(Guangzhou Marine Geological Survey, Guangzhou, 5 10760)

Brief introduction of the first author: Wan Ling, female, graduated from the Department of Earth Sciences of Nanjing University 1998, Ph.D., senior engineer, now mainly engaged in marine geological research and investigation.

According to the different characteristics of seismic reflection interface, seismic sequence, faults and folds in the sea area, combined with the existing drilling data and land geology in the surrounding areas, Nansha area can be roughly divided into four important tectonic movements during Cenozoic: ritual and music movement, westward movement, South China Sea movement and Nansha movement. Frequent tectonic movements reflect the Cenozoic tectonic activities in Nansha area, and also cause the diversity of structural styles in this area. The appearance of this series of movements is closely related to the relative movement and interaction between the Pacific plate, the Philippine Sea plate and the Indo-Australian plate and the Eurasian plate around the South China Sea during the Cenozoic era.

Nansha tectonic movement; Cenozoic seismic reflection; Earthquake sequence

1 preface

Nansha sea area is located in the south of the South China Sea, and belongs to the junction of Eurasian plate, Pacific plate, Philippine plate and India-Australia plate in structural position. In Cenozoic, the Pacific plate subducted Eurasia, the Indo-Australian plate collided with Eurasia in Tibet, converged with Southeast Asia plate along Sumatra Trench and Java Trench, the Philippine plate drifted counterclockwise to the north, and collided with Eurasia plate in Pliocene. Under this regional tectonic background, the South China Sea split and dispersed in Cenozoic, forming the southwest sub-basin and the east sub-basin, leaving many traces of tectonic movement in the strata of the north and south continental margins. In this paper, the characteristics of Cenozoic tectonic movements in Nansha sea area are comprehensively reflected by the strata contact relationship revealed by drilling, the characteristics of seismic reflection layer and the surrounding land geological data, which can be divided into four tectonic movements (table 1), namely, the ritual and music movement from the end of Mesozoic to the beginning of Cenozoic, the westernization movement in the middle and late Eocene, the South China Sea movement in the late Oligocene and the Nansha movement in the Miocene. Compared with Nansha adjacent areas, the four regional tectonic movements in Nansha sea area are discussed.

2 Characteristics of Cenozoic tectonic movement in Nansha sea area

2. 1 ritual and music movement

The ritual and music movement began in the late Cretaceous, and some areas continued to the Eocene. Because of its outstanding performance in the ritual and music area in Nansha sea area, it was named the ritual and music movement. Seismic profile (Figure 1) is characterized by Tg interface, which is equivalent to basement unconformity surface, but this interface is diachronic, and Cenozoic strata in different development stages cover this denudation surface.

Well Sampaguita 1 in Liletan encountered this unconformity, and the Paleocene fluvial-delta facies strata covered the marginal marine strata of the Early Cretaceous. On the Palawan shelf in the northwest of China, the late Eocene clastic rocks are covered on the strata before the Early Cretaceous. This regional unconformity is also found in the northern South China Sea. For example, drilling in the southwest basin of Taiwan Province revealed that the Oligocene shallow-sea strata were not integrated on the fluvial-shallow-sea facies strata in the early Cretaceous and the middle Cretaceous. The obvious changes of sedimentary facies and formation lithology above and below the unconformity surface reflect that this is a very strong large-scale tectonic movement.

Table 1 Cenozoic tectonic movement in Nansha sea area and its adjacent areas

Figure 1 Selected seismic profile of Liletan in the north-central Nansha sea area.

Figure 1 seismic profile of reed beach in north-central Nansha

The performance of this tectonic movement is different in different areas of Nansha sea area. North-central Nansha is characterized by tensile faults, forming a series of NE and NNE aulacogen. In the south and northwest of Borneo, the Kuching orogenic belt is characterized by subduction and orogeny. During this period of tectonic movement, most of the strata in the pre-Cenozoic era experienced different degrees of metamorphism and magmatic emplacement. Late Jurassic-Early Cretaceous metamorphic rocks were collected by Liletan trawl, and early and middle Cretaceous shallow metamorphic shale was drilled in southwest Taiwan Province. A large number of Mesozoic granites and granites were drilled in the Pearl River Estuary, Wan 'an Basin and Zengmu Basin, with an age of (80 2.4) ma ~129 7) ma.

2.2 Westguard movement

The westward movement of the satellite began in the middle and late Eocene, and the T5 interface was shown on the seismic profile. This tectonic movement has formed a regional unconformity in Nansha area, but the movements in different areas are not isochronous, and the horizons change slightly from time to time. In Wan 'an Basin and the north-central Nansha area, T5 often coincides with the Tg interface in the uplift area, and in the depression area, T5 interface shows cutting and overlapping phenomena. Well Sampaguita- 1 in Liletan is semi-deep marine mudstone, siltstone and shallow marine coarse clastic rock. The sedimentary environment and sedimentary formation reflected by the upper and lower layers are quite different. The unconformity surface is the interface between semi-deep sea facies and marginal marine facies; In Sarawak area, there is an obvious angular unconformity between the Begara Formation in Lang Group and the overlying Tatao Formation. The reconstructed late Cretaceous-early Eocene nannofossils were found in the sediments of Luoke Formation in Sabac, indicating that there was an active uplift and erosion period. On the seismic profile, this unconformity is equivalent to the deep unconformity between Sabah and Sarawak. In Zengmu basin, this unconformity separates the late Cretaceous-Middle Eocene folded metamorphic rocks from the late Eocene-Oligocene non-metamorphic rock series. In Luzon area, before the late Eocene, the strata were folded and uplifted, and the strong erosion made the early and middle Oligocene strata missing.

On the seismic profile (Figure 2), the deposition between T5 and TG is wedge-filled, and its boundary is often controlled by faults, with great thickness variation and local folding deformation in different degrees, which is equivalent to the coherent rift sedimentary system.

Fig. 2 Selection of seismic profile in Nansha sea area (Yao Yongjian et al., 1998)

Fig. 2 Seismic profile of Nansha area (Yao Yongjian et al., after 1998)

2.3 South China Sea Movement

The South China Sea Movement began in the late Oligocene and was first named by He Liansheng et al. (1979). On the seismic profile, the T4 interface, as a representative, can be tracked and compared in Nansha area, which is equivalent to the fault unconformity surface before the expansion of the sub-basin in the eastern South China Sea, but the formation of this interface is diachronic. In Wan 'an Basin and Zengmu Basin, the T4 interface is located between Oligocene and Miocene, and in Namwisi Basin, Kang Bei Basin, Lille Basin and the southeastern part of Nansha, its time is between the early and late Oligocene, indicating that this movement occurred earlier in the eastern part of Nansha Sea and later in the western part. This interface has different forms in different areas of the South China Sea. In Kang Bei basin, Zengmu basin, Nanweixi basin and Wan 'an basin (Figure 3), the reflection characteristics of the upper and lower strata on the interface are obviously different, with weak amplitude and intermittent reflection in the lower part and medium-strong amplitude, medium-high frequency and continuous reflection in the upper part, which reflects the variation law of sediments from bottom to top. In the South Weidong Basin, Jiuzhang Basin and Andubei Basin, it is an upper overcharged deposit. Well Sampaguita- 1 in Liletan reveals that there are two sets of completely different sediments above and below the interface, with marginal marine sandstone mixed with mudstone and siltstone below and limestone above; In Zengmu basin, there is an obvious angular unconformity between the second cycle and the first cycle in East Balinjian sag. In the Nansha Trough area, it is equivalent to the blue unconformity shown by German "SONNE" on the seismic profile of Nansha Trough. The strata below this interface have obvious erosion phenomenon, and the upper and lower earthquake sequences have completely different characteristics. However, in the structural framework, except Wan 'an Basin, the sedimentary subsidence centers above T4 do not coincide with those below T4, and the upper and lower structures in the middle interface of other basins have certain inheritance, which shows that the fault subsidence is further deepened and the sedimentary scope of the basin is gradually expanded.

Fig. 3 Selected seismic profile of Namwisi Basin (according to Bai et al., 2000).

Fig. 3 Seismic profile of the western Nanwei Basin (Bai et al., after 2000)

2.4 Nansha Movement

Nansha movement is a strong tectonic movement in Nansha area, which is characterized by unconformity interface with T3 on seismic profile (Figure 4). This unconformity was first discovered in Wan 'an Basin and named as Wan 'an Movement. Later, the scope of investigation was extended to the whole Nansha sea area, indicating that the unconformity surface representing this movement can be traced continuously in the whole area, indicating that this is a tectonic movement with wide influence, so it was renamed Nansha Movement.

Geological research in the South China Sea. 2003

Fig. 4 Seismic profile of unconformity A and T3 of ZML3 12 line in Nansha area.

Nansha Movement consists of two parts. The first scene took place between the Early Miocene and the Middle Miocene, and was an interface on seismic profile, especially in Wan 'an Basin, Zengmu Basin and Kang Bei Basin. The tectonic deformation in Namwisi and Li Le basins is strong, and it is difficult to trace. The second act took place in the middle and late Miocene and was the main act of this tectonic movement. On the seismic profile, there is T3 interface, which is a regional unconformity surface that can be traced continuously. The most striking feature is that the strata below T3 interface are widely folded and deformed, while the overlying strata are not deformed horizontally-nearly horizontally, and the structural styles above and below the unconformity surface are completely different. The early extensional faults changed into compression and compression-torsion, indicating that the regional tectonic stress field has changed from early extensional to compressive.

The influence degree and manifestation of Nansha movement are different in different areas of Nansha, with the strongest in the western and southern areas of Nansha, the most obvious in Kang Bei Basin, the western basin in the south latitude and Wan 'an North Basin, and even structural inversion, with the maximum erosion amount of Wan 'an Basin reaching 2000 m; Especially in the north-central Nansha sea area, T3 is a set of reflective layers with high inclination (Chen Ling et al., 1997), and the reflective layers below T3 have been eroded and leveled on a large scale. In the west of Kang Bei basin, the calculated maximum denudation thickness exceeds1500m (Liang Jinqiang, 1998). A similar phenomenon was found in the northeast of Zhongjiannan Basin (Chen Ling et al., 2002).

In the south latitude uplift area, south latitude east basin, Kangtai basin and Jiuzhang basin in the north-central Nansha, uplift, denudation, fracture and magmatic eruption of strata below T3 were caused. In Lille basin, the influence of this movement is weak, mainly in the depression in the southwest of the basin, and the platform shoal area in the east of the basin is not obviously transformed by compression, which is generally characterized by strong in the south and weak in the north (Zeng Xianghui et al., 2002); The Palawan shelf and Nansha Trough in the southwest are equivalent to the blue unconformity surface, with Oligocene-Early Miocene carbonate rocks under it and a set of coarse and disorderly deformed sedimentary wedges above it (Figure 5).

Fig. 5 Reflected seismic record (monitoring record) and SO-27-007 line interpretation profile (according to Hinz et al., 1985) (the+symbol in the figure indicates carbonate strata).

Fig. 5 Cross section of reflection seismic recording and interpretation along the line SO-27-007 (showing the cross section of carbonate) (Hinz et al., after 1985)

3 Comparison of Cenozoic tectonic movement in Nansha sea area and adjacent tectonic movement

The Cenozoic tectonic movement in Nansha area can correspond to the tectonic movement in the northern South China Sea and Taiwan Province Province (Table 1), indicating its universality.

The ritual and music movement can be compared with the Shenhu movement in the northern South China Sea and the Taiping Heavenly Kingdom movement in Taiwan Province. These three movements essentially belong to the late Yanshanian tectonic movement in the southeastern margin of South China. The horizon of Taiping Movement may have been elevated to the end of Mesozoic, and the Liletan Movement and Shenhu Movement should belong to the same tectonic movement, but Liletan was separated from the northern continental margin of the South China Sea in Oligocene and drifted to the present Nansha area. The unconformity seen in Palawan area in the northwest of Liletan is similar to the drilling data in Taiwan Province province shoal and southwest basin of Taiwan Province, showing strong uplift and erosion. The seismic profile is characterized by extensional fault blocks, which is the main feature of the late Yanshanian tectonic movement in eastern China. On this basis, a series of fault basins have been developed.

There is also a westward movement of satellites in the northern continental margin of the South China Sea. After Li Pinglu (1992) studied the tectonic movement in the Pearl River Mouth Basin, he called it the second act of the Zhu Qiong Movement. This movement is most obvious in the northern edge of Borneo in Nansha area. The Lajiang Group has undergone intense compressive deformation with an inclination angle of 80 ~ 90, in which quartzite veins are filled. Hutcheson (1996) called it Sarawak orogeny.

The movement of the South China Sea occurred between the early and late Oligocene in the northern margin of the South China Sea, but the eastern part was earlier than the western part, that is, the movement was generated from east to west, and the unconformity formed by this movement was diachronic. In the Pearl River Mouth Basin, the seismic reflection characteristics of the unconformity surface formed by this movement are quite different, and the reflection continuity above the interface is good, generally parallel to sub-parallel, with low intermediate frequency and moderate amplitude, reflecting the characteristics of marine deposition; The reflection layer below the interface is slightly inclined, with weak amplitude and low frequency, and the reflection is discontinuous or divergent, reflecting the continental sedimentary characteristics. In Taiwan Province Province, the time limit of this movement is roughly between Paleogene and Neogene, and it is called Puli Movement, which caused the shallow metamorphism of Paleogene strata in the west wing of the central mountain range.

Nansha Movement is called Dongsha Movement in the northern South China Sea and coastal mountain movement in Taiwan Province Province. This movement began at the end of Middle Miocene. Although the time and form of sustainable development are different in different regions, it is usually a common tectonic inversion event in sedimentary basins around the South China Sea. The sediments developed in the early tensile environment generally produce fold deformation, which is stronger in the eastern part of the northern continental margin of the South China Sea and weaker in the western part. In Sabah area, this movement is called Sabah orogeny, and Kela Formation was uplifted and deformed in the middle and late Miocene.

4 reason discussion] >

4. 1 ritual and music movement

Before 185Ma, the Pacific plate was generated at the triple point of the mid-ocean ridge of Kula plate, Fenghuang plate and Faralon plate, which promoted the subduction of Kula plate to Eurasia. According to the research of Northrup et al. (1995) and Konov( 1984), the Pacific plate moved relative to the Eurasian plate along the NWW direction in the late Cretaceous, with an average convergence rate of 130mm/a (Figure 6), and in the late Cretaceous-early Eocene (68.5 ~ 53.5). Moving northward from the initial NWW direction, the average convergence rate suddenly dropped to the level of 78mm/a, which led to the transformation of the regional tectonic stress field in the eastern and southeastern edges of Eurasia from pre-compression to post-Late Cretaceous extension, forming a series of initial rifts. Therefore, the ritual and music movement is a separatist movement, and the unconformity surface formed is equivalent to that of Falway (1974). Regional extensional faults produce a series of NE-trending extensional faults, which form a series of NE-trending graben and semi-graben separated from each other on the surface. Paleocene-Eocene sediments developed in graben, but the corresponding sediments were often missing on the horst between graben. This may be the root cause of this disunity.

Fig. 6 average convergence rate of Cenozoic Pacific plate relative to Eurasian plate (according to Northrup et al., 1995).

Fig. 6 Average convergence speed of the Pacific plate relative to the Eurasian plate (after Northrup et al., 1995).

4.2 Westguard movement

In the middle and late Eocene, the global plate pattern changed greatly. Several important tectonic events occurred around the South China Sea. At 44Ma, the Indian plate collided head-on with Eurasia, resulting in the uplift of the Qinghai-Tibet Plateau as a whole and the compression of Indo-China Peninsula to the southeast. The sliding boundary in the north is the Red River Fault. At about 43Ma, the subduction direction of Kula plate turned to the northwest, subducted under the East Asian continent, and began to form the western Pacific trench-arc basin system. Around 44Ma, the Indian Ocean began to expand for the third time, and the Indo-Australian plate moved northward rapidly, and began to dive to the Southeast Asian continent along the Sunda Trench. Under this tectonic background, the tectonic framework of the South China Sea has undergone new changes. On the basis of the early northwest-southeast extension, the extension was further developed and strengthened. The crust in the central Xisha area in the northern South China Sea has become thinner obviously. The Nansha block in the southeast of zhongsha islands splits from the margin of South China continent and drifts to the southeast. After the rift block, the southwest sub-basin began to expand on the basis of the early NE-aulacogue.

In addition, in the southeast of Borneo, the Sulawesi basin and the Makassar basin also cracked almost simultaneously with the southwest sub-basin. The difference is that the extension of Makassar basin did not stop until the early Miocene, while the opening of Sulawesi Sea continued until Oligocene or even Miocene. The expansion of the Sulawesi Sea may be the result of the back-arc expansion caused by the northward subduction of the Australian plate along the Java Trench (Lee, 1995), and a magmatic arc with genetic connection has been formed in West Sulawesi, North Sulawesi and Sulu Ridge. With the expansion of the Sulawesi Sea, West Sulawesi turned clockwise and gradually separated from Kalimantan.

With the expansion of the southwest sub-basin of the South China Sea, the Nansha block was pushed southward, which accelerated the disappearance of the ancient South China Sea crust on its south side. At the same time, the expansion of the Sulawesi Sea pushed Sulu Block northward, and the Nansha area between them was in a tectonic environment of north-south compression, resulting in regional uplift. Therefore, the westward movement of the satellite is more prominent in Nansha sea area than in the northern South China Sea, especially between Nansha and Borneo, and the Paleocene-Eocene strata even experienced a certain degree of shallow metamorphism. The T5 interface produced by this tectonic movement is equivalent to separation and unconformity.

4.3 South China Sea Movement

In the late Oligocene, the sub-basin in the eastern South China Sea began to expand, and the direction of expansion was different from the northwest-southeast direction of the southwest sub-basin, but it expanded in the north-south direction (B.Taylor and D E Hayes, 1983). The expansion of the eastern sub-basin pushed the Li Le-North Palawan block to drift southward, and the ancient South China Sea oceanic crust dived southward under the southwest Palawan shelf. Correspondingly, the global sea level dropped sharply during this period, resulting in low sea level (Vail et al., 1977, 1979). Therefore, the formation of this unconformity/false conformity surface is influenced by both structural factors and sea level changes. The movement of the South China Sea mainly reflects that the expansion of the eastern sub-basin caused the compression and uplift of the northern and southern continental margins of the South China Sea, which had the greatest impact on the early strata in the south, with the southern and eastern movements earlier and the western and northern movements later.

4.4 Nansha Movement

During the Miocene, the Australian plate and the Philippine Sea plate drifting northward began to encounter obstacles. According to research, at 25Ma, the southern end of the Philippine plate collided with the northern end of the Australian plate; In the middle and late Miocene, the Philippine island arc and Palawan block, accompanied by rapid counterclockwise rotation, collided between Mindoro Island and Bannai Island, and southeast Sulawesi and Sisula Wei collided on the Australian plate (Longli, 1997). This series of collision events caused the southeast edge of Nansha block to suffer regional compression, which accelerated the southward subduction and subduction of the oceanic crust of the ancient South China Sea, and finally led to the collision between Li Le-North Palawan block and Kalimantan-Sulu block and the complete extinction of the ancient South China Sea.

On the continental margin of the northern South China Sea, the Philippine Sea Plate rotates counterclockwise, moves to the northwest, subducts to Eurasia, and squeezes the northern South China Sea, resulting in strong deformation of the Middle Miocene and pre-Miocene strata in Dongsha area. The deformation degree is stronger in the east and weaker in the west, and the tectonic stress generated by the reaction movement comes from the east.

In the late Middle Miocene (12 ~ 15 Ma), the eastern edge of the Indian plate collided obliquely with the western edge of Southeast Asia, which led to the formation and uplift of the Indo-Myanmar Mountains. Therefore, in the middle Miocene, the whole periphery of the South China Sea was under the action of compressive tectonic stress field, which was the fundamental reason for the Nansha movement. This movement caused extensive tectonic inversion in the sedimentary basins around the South China Sea, so the Nansha movement is the strongest, most extensive and most influential tectonic movement in the Nansha sea area. After this movement, the submarine expansion of the South China Sea stopped, the crust of the ancient South China Sea shrank completely, and the evolution of the South China Sea entered a new historical stage.

5 conclusion

During the Cenozoic, there were four tectonic movements in Nansha sea area, namely, ritual movement from the end of Mesozoic to the beginning of Cenozoic, satellite movement in the middle and late Eocene, South China Sea movement in the late Oligocene and Nansha movement in the middle Miocene. These movements are related to the interaction between plates (or blocks) around the South China Sea. Among these four movements, the Westernization Movement in the middle and late Eocene and the Nansha Movement in the middle and late Miocene had the greatest influence on the tectonic development of the South China Sea, corresponding to the initial expansion of the South China Sea basin (southwest sub-basin) and the end of the expansion of the basin (east sub-basin) respectively. An in-depth study of these two movements will help reveal the origin of the South China Sea Basin.

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Cenozoic tectonic movement and its genesis in Nansha sea area

Wan Ling, Wu Nengyou, Yao Bochu, Zeng

(Guangzhou Marine Geological Survey, Guangzhou, 5 10760)

Abstract: The newly acquired seismic data and previous drilling data show that Nansha area experienced multiple tectonic movements in Cenozoic, namely, Li Yue movement, Western Wei movement, South China Sea movement and Nansha movement. These movements are related to the movements of the Pacific plate, the Philippine Sea plate and the Indo-Australian plate relative to Eurasia.

Keywords: Cenozoic seismic reflection seismic sequence of Nansha tectonic movement