(1. China Geo University Beijing100083; 2. Guangzhou Marine Geological Survey Bureau Guangzhou 5 10760)

Xisha trough basin is a Cenozoic sedimentary basin developed in the deep water area of the northern continental slope of the South China Sea, which has received sediments with a thickness of 1500 ~ 8000 m, thick in the middle and thin in the north and south, showing the characteristics of north-south zoning. The seismic profile shows the characteristics of lower fault and upper depression, and the structural style of the basin in rift period shows the characteristics of "domino half graben" or "graben" The development of the basin has experienced two main evolutionary stages: Paleocene-Oligocene fault depression and Miocene-Quaternary depression. Continental river and lake deposits are developed in the fault depression stage, and shallow sea and semi-deep sea deposits are developed in the depression stage.

Geological structure in deep water area of Xisha trough basin

Xisha Trough has been debated for a long time because of its special tectonic significance, but there is little research on Xisha Trough Basin. 1987, when the Second Marine Geological Survey Brigade of the former Ministry of Geology and Minerals compiled the Geological Geophysical Atlas of the South China Sea (1:2 million), the Xisha Trough Basin was delineated by gravity data, and it was considered as a continental margin extension basin [1]. Yao Bochu (1994) thinks that Xisha seawall is an ancient suture line of Indosinian, and the sediments in the trough are mostly the products of the middle and late Cenozoic, with early Cenozoic sediments locally developed [2]. Xisha sea area is an integral part of continental margin graben system (Liu, 2002) [3]. Wan Ling et al. (2009) think that there is an east-west Xisha trough fault between the central Xisha block and the continental margin block in the northern South China Sea, and the Xisha trough is a strip-shaped high-value positive magnetic anomaly zone in magnetic force, and the spatial gravity anomaly is a low-value negative anomaly zone; The crustal velocity structure reflects that the crust in the trough is thinner than its two sides, and there is an abnormal high-speed layer in the lower part of the lower crust. The heat flow value of the groove is relatively high; The average is 78 MW/m2. It reflects that this is a Cenozoic tectonic active area [4]. Qiu (2000) thinks that the crustal structure of Xisha Trough is characterized by Cenozoic extensional rift, and the crustal structure on both sides of the trough is similar and symmetrically distributed from north to south [5]. Yuan et al. (2008) and Zhang Gongcheng et al. (2009) divided the Xisha trough in Qiongdongnan Basin into Changchang sag [6,7]. Is Xisha Trough Basin an independent sedimentary basin or a part of Qiongdongnan Basin? Its geological structure characteristics and evolution? According to the latest seismic data, this paper holds that Xisha trough basin is a Cenozoic continental margin rift basin.

This paper is supported by the general project of National Natural Science Foundation "Study on Seismic Refractive Waves of Cenozoic Sedimentary Strata in South China Sea" (No.:4 1 176056).

Brief introduction of the first author: Zhong Guangjian (1965-), Ph.D., professor-level senior engineer, mainly engaged in marine geological research, e-mail: guangjianz @ 21cn.com.

1 regional geological background

Xisha Trough Basin is located in the western part of the continental slope in the northern South China Sea, connected to Qiongdongnan Basin in the northwest, Pearl River Mouth Basin in the northeast and Xisha Islands in the south, spanning the eastern part of Xisha Trough, with a water depth of 200 ~ 3,000 m and an area of about 20,000 km2 (Figure 1). The northern continental margin of the South China Sea is the junction of the South China block and the central basin of the South China Sea. The crust thickness is 14 ~24 km, which belongs to the transitional crust of Mesozoic and Cenozoic strata. Due to the evolution of geological history and the transformation of multi-stage tectonic movement, the development degree, lithology and lithofacies of strata are quite different. In the late Cretaceous, the subduction zone of the western Pacific retreated, and the southeast margin of Eurasia began to crack and continued to Paleogene, forming a series of Cenozoic sedimentary basins. This tectonic movement, Shenhu Movement, covered the whole South China Sea and formed a series of large continental margin basins, such as Qiongdongnan, Xisha Trough, Pearl River Mouth, Jianfengbei, Jiabi and Southwest Platform. The structural characteristics of these basins are: Paleogene belongs to continental margin rift basin, and Neogene developed into continental shelf-slope depression basin. The formation and evolution of the central basin in the South China Sea plays an important role in controlling the evolution of Cenozoic sedimentary basins in the northern margin of the South China Sea.

Figure 1 basin location map

2 Geological characteristics of the basin

2. 1 sedimentary characteristics

Well Xiyong 1 in Xisha Sea reveals that the basement of the central Xisha block is a set of granite gneiss, gneiss and gneiss granite, which is similar to that of Song Kunlong. Drilling data show that the pre-Tertiary bedrock in Qiongdongnan basin is composed of Paleozoic metamorphic rocks, dolomite, Cretaceous intermediate-acid granite, diorite and pyroclastic rocks, and its lithology can be compared with the pre-Cenozoic strata in Hainan Island. According to the reflection characteristics of basement revealed by seismic profile, the basement of Xisha trough basin may be consistent with Qiongdongnan basin.

On the pre-Cenozoic basement, the basin has developed sediments since Paleocene, with the thickness of 1500 ~8000 m, and the general trend is that it is thick in the middle and thin in the north and south, and the sedimentary center is located in the south-central part of the basin. From the seismic reflection profile of the basin, two tectonic sequences can be clearly divided (Table 1): TG-T6, rift tectonic sequence, whose main feature is that faults are very developed and control stratigraphic deposition; After T6, it is a rift thermal subsidence sedimentary sequence, in which faults are less developed and the deposition is mainly caused by thermal subsidence.

Table 1 structural sequence table

Paleocene-Eocene was only deposited in the middle of the basin, and developed the characteristics of continental lake basin and lake sedimentary system. At the beginning of Oligocene, seawater invaded completely, and coastal facies and shallow marine facies were generally accepted. From the early Miocene to the middle Miocene, the basin turned into a continental slope deep-water sedimentary environment, and a certain scale of slope fans, deep-water fans and turbidite fans developed. Since the late Miocene, it has entered a stable regional subsidence stage, and mainly deposited a set of semi-deep-sea facies deposits (Figures 2 and 3).

2.2 structural characteristics

Mature rift basins generally go through two stages: extension and post-fault (Ruke, 1990) [8]. Xisha trough basin is divided into two sets of structural sequences, upper and lower, with the overall feature of lower fault and upper depression (Figures 2 and 3). The lower structural layer represents the product of half graben filling formed by early extension, which fills the rift structural sequence; The upper structural layer is the product of depression, which is filled with thermal subsidence sedimentary strata after rifting.

The basin as a whole is NE-trending, controlled by the fault system, and can be further divided into five secondary structural units: the northern fault terrace belt, the central depression, the southern fault terrace belt, the eastern slope belt and the southern depression (Figure 4). The secondary structural units have obvious north-south zoning characteristics, which are similar to Qiongdongnan Basin and Pearl River Mouth Basin in the northern South China Sea.

The structural style of rift basin depends on the geometric and kinematic characteristics of main faults and their combinations. The differences in geometric shape and kinematic characteristics of normal faults at the boundary of basins or depressions lead to different structural styles of extensional basins. The profile structural styles of extensional fault basins can be divided into graben and horst. Controlled by non-rotating plane normal fault; Domino half graben system); Controlled by rotating normal fault; "Half graben" or "rolling half graben") is controlled by shovel normal fault; Half graben and slope depression controlled by slope flat normal fault (Lu Kezheng, 2006) [9]. According to the characteristics of seismic profile, the structural style of Xisha trough basin during the rifting period is "domino half graben" or "graben" (Figure 2 and Figure 3), and the main mechanism of its structural deformation is simple shear deformation. The differential settlement in the half graben is obvious, and the reflection structure of the filling stratum is wedge-shaped, diverging to the boundary fault and converging to the slope zone. The settlement is mainly controlled by a group of boundary faults.

The main faults that control the development of half graben or graben are F 1, F2, F3, F4 and F5.

Fig. 2 Structural Diagram of Section A-A' of Xisha Trough Basin

Fig. 3 Structural Diagram of Section B-B' of Xisha Trough Basin

Fig. 4 Structural Map of Xisha Trough Basin

F 1: It is the northern boundary fault of the basin, with the strike of NEE and the dip of S, the extension length is nearly 60 km, the vertical and horizontal faults are 142 ~ 3 1 13 m and 375~2400 m, respectively, and the Cenozoic uplifting plate thickness is about 448 m, while the declining plate thickness. The fault uplift plate is an ancient uplift area, lacking T6-Tg stratum, and the fault is undeveloped. The basement cut into the fault plays an extremely obvious control role on the deposition of T6-Tg stratum. The fault was formed in the early Oligocene, with syngenetic nature, and its main active period was from the early Oligocene to the middle Miocene.

F2: the northern boundary fault of the basin, striking northeast and dipping southeast, with an extension length of 120 km and vertical and horizontal faults of 568~2694 m and 925~4850 m respectively. The Cenozoic thickness of the ascending wall is about 694m, and that of the descending wall is 4366m ... This fault is normal, with steep top, gentle bottom and slightly convex middle (Figure 2). The rising plate lacks T6-Tg stratum; The descending plate obviously controls the deposition of T4-Tg stratum. The fault was formed in the early Oligocene, the main active period was from the early Oligocene to the early Miocene, and the activity ended in the late Miocene.

F3: The general trend is NE and tends to NW. The extension length in the area is about 150 km, and the vertical and horizontal fault distances are 559 m ~ 2235 m and 475 m ~ 2850 m respectively. It appears as a steep normal fault on the seismic profile (Figures 2 and 3). The western section is the boundary fault between the southern fault step belt and the central depression, the middle section is magmatic activity, which separates the central depression from the southern depression, and the eastern section is NWW, which is the boundary fault in the southeast of the basin. The fault cuts into the basement and across the seabed, and the descending plate is invaded by magma, and the igneous body cuts through T8, which makes the T7-Tg stratum uplift and bend. The fault was formed in Paleocene, and its main active period was Paleocene-Late Oligocene.

F4: It strikes NWW, inclines to NNE, and extends for about 50 kilometers in the area, which is the southern boundary fault of the basin. The vertical and horizontal faults are 2900 ~ 47 10/0m and 3775 ~ 5225m respectively. The Cenozoic boundary of the ascending wall is about 400 meters thick and the descending wall is 4669 meters thick. This fault is a normal fault with a gentle upper part and a steep lower part and a raised middle part (Figure 2). Fault uplift plate is an ancient uplift area, lacking T6-Tg stratum. The fault cuts into the basement until T0, which obviously controls the deposition of T4-Tg stratum. The fault was formed in Paleocene and is still active today, with the main active period from Paleocene to the middle Miocene.

F5: It is a boundary fault in the southwest of the basin, striking NW -NWW and trending northeast. The extension length in the area is 100 km, the vertical and horizontal fault distances are 677 ~ 1222 m and 725~2600 m respectively, and the Cenozoic uplifting plate thickness is 669 m and the descending plate thickness is1790 m. This fault was formed in Paleocene, and it is still active today, with contemporaneous nature. Its main active period is from Paleocene to middle Miocene, with little activity since late Miocene (Figure 5).

Fig. 5 Structural Diagram of Section C-C' of Xisha Trough Basin

2.3 tectonic evolution

According to the stratigraphic and structural characteristics, Xisha trough basin can be divided into two main evolution stages: Paleocene-Oligocene fault depression and Miocene-Quaternary depression.

2.3. 1 evolution stage of fault depression

Paleocene-Eocene initial fault depression period: this period corresponds to the first act of Shenhu Movement and Zhu Qiong Movement. Under the combined action of the NW-trending tensile stress field generated by the Pacific-Eurasia plate interaction and the near-SN- trending tensile stress field generated by the India-Eurasia plate interaction, the Xisha Trough Basin was subjected to tensile cracking, and some Neogene ne-trending and NW-trending faults in the basin were revived, such as the faults F3 and F4 at the southern boundary of the basin began to move, and shallow lakes, semi-deep lakes and deltas developed. During this period, the distribution of sedimentary strata was limited, mainly located in the current central depression and southern depression; Shenhu uplift and southern uplift on both sides of the basin are basically exposed to water, which are provenance supply areas. In this fault depression period, the central part of the basin is about 900 m, and the subsidence rate is 30 m/Ma (Figure 6), which reflects that the overall tectonic activity is not strong, and the subsidence amplitude is strong in the west and weak in the east. The overall performance of the basin is a half graben with east fault and west overload.

Oligocene rapid subsidence period: The second act of Zhu Qiong Movement occurred in the early Oligocene, and the subsidence of Xisha trough basin accelerated rapidly, and seawater flooded into the basin, expanding the sedimentary range and filling a set of coastal and shallow sea sediments. The deposition thickness of Oligocene in the west of the basin is 1 500 m, and the deposition rate is close to 250 m/Ma, which is the highest in all stages of the basin. The subsidence intensity in the east is still much weaker than that in the west. The deposition thickness and rate of Oligocene are about 680 m/Ma and 1 10 m/Ma, which is only half of that in the west (Figure 6). Faults F 1 and F2 at the northern boundary of the basin began to move in the early Oligocene, which laid the overall structural framework of the basin and transformed the whole basin into a double-fault graben (Figure 7).

In the late Oligocene, influenced by the South China Sea movement, seawater overflowed Shenhu Uplift and South Uplift, and the sedimentary range of the basin was further expanded. The sedimentary strata in this period were developed throughout the investigation area. The sedimentation rate in the west of the basin is slightly lower than that in the previous period, but it is still high, about 160 m/Ma, and the sedimentation thickness is nearly1000 m. The sedimentation rate in the east is basically the same as that in the previous period, about 1 10 m/Ma, and the sedimentation thickness is nearly 700 m. The sedimentary environment of the late Oligocene strata is the same as that in the previous period, and it is a coastal-shallow sea facies deposit.

Fig. 6 tectonic subsidence curve (top) and subsidence rate (bottom) of Xisha trough basin

At the end of Neogene, the crustal uplift caused by compression occurred in the investigation area, forming the T6 fault unconformity surface of Xisha trough basin. Affected by this tectonic movement, the strata between T6 and TG are folded and bent, especially in the eastern part of the basin, and the strata deformation in the western part of the basin is relatively weak (Figure 7).

2.3.2 Evolution stage of depression

Early-middle Miocene rapid subsidence period: In the early Miocene, with the stop of the expansion of the central basin in the South China Sea, the expanding abnormal mantle gradually contracted with the rapid thermal diffusion, and the mantle lithosphere gradually thickened with the cooling of the mantle from shallow to deep, resulting in regional balanced subsidence. Xisha trough basin entered a typical post-split depression period from early Miocene. During this period, the subsidence speed of the basin gradually accelerated, the seawater rose, and the sedimentary environment evolved into an open shallow sea-continental slope semi-deep sea. The sedimentation rates of the Early-Middle Miocene strata in the east and west of the basin are about 40 m/Ma and 60 m/Ma respectively, and the sedimentation thickness is about 550 m and 820 m respectively (Figure 6). During this period, the fault activities in the eastern and western parts of the basin were slightly different: there were basically no new faults in the eastern part, and the early main fault activities weakened or even stopped (Figure 7); On the other hand, in the west of the basin, in addition to the early inherited fault revival, a series of NE-trending normal faults inclined to the center of the depression were newly formed at the edge of the depression (Figure 7).

Profile of structural development and evolution along line A-A' in Xisha Trough Basin.

Late Miocene-Quaternary stable regional subsidence period: From late Miocene to Quaternary, the survey area entered the stable regional subsidence period, mainly depositing a set of semi-deep-sea facies deposits. The sedimentation rates of the eastern and western strata are 56 m/Ma and 60 m/Ma respectively, and the sedimentation thickness is about 580 m and 630 m respectively. The south boundary fault F3 continues to move up to now, but because the basin is far away from the provenance, the thickness of sedimentary strata on the upper and lower walls of the fault is similar, and the fault activity has weak control over sedimentation, only controlling the seabed topography (Figure 7).

3 Conclusion

Xisha trough basin, like the fault depression in the south of Qiongdongnan basin and Baiyun sag in the Pearl River Mouth basin, is a Cenozoic sedimentary basin developed in the deep water area on the northern slope of the South China Sea, with two sets of structural sequences: lower fault and upper sag. The main faults controlling the development of the basin are F 1, F2, F3, F4 and F5. In space, the basin is characterized by north-south zoning, which is characterized by "domino half graben" or "graben" The development of the basin has experienced two main evolutionary stages: Paleocene-Oligocene fault depression and Miocene-Quaternary depression. Continental river and lake deposits are developed in the fault depression stage, and shallow sea and semi-deep sea deposits are developed in the depression stage. According to the geological structural characteristics of the basin, Xisha trough basin is a favorable area for oil and gas exploration.

refer to

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Structural characteristics of Xisha trough basin

Zhong Guangjian 1, 2, Feng Changmao 2, Zhang Baojin 2, Wei Zhenquan 2

(1. China Geo University, Beijing,100083; 2. Guangzhou Maritime Survey, Guangzhou, 5 10760)

Abstract: Xisha trough basin is located on the northern slope of the South China Sea, which is a deep-water Cenozoic basin. The sedimentary thickness of the basin is1500 ~ 8000m. The sedimentary center is located in the middle of the basin, and the sedimentary thickness becomes thinner from north to south. The basin has the characteristics of North sout zoning. The seismic profile is characterized by two layers, the upper part is subsidence layer and the lower part is rift layer. During the rifting period, the structural style is "Domino Half Graben", which is characterized by half graben controlled by main faults (F 1, F2, F3, F4, F5). The evolution of the basin has gone through two stages: the first stage experienced Paleocene-Oligocene subsidence and accepted lacustrine deposition; The second stage experienced Miocene-Quaternary subsidence and accepted shallow or semi-deep sea deposits.

Keywords: deepwater geological structure of Xisha trough basin