(1) Central Uplift Zone (Ⅰ 2)
The central uplift belt is a unique tectonic unit in the northern South China Sea, which has three characteristics: Dongsha uplift belt is different from Tanweigu uplift belt, with high gravity and magnetic anomaly background and special lower crust high-speed layer.
1. Tanweigu uplift belt is different from Dongsha uplift.
The Tanweigu uplift belt on the north side of Dongsha Island constitutes the core of the central uplift belt, which runs through the shoal of Taiwan Province Province, Penghu Islands and Beigang of Taiwan Province Province to the northeast, with a total length of about 700km and a width of 50 ~ 80 km. It should be emphasized that this area does not include Dongsha Uplift with Dongsha Island as the center (Figure 4- 17), because they are products of different times and show different development characteristics. Tanwei ancient uplift is a basement uplift type composite anticline structure (Figure 4- 18), with few faults, gentle northwest wing and steep southeast wing. The variation characteristics of stratum thickness and occurrence from the top to the wing of the anticline show that this is a pre-Mesozoic basement uplift zone, and the Mesozoic and Cenozoic sediments basically inherited this stable paleotectonic environment, with greatly reduced thickness, and Miocene platform facies carbonate rocks are often directly seen. The strong truncation of T7 unconformity shows that there was a great difference in uplift in this area before the late Oligocene. According to the cutoff mark of its southeast wing (Figure 4- 18 A), the stratum denudation caused by uplift can reach about 2000 meters, and the overburden (structural layer I) with T7 as the bottom boundary is widely distributed throughout the South China Sea. Therefore, T7 essentially reflects a time-consuming unconformity event, corresponding to the expansion movement of the South China Sea, so it was called the "South China Sea Movement" in the past, equivalent to the "Puli Movement" in Taiwan Province Province and the "Yuquan Movement" in the East China Sea. Dongsha uplift with Dongsha Island as the center is located in the northern part of the southern margin depression fold belt. It is actually a anticline high belt on the southeast side of Tanweigu uplift belt, about 150km long. It is more appropriate to call it "Dongsha structure". The Dongsha structure reflected in Figure 4- 18 is close to the northeast dip end of the anticline. The arch phenomenon of this structure on Dongsha Island is particularly prominent, which is the result of being pierced by a large volcano. There are many similar anticline structures (Figure 4- 17) on the southern edge of the depression fold belt, and most of them are distributed in the northeast in the form of geese. The angular unconformity contact relationship between structural layers ⅱ and ⅲ shows that they were formed in the folds of structural layer ⅲ, which is the product of Yanshan movement, and the thickness of the involved structural layer ⅲ can reach 6000 ~ 8000 meters. Strong deformation under T2 unconformity can be seen in both the central uplift belt and the southern margin depression belt, indicating that a tectonic event dominated by lateral compression occurred before the late Miocene, which is equivalent to the so-called "Dongsha movement". Some common shallow local folds on seismic profiles are the products of this movement. For example, on the stable Dongsha uplift belt, two rows of German shallow anticline structures with ladder distribution have been preliminarily identified (Figure 4- 17). The phenomenon that T2 unconformity is eroded by modern seabed can be seen in both Tanweigu uplift belt and Dongsha structure, which shows that neotectonic movement in this area is strong, and Dongsha structure only fully shows its uplift appearance during this period.
Fig. 4- 17 Mesozoic tectonic outline in the northeastern South China Sea
ⅰ 1- northern margin fold belt; Ⅰ 2—Central fold belt; Ⅰ 3-southern margin depression and fold belt; Ⅰ 4—Baiyun fold belt A—ESP—E; Section selection of B-365 section: c-OBS 2006-3
2. High value weight and magnetic field background
The background of gravity and magnetic anomalies in the central uplift zone has been discussed many times. It is generally believed that the gravity and magnetic anomaly belt is the product of Mesozoic magmatic arc and the extension of the late Mesozoic magmatic belt along the coast of Fujian and Zhejiang to the South China Sea. We think this explanation needs to be discussed. The reasons are as follows: first, the abnormal characteristics of the two are very different. The land area is a chaotic anomaly with wavy and high-frequency changes, which belongs to shallow source anomaly, mainly reflecting the late Mesozoic intermediate-acid magmatic rocks, especially the large set of volcanic rocks, while this zone is a low-bandwidth, slow and high-value positive anomaly with no obvious associated phenomenon. The extension of 20 ~ 40 km upward is still outstanding, which should reflect the deep buried basic-ultrabasic rocks. According to the field splitting results of differential cutting method (Luan Xiwu et al., 20 1 1), the field source of high positive anomaly in Tanweigu uplift belt began to show weak anomaly at 12km depth, and obvious anomaly at 20km depth, reaching the maximum at 28km depth; Second, they are distributed in different directions, and the chaotic magnetic anomalies in the land area are generally distributed in the northeast, while the high-value positive magnetic anomalies in this belt are in the same direction as the central uplift belt, and generally distributed in the northeast; Third, the source time is different. The contiguous magmatic rocks in the land were formed in the late Mesozoic, but only a few wells in the central uplift belt saw the late Mesozoic magmatic rocks, which was far less than the large-scale distribution along the coast of Fujian and Zhejiang. It is worth noting that, on the seismic profile, there are few faults along the Tan Wei ancient uplift belt, and no volcanic puncture phenomenon is found, but faults are developed in the southern margin of the depression fold belt, accompanied by a large number of volcanic puncture phenomena, such as the strong uplift of Dongsha Island, and no stratigraphic interface reflection is found below T7, which is interpreted as large-scale volcanic puncture (Luan Xiwu et al., 20 1 1). Dongsha structure seen in Figure 4- 18 is far from Dongsha Island. Although a large set of stratum interface reflections can be seen below T7, it is very chaotic, probably caused by faults and volcanic puncture. The above phenomenon shows that the background of gravity and magnetic anomalies in the central uplift belt is not the extension of Mesozoic magmatic rock belt along the coast of Fujian and Zhejiang, and the formation mechanism of the two should be different.
Fig. 4- 18 Seismic Profile Crossing Tanweigu Uplift and Dongsha Structure
(See Part B of Figure 4- 17 for location; See Table 4-8 for seismic reflection sequence division; F 1 Fault corresponds to F 1 in Figure 4- 17).
Table 4-8 Division of Main Structural Layers in the Northern South China Sea
3. Special lower crust high-speed layer
Fig. 4- 19 Interpretation Map of Crustal Velocity Structure in Northeast South China Sea
A-ESP-E crustal velocity structure profile (according to Nissen et al., 1995) (see Figure 4- 17 A profile for location); B— Schematic Diagram of Geological Interpretation 1— Coherent—Post-extensional Deposition; 2- Upper crust (v < 6.4 km/s); 3- lower crust (6.4 km/s < v < 7.0 km/s); 4- lower crust (v > 7.0/s); 5- continental crust; 6- oceanic crust; 7- Shangdi slow lithosphere; 8— Serpentine peridotite
The particularity of the crustal velocity structure in the northeastern South China Sea is that the lower crust layer is much thicker than the upper crust layer, and the high-speed layer changes suddenly laterally. In ESP7, there is a high-speed layer with a thickness of 15km in the lower part of the lower crust with a thickness of about 22km (the velocity value is 7.0 ~ 7.2 km/s), but all the high-speed layers suddenly disappear as far as the north of ESP7 A (Figure 4- 19a). Coincidentally, a similar OBS wide-angle seismic profile was obtained at the western end of the central uplift belt (Figure 4-20). Coincidentally, this mutation site corresponds to the peak area of the high-value positive magnetic anomaly area, indicating that there should be a connection between the two. There are generally two explanations for the appearance of the high-speed layer in the lower crust on the continental margin: on the passive continental margin, it is often explained as a lava pad formed by crustal extension and subsidence or slow bottom erosion; In the active edge area, it is attributed to a large volcanic arc. There have been many discussions on the phenomenon of high-speed layer in the lower crust of the northern South China Sea (Nissen et al.,1995; Zou Heping, 20065438+0; Qiu et al., 2003; Cai et al., 2007; Wei Xiaodong et al., 20 1 1), is generally believed to be the result of lithospheric subsidence or underplating (denudation) under the background of regional extension since the late Cretaceous. However, these discussions are contrary to the actual situation in this area: first, as mentioned above, this area has nothing to do with the magmatic rock belt along the coast of Fujian and Zhejiang, thus denying the theory of magmatic arc on the active edge of the late Mesozoic; Second, lithospheric detachment or underplating should occur in the passive marginal zone with the strongest extension. If it is related to the expansion of the South China Sea, it should be accompanied by strong magmatic activity and high geothermal background in the middle and late Cenozoic. On the contrary, this area is a low-value heat flow background (Figure 4-2 1), the crust and lithosphere are obviously thickened (Figure 2-26), the fault activity is weak, and there is no volcanic puncture. Thirdly, these discussions did not involve the reasons for the sudden change of the inner crust structure between ESP7 and ESP7 A and the rapid thickening of the high-speed layer of the lower crust.
Figure 4-20 P-wave velocity structure of OBS 2006-3 seismic profile
(According to Wei Xiaodong 20 1 1)
(See Part C of Figure 4- 17 for location)
As mentioned above, there is a unified lithospheric detachment from the northeast of the South China Sea to Taiwan Province Island, and the suture zone between the Caledonian and the Taiwan Province Strait constitutes the boundary between the northern margin of the South China Sea-Taiwan Province Province block and the Fujian-Zhejiang-East China Sea block. Tanweigu Uplift Zone is basically parallel to the suture zone of Zhuwai-Taiwan Province Strait, and there may be genetic relationship between them, which can be used for reference by Aya Kamimura and others (2002). According to this model, dehydration will occur when the subduction plate with water is inserted into the depth of 25 ~ 50 km, so that the slow peridotite rising along the subduction zone on the thrust plate can get water and form serpentine zone. The biggest feature of this model is that there are no volcanic rocks on the surface, so there are no common positive and negative magnetic anomalies similar to those in Zhejiang and Fujian coastal areas. According to this model, it can be inferred that the plate subducting southward along the suture zone of Zhuwai-Taiwan Province Strait may suddenly turn into a high-angle subduction near ESP7, which on the one hand promotes the serpentinization of the slow peridotite on the south thrust plate, and on the other hand causes the abnormal slow intrusion of the lower crust, thus forming the phenomenon that the high-speed layer of the lower crust increases rapidly in a short distance (Figure 4- 19 B). As for the southern part of the belt, a large area of lower crust high-speed layer can still be seen up to the edge of the central basin, which may be caused by large-scale lithospheric subsidence or underplating in the northern margin of the South China Sea under the extension background since the late Cretaceous. In other words, it is not excluded that the high-speed layer of the lower crust shown in Figure 4- 19 A may have been formed in two periods or two mechanisms, namely, Caledonian plate subduction mechanism and lithospheric subsidence or underplating mechanism since the late Cretaceous.
To sum up, the central uplift belt is the basement uplift belt connecting Tanweigu Uplift, Taiwan Province Shoal and Penghu-Beigang Uplift, extending in the northeast direction as a whole, which is roughly parallel to the suture belt of Zhuwai-Taiwan Province Strait. The deep source of high gravity and magnetic anomalies and the development of a large set of high-speed layers in the lower crust along the belt indicate that the uplift belt was formed in the pre-Mesozoic, which may be related to the subduction of the suture zone along the Taiwan Province Strait and outside the pearl during Caledonian orogeny in the eastern Fujian-Zhejiang sea-land block. It is the existence of this area that controls the obvious differences of Mesozoic and Cenozoic geological structure characteristics between the northern and southern margins.
Figure 4-2 1 geothermal flow trend map in the northern South China Sea
(According to Zeng et al., 1995, adapted)
(2) Fold zone of northern margin depression (Ⅰ1)
The fold belt of the northern margin depression is located on the north side of the central uplift belt, and its main body roughly overlaps with the eastern part of Zhu Yi sag, and it is generally distributed in the northeast direction. Zhu 1 sag is a structural unit divided according to Cenozoic sedimentary thickness, but its development characteristics (distribution range, thickness change, etc. ) is different from the Cenozoic era. In the early stage of geophysical survey in the Pearl River Mouth Basin, it was found that there was a set of large fold reflection sequence under the relatively gentle reflection sequence (confirmed by drilling as Cenozoic), which may be equivalent to the Upper Triassic-Lower Jurassic developed in southwestern Fujian and eastern Guangdong. Later, more and more seismic survey results prove that this reflection sequence is common in the eastern part of Zhu Yi sag and Tanweigu uplift, but its shape, thickness and fracture development degree vary greatly laterally, and the common sequence suddenly contacts with some random reflection or reflection blank zones, which may be the result of the interference of Yanshanian magmatic active zones or dynamic metamorphic zones, so it is difficult to make a large-scale tracking and comparison. However, macroscopically, it can be confirmed that although this sequence is common, its main body develops in the south of the suture zone between the Pearl River and the Taiwan Province Strait (Figure 4-22 and Figure 4-23), showing a belt-like distribution in the northeast direction, and the north and south are obviously thinned, which is roughly equivalent to the northern zone of the Mesozoic distribution area delineated by the vertical first derivative of Bouguer gravity anomaly and the vertical first derivative of polarized magnetic anomaly in the basement. It should be pointed out that there is still a phenomenon of east-west division along this belt, which may be related to its subsequent sedimentary environment, magmatic activity, metamorphism and tectonism.
The Mesozoic developed in the northern margin of the depression fold belt is equivalent to the Mesozoic exposed in Fujian and Guangdong. In view of the fact that the Upper Triassic-Lower Jurassic is mainly exposed in the eastern Guangdong-southwestern Fujian region, and the main body is distributed in a northeast belt, its development characteristics seem to be controlled by the water-Haifeng suture zone in the land. Upper Triassic-Lower Jurassic marine strata have also been found in some islands near the Pearl River Estuary (such as Hong Kong and Hebao Island). This phenomenon shows that this area has extended into the sea area, and it is bound to merge with the northern fold belt (Ⅰ1) distributed in the northeast of the sea area outside the Pearl River Estuary. Therefore, it can be considered that the Mesozoic widely distributed in the northern margin of the fold belt and the eastern Guangdong-southwestern Fujian region mainly belongs to the Upper Triassic-Lower Jurassic, and its main development intervals are controlled by the Zhuwai-Taiwan Province Strait suture zone and Lishui-Haifeng-Qiongdongnan suture zone.
Figure 4-22 Seismic Interpretation Profile of Line 79PR 1788 in Pearl River Mouth Basin
See Table 4-8 for descriptions of structural layers I-III.
Figure 4-23 Seismic Interpretation Profile of Line 79PR 1827 in Pearl River Mouth Basin
See Table 4-8 for descriptions of structural layers I-III.
(3) southern depression fold belt (Ⅰ 3)
Development characteristics of 1. boundary faults
The basement fault zone on the south side of the central uplift belt is an important dividing line between this belt and the southern margin depression belt.
The seismic reflection profile (Figure 4- 18) reveals that the evolution of the slope belt on the south side of Tanwei Ancient Uplift is obviously controlled by the basement fault zone (F 1) existing in its depth. This belt is equivalent to the F2 fault classified by Wu et al. (20 1 1) according to the collision characteristics in this area, so the magnetic difference between the two sides of this belt is the strongest, which may be formed before the underplating related to continental margin stretching. It is worth noting that this zone is just between ESP7 and ESP7 A, that is, the abrupt change of the high-speed layer in the lower crust, which roughly corresponds to the section where the truncation phenomenon of T7 unconformity is most concentrated (Figure 4- 18 A). According to the estimation of eroded stratum thickness, the differential settlement at close range is about 2000m. The above phenomenon shows that the southern boundary of Tanwei ancient uplift is controlled by a basement fault zone that moves southward for a long time, and its development history can be traced back to the Caledonian period when the high-speed layer of the lower crust changed in this area. In the Late Paleozoic, the belt developed into a fault-step belt with high north and low south, and the uplift belt on the north side accepted the platform facies carbonate deposition during the maximum period of global transgression in the Late Permian. Mesozoic and Cenozoic still inherited this trend of high in the north and low in the south, and the sedimentary thickness on both sides of F 1 is very different. It was not until the "South China Sea Movement" marked by T7 unconformity that this difference rose and fell.
Meng (1970) first interpreted the Penghu-Beigang uplift as a "structural rock wall". The results of gravity and magnetic survey (Xie Shixiong et al., 1972) show that there is a large NE-trending fault in the southern boundary of the uplift (that is, the transition zone between the continental shelf and the continental slope), which can be successfully connected with the Zhu Yi edge hinge fault in the coastal area of Taiwan Province Province. The development characteristics of Zhu Yi marginal hub fault and F 1 fault are similar, so it is speculated that they belong to the same fault zone, and the isomorphism becomes an important dividing line across the south side of Tanweigu Uplift, Taiwan Province Shoal and Penghu-Beigang Uplift. The formation of this zone may be related to the subduction of the suture zone in the Zhutai Strait in Caledonian. In the early Yanshan period, the uplift belt was dislocated by the left-lateral shearing action that obliquely crossed the fault zone between the western edge of the East China Sea and the eastern edge of Dongsha (Figure 2-2 1). According to the distance from the fault in the suture zone between Zhuwai and Taiwan Province Strait, the shear displacement is about 100km. If the position before the fault is restored, the shoal in Taiwan Province Province and Penghu Islands belong to the same basement uplift block. Because the fault zone between the western edge of the East China Sea and the eastern edge of Dongsha is a large basement fault zone, it often appears zigzag in the Mesozoic and Cenozoic caprock development areas. F2, F3, F4 and F5 in Figure 4- 17 all reflect the large basement fault between the western edge of the East China Sea and the eastern edge of Dongsha, in which F2 intercepts the Zhu Yi marginal hub fault and the F 1 fault.
Figure 4-24 Seismic Interpretation Profile of NHD96 Line in Chaoshan Depression
See Table 4-8 for descriptions of structural layers I-IV.
2. The development characteristics of the fold belt in the southern margin depression.
Figure 4-25 Seismic Interpretation Profile of 973 Scientific Research Line in Southwest Basin
See Table 4-8 for descriptions of structural layers I-III.
Figure 4-26 Seismic Interpretation Profile of Line NHD-4 16 in Southwest Basin
See Table 4-8 for descriptions of structural layers I-III.
The southern fold belt is located in the south of the central fold belt, and its range extends from the southern slope of the fold belt to the edge of the South China Sea basin, including Chaoshan depression and southwest Taiwan basin. A series of folds and faults were formed in Mesozoic in this area, which were staggered in NE direction, reflecting the strong NW-SE compression. Another important feature of Mesozoic in this area is that there is obvious difference in its development degree between east and west: a set of Mesozoic can be widely seen in Chaoshan Depression (Figure 4-24), and its maximum thickness is estimated to be 8000 ~ 9000 m; The southwest basin of Taiwan Province is characterized by the Cenozoic era, with a maximum thickness of 7,000-8,000 meters. Under this stratum, some reflection sequences (Figure 4-25 and Figure 4-26) can be seen intermittently, which may belong to Mesozoic, but their appearance is different from that of Chaoshan Depression, reflecting that not only the sedimentary environment and tectonic action are different, but also the formation time may not be completely comparable. The Mesozoic in the southern margin of the depression and fold belt is generally thick in the west and thin in the east, which roughly corresponds to the distribution area of Mesozoic delineated by the vertical first derivative diagram of Bouguer gravity anomaly in the basement and the vertical first derivative diagram of polarized magnetic anomaly. There is a large area of low-value area in the south of Dongsha, but in the southwest basin of Taiwan Province, the low-value area only appears in the southern margin of Penghu-Beigang uplift. It is worth noting that this difference between the east and the west is roughly bounded by the basement fault zone on the western edge of the East China Sea and the eastern edge of Dongsha. This belt is represented by a series of NNE faults in Mesozoic and Cenozoic caprocks.
(4) Baiyun 'ao Fold Belt (Ⅰ 4)
Baiyun fold belt is connected with the southern margin fold belt in the east, but the two are separated by a fault barrier-like bulge in the near north-south direction. The direction of tectonic lines in this area is changeable, mainly NW-NW (Figure 4- 17), and the Mesozoic thickness tends to be thinner, with strong tectonic activity (Figures 4-27 and 4-28). As mentioned earlier, this area is located near the boundary of the east and west blocks on the northern margin of the South China Sea (Figure 4- 13). This NW-trending block boundary has the nature of transformation, and the earliest giant eastern stage appeared, which will undoubtedly affect the development of Mesozoic.
Figure 4-27 Seismic Interpretation Profile of Line NHDL 1 12 in Pearl River Mouth Basin
See Table 4-8 for descriptions of structural layers I-III.
Figure 4-28 Seismic Interpretation Profile of NHD 16 Line in Pearl River Mouth Basin
See Table 4-8 for descriptions of structural layers I-III.