Tectonic Framework and Dynamic Characteristics before Yinggehai Basin Formation

The southeastward extrusion of the Ailaoshan-Truong Son extruded terrane and the transform-type sinistral strike-slip motion of the Red River fault zone occurred before the basin forming stage in the Early Eocene.

4.1.1   The extrusion of the Ailaoshan-Truong Son extruded terrane

There were various theories explaining the unique structure of the eastern Indochina Block and many hypotheses regarding the nature of the basin and the Red River fault zone. We found that the Early Eocene formation of the Ailaoshan-Truong Son extruded terrane and the Yinggehai mantle body is closely related to the tectonic activity along the Red River fault zone. While the Indochina Block extruded gradually (Mo et al., 2006; Fukao et al., 1992), the rapid extrusion of the Ailaoshan-Truong Son extruded terrane is constrained by the northeastern bends of the Red River fault zone (He et al., 2020). As a result, it escaped to the southeast and formed a large linguloid body that converged in the northwestern direction, and then moved southeastward (Fig. 1). However, typical NW-oriented strike-slip faults and shear tectonic schists are not commonly found in the Ailaoshan-Truong Son extruded terrane; similarly, no clockwise rotation has been observed in this region (Guo et al., 2001; Ru, 1988). Instead, the interior of the Ailaoshan-Truong Son extruded terrane consists of a group of six NW-trending lenticular secondary extrusion bodies, including the Ailaoshan, Hoang Lien Son, Sip Song Chau Thai, Sam Neue-Nghe Tinh, Truong Son, and Kon Tum lenticular extrudates (Fig. 1). Each of the secondary extrusions exhibits left row iteration characteristics, and has a NW-SE orientation. Extrusion bodies generally tilt to the northwest and incline towards the southeast; these features are evidence of a state of repeated compression accommodated by transform-type sinistral strike-slip fault zones such as the Da River, Ma River, Lam River, and Truong Son fault zones.

4.1.2   The transform-type sinistral strike-slip motion of the Red River fault zone

The NW-striking Red River fault zone is located to the north of Hanoi City, runs parallel to the Lo River fault zone, and is bounded by the Ailaoshan-Truong Son extruded terrane to the southwest (Fig. 1). The city of Hanoi as the dividing point divides the Red River fault zone into two sections in the north and south. After passing through Hanoi City, the Red River fault zone gradually deflects to the SSE and bends slightly eastward, where it obliquely intersects the secondary extruded bodies of the Ailaoshan-Truong Son extruded terrane and the transform strike-slip fault zones of the Da River, the Ma River, and the Lam River, forming the front margin of the Ailaoshan-Truong Son extruded terrane (Fig. 1). Due to the extrusion of the Indo-China Peninsula, the Ailaoshan-Truong Son extruded terrane is extruded along the southwest side of the Red River fault zone to a large extent. The Yangtze Plate on the northeast side of the northern Red River fault zone extruded and escaped to a small extent in the same time. As a result, the northern Red River fault zone constitutes an intracontinental transform-type sinistral strike-slip extrusion pattern characterized by "weak in the north and strong in the south". The South China continental uplift on the east side of the southern section of the Red River fault zone extends in the south-south-east direction, forming the Beibu Gulf fault basin, the Hainan Island fault uplift and the Qiongdongnan fault basin, which makes the southern Red River fault zone in a transitional state of "squeezing in the west and stretching in the east, escaping in the same direction". Previously, due to the lack of attention to the extrusion and extension of the eastern side of the Red River fault zone. The Red River fracture zone was believed to be simply sinistral strike-slip fault zone without transformation effect. This study suggests that the northern and southern sections of the Red River fault zone presented two different transform-type sinistral strike-slip modes prior to the formation of the Yinggehai Basin. At this time, upper mantle fluid was extruded along with the Ailaoshan-Truong Son extruded terrane, forming an NNW-trending Yinggehai ridge-like mantle plume along the Red River fault zone (Fig. 6a).

Figure  6.  Basin-forming models. (a) Yinggehai Basin mantle ridge formation. (b) Evolution model of the Yinggehai active rifting extensional Basin.

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4.2 Formation, Evolution, and Dynamic Process of Yinggehai Active Rift Basin

After the formation of the Yinggehai Basin mantle ridge, the basin entered the dynamic process of mantle ridge overarching and crustal extension (Fig. 6).

4.2.1   Yinggehai extensional active rift basin formation

Both seismic (Figs. 3 and 4) and geological survey data indicate that the period from the Eocene to the Miocene was characterized by strong rift depression, large fault depressions, and fault subsidence (Fig. 6b). At the bottom of the basin, we observe that the NNW-trending normal faults control the basin, which is characterized by a series of convex and concave graben and half graben structures. At the upper part of the basin, the number of faults is reduced at the basin margin, controlled basin edge faults are overlapping, and the internal structure tends to be simple. Neither the NW-trending fault zones of the Ailaoshan-Truong Son extruded terrane to the west nor the EW- and NE-trending fault zones of the Hainan Island uplift to the east enter the basin, and this symmetrical spindle-shaped basin has a slightly steeper slope to the east. According to the research of Wang et al. (2019) and Sun et al. (2014), the river deltas deposited in the peripheral structures during the Miocene Huangliu Formation all converged towards the middle of the Yinggehai Basin, which also shows the control of extensional basin on sedimentation. Overall, the lack of strike-slip pull-apart sedimentary structural features in the basin formation period indicates that it is a typical extensional basin formed by active rifting.

Ren and Lei (2011) and Zhu et al. (2015) attributed the Miocene fault depression stage of the Yinggehai Basin to thermal subsidence. However, our study revealed that, from the Late Oligocene to the Early Miocene, the central basin of the South China Sea entered a period of basin expansion and mantle uplift (Yao et al., 2004), which is characterized by the rapid filling and thick sediment layers in the basin. The Yinggehai ridge-shaped mantle uplift is a part of the South China Sea mantle uplift area. From the Late Oligocene rift depression and fault depression stage to the Miocene fault subsidence stage, the Yinggehai main basin had a high sediment deposition rate. The maximum thickness of the formation is about 7 700 m, so it is unlikely that the subsidence was caused by the cold shrinkage of the mantle lithosphere. In the Miocene, the Yinggehai mantle was still uplifting, the center of the basin was sinking, and the mantle ridge was arching upward in the center. After the arching subsided, the basin entered the post-depression stage. The depression gradually widened and expanded. According to Gong (2004), most of the Cenozoic petroliferous basins in China's offshore areas (including the Yinggehai Basin) have entered the most active period of thermal subsidence since the end of the Miocene (approximately 5.3 Ma).

As discussed above, the Yinggehai mantle ridge is a protruding mantle plume in the South China Sea mantle uplift area, with a high kinetic energy and a strong thermal flow. Mantle uplift and geothermal activities throughout the basin formation process triggered the large-scale adjustment of crustal materials. The Yinggehai Basin ultimately subsided to a depth of 17 000 m, and the thinnest part of the basement crust was only 4 to 5 km. A large amount of crustal material migrated to the uplifted areas on both sides of the basin. As a result, the Ailaoshan-Truong Son uplift on the west side lifted continuously, leading to an elevation of 3 142 m of the Hoang Lien Son. The Hainan faulted uplift on the east side also experienced rapid uplift; it is speculated that height of the main peak, Wuzhi-shan, has risen by approximately 1 500 m since the Eocene, reaching an elevation of 1 867.1 m. The mountain is strongly denudated, and the Late Cretaceous granite is exposed to the surface, while denudation of the Early Cretaceous granite created a batholith. Basin filling and sedimentation formed a thick Cenozoic crust; mantle uplift is the main cause of the thin crust, deep subsidence, rapid sedimentary filling, high geothermal gradient, and high-pressure values observed in the Yinggehai Basin.

4.2.2   Strike-slip superposition and thermal subsidence

By the end of the Miocene, the Yinggehai Basin had become a large-scale extensional active rifting basin, the overarching of the mantle uplift had largely ended, the extension had weakened and the basin entered the thermal subsidence stage in the Pliocene. At this time, the dynamics in the two basins were very different. Compressional dynamics began to prevail in the western part of the Yinggehai Basin, and extensional dynamics dominated in the east.

The Hanoi sub-basin in the northwestern part of the basin was compressed by the extrusion of the Hoang Lien Son body, which resulted in the thrust sinistral strike-slip tectonic superposition and reworking, complicating the structure of the basin. The sinistral strike-slip motion is evidenced by the appearance of the thrust fracture zone, the near north-south small folds, flower-like structures (Fig. 3) as well as the east-west normal faults in the basin. The faults in this period staggered the Miocene and were covered by the Pliocene. This geological event occurred after the end of the Miocene, not during the period of extension into a basin.

The dextral strike-slip motion of the Yinggehai main basin could be explained by the change of the tectonic setting on the eastern side of the basin. From the Eocene to the Miocene in the basin formation period, the eastern side of the basin, including the Hainan Island uplift, generally escaped in a south-southeast direction and expanded eastward, which was in an extensional state and stretched out. However, in the Pliocene, tectonic setting on the eastern side of the basin was strongly influenced by the expansion of the Philippine Sea Plate from the east and converged towards the Yinggehai Basin. Thus, the SSE pulling occurred (Yu et al., 2017) and transformed the Yinggehai main basin into an extensional dextral strike-slip environment, activated the internal structure of the basin and formed an extensive NS-oriented tensile fracture zone. The dextral strike-slip motion is firstly evidenced by the appearance of a series of nearly NS-oriented tensile fracture and mud diapir zones appeared in the basin (Fig. 2a). According to the study of previous seismic data, there were at least 18 diapirs distributed in the diapir zone in the central depression (Figs. 2a and 4b), which are linearly aligned along six tensile fracture zones with an NS en échelon striking orientation (Lei et al., 2011). This is consistent with the east-west tensile stress field derived from the dextral strike-slip motion (Fan, 2018). Secondly, the basin's sedimentary center shifted to the south-eastern Yingdong slope and Ledong area with a large drift in the Pliocene (Li X S et al., 2017; Li S T et al., 1998). The central depression and the sedimentary center of the basin are obviously controlled by the Red River fault zone and migrate from NNW to SSE, indicating that the basin changed from extensional active rifting to extensional dextral strike-slip state. This dynamic transformation is vital for mud diapir activity and natural gas accumulation. While hydrocarbons were deposited during extensional basin formation, the extensional strike-slip activity promoted natural gas accumulation, resulting in Yinggehai Basin hosting significant hydrocarbon resources.

4.3 Basin Response to Plate Activity and the Qiang-Ying Mantle Flow Channel

In the course of studying the geological structure of the Yinggehai Basin, we noticed that the Red River fault zone is a link between the Qinghai-Tibet orogenic belt and the South China Sea Basin, intersecting the Jinshajiang fault zone to the northwest (Fig. 7a) (Ren and Lei, 2011; Li et al., 1998). With the Longmenshan fault zone in the north, the Red River fault zone forms the boundary between the Qinghai-Tibet orogenic belt and the South China continent extensional region. To the south of the Jingshajiang-Red River fault zone lies a series of Mesozoic–Cenozoic basins, including the North Qiangtang, Changdu, Simao, and Yinggehai basins, which we called "Qiang-Ying basin zone" (Fig. 7a) in this paper (Li et al., 1998). Yinggehai Basin is just at the south end of this basin zone, at the junction of the Qiang-Ying basin zone and the South China Sea Basin (Wang et al., 2020). Therefore, we find that Yinggehai Basin is not only a Cenozoic member of the Qiang-Ying basin zone, but also a unique NNW-trending member in the northwest part of the Cenozoic basin zone in the South China Sea. The formation and evolution of the basin is closely related to the convergence of the Indian, Eurasian, and Pacific plates. The tectonic activity between and within these plates is responsible for the subduction of the Indian Plate, the Qinghai-Tibet orogeny, the extrusion of the Indochina Block, and the eastward extension of South China. During the eastward extension of the South China continent, the movement was blocked by Yuli belt in the Pacific Plate, which led to the enhancement of the southward extension and escaped towards the SSE direction. And thus, making the Red River fault zone a transform-type of strike-slip dynamic pattern.

Figure  7.  (a) Tectonic and dynamic setting during the Yinggehai basin formation process. Fault zones: ① Yarlung Zangbo River; ② Nujiang; ③ Lancangjiang; ④ Jinshajiang-Red River; ⑤ Longmen Mountain; ⑥ Yuli. The Qiang-Ying basin zone is composed of four basins: Ⅰ. North Qiangtang Meso–Cenozoic Basin; Ⅱ. Changdu Meso–Cenozoic Basin; Ⅲ. Simao Cenozoic Basin; Ⅳ. Yinggehai Basin. (b) Cenozoic volcanic rocks in the Qinghai-Tibet Plateau (modified after Mo et al., 2009).

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Previous work related to the Qinghai-Tibet orogeny and mantle extrusion (Fukao et al., 1992) suggests that the Indian Plate wedged northward and caused the mantle material to be extruded to the east. Li et al. (1998) also believes that there is the possibility of lateral mantle flow. Based on the trend of Cenozoic volcanic activity, Mo et al. (2009) concluded that, during the Late Eocene to the Miocene (48 to 16 Ma), a volcanic rock belt migrated to the southeast along the Jinshajiang-Red River fault zone (Fig. 7b). This mantle flow consists of potassic magmatic rocks, Na-basanite, and alkaline basalt. This material, in addition to the mantle xenoliths found in the Cenozoic potassic basalt from the Wenshan area in the northeastern part of the Red River fault zone, is considered to be the product of lateral mantle flow extrusion due to the subduction of the Indian plate and the extrusion of the Qinghai-Tibet region. In particular, the Sanjiang (Nujiang, Lancangjiang and Jingshajiang) belt is an important deep material flow channel.

Following a similar train of logic, we found that the Yinggehai Basin lies within the extension direction of the Sanjiang belt mantle flow channel, and coincides with the northwestward mantle ridge. Within the Qiang-Ying basin zone, the channel has a length of ~5 000 km and is a trinity tectonic active belt integrating basin-forming belt, volcanic rock belt and mantle flow channel. We call this area "the Qiang-Ying mantle flow channel and basin zone" (Fig. 7a). Due to the extrusion of the Indochina Block and the extension of the South China continental margin, the mantle material flowed along the Red River fault zone, forming a ridge-shaped mantle uplift and extensional basin in Yinggehai area. As a result, the typical Yinggehai active rift basin is formed.

5. CONCLUSIONS

(1) The Yinggehai Basin is a typical active rift basin, overlying the Ailaoshan-Truong Son extruded terrane, Red River fault zone, and the Hainan Island uplift. In the Early Eocene, before the basin forming stage, the Ailaoshan-Truong Son terrane extruded in the SSE direction and the South China continental margin escaped and extended towards south-southeast at the same time. The northern and southern section of the Red River fault zone experienced two different transform-type sinistral strike-slip tectonic activities prior to the formation of the Yinggehai Basin and a mantle ridge formed along the Red River fault zone.

(2) Yinggehai active rift Basin formation and extension occurred from the Eocene to the Miocene. The extruded ridge-like mantle uplift has a high kinetic energy and caused high thermal flow. The uplift arching process is present throughout the entire basin formation process. The basin, which is characterized by its large size, deep depression, rapid sedimentary filling, and huge deposition thickness, has experienced rift depression, fault depression, and fault subsidence, making it a likely location for hydrocarbon generation.

(3) Since the beginning of the Pliocene, after the basin forming stage, the tectonic environment has shifted towards thermal subsidence, tectonic superposition, and reworking. While the northern Hanoi sub-basin became a thrust sinistral strike-slip tectonic environment, the southeastern Yinggehai main basin was transformed into an active extensional dextral strike-slip environment. These conditions promoted thermal flow in the mud diaper, resulting in natural gas accumulation.

(4) The Yinggehai Basin is located at the junction of the Qinghai-Tibet orogenic belt and the South China Sea basin. It is not only a part of the South China Sea basin but also an important member of the Qiang-Ying basin zone. It has a unique geological structure, a complex formation history, and a high potential for natural oil and gas exploration. Based on previous studies, the proposed escape regime consisting of the unique tectonic style of the Ailaoshan-Truong Son extrusion and the South China continental margin extension, together with the Qiang-Ying mantle flow channel and basin zone will provide useful insights into future scientific work and hydrocarbon exploration in the Yinggehai Basin.