The characteristics of the Geological structures of the Tertiary basins on the continental margin in the northern part of the South China Sea

The characteristics of the Geological structures of the Tertiary basins on the continental margin in the northern part of the South China Sea

EnergvVol.IO.No.3/4, pp,359-372. 1985 Printed I"GreatBritam 0360-5442185 $3.00+ 00 Q 1985PergamonPress Ltd. THE CHARACTERISTICS OF THE GEOLOGICAL ST...

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EnergvVol.IO.No.3/4, pp,359-372. 1985 Printed I"GreatBritam

0360-5442185 $3.00+ 00 Q 1985PergamonPress Ltd.

THE CHARACTERISTICS OF THE GEOLOGICAL STRUCTURES OF THE TERTIARY BASINS ON THE CONTINENTAL MARGIN IN THE NORTHERN PART OF THE SOUTH CHINA SEA JIN-MIN WV South China Sea Geological Investigation Headquarters, Ministry of Geology and Mineral Resources, Guangzhou, China (Received August 1983) Abstract-The superimposed assemblages of structural lines trending northeast, east-northeast, and west-northwest form the major structural framework of the continental margin in the northern South China Sea. The northeast structural trend represents Mesozoic basement trends, those in early Paleogene sedimentary layers. The east-northeast or west-northwest structural lines developed in post-Eocene-Oligocene sedimentary layers as a result of superimposition of northeast trends. The Tertiary sedimentary basins have been produced and developed in block structures formed by these structural trends. Tertiary deposits at the continental margin of the northern South China Sea reach a maximum thickness of 10 km. Lower Tertiary sequences are usually absent from the high points but are commonly present in the splintered segments, with continental deposits dominating. The Upper Tertiary section is well developed, and extensively overlaps the high points and the splintered segments, with marine deposits dominating. Tertiary sediments can be divided into four subcycles of Paleocene-Early Oligocene, Late Oligocene-Early Miocene, Mid-MioceneLate Miocene, and Late Miocene-Pliocene ages. The early Pliocene transgression was the greatest since late Oligocene time. During Early-Late Miocene time, regression was most extensive. All or part of the Upper Miocene is absent in many basins. The known productive formations (EoceneLower Oligocene, Upper Oligocene-Lower Miocene and Mid-Miocene) are all closely related to the middle stages of the sedimentary subcycles. Tertiary basins have different structural positions depending on basement features. Multisite-type faulting depressions are developed within the basin causing splintering. The basins are shaped like grab-ens and pan-type depressions, and had an initial stage of faulting, a middle stage of depression, and a late stage of overlapping and coverage by sediments. The development of normal faults within the basin, high heat flow, and the lack of compressive folding show that the basins were formed by extension. The development of basins was influenced by four plate movements during Cenozoic time. The Tertiary basins in the north of the South China Sea are related to the gradual disintegration of the northern continental margin or the South China Sea, the frequent movement of the plates around the South China Sea, and the spreading of the South China Sea Basin. Thus basins were formed with different structural attributes but similar sedimentary cycles.

INTRODUCTION

Seventeen Tertiary sedimentary basins have been discovered and defined on the continental margin of the northern South China Sea (Fig. 1). All of the basins trend in a northeast direction or east-northeast except for the Yinggehai Basin. The Beibu Gulf and the Yinggehai basins are wholly located on the continental shelf. Basins partly occupying the continental shelf and partly the upper continental slope are the Southeast Hainan Basin, the Zhujiangkou Basin, and the Southwest Taiwan Basin. The basins on the lower continental slope are the Xisha Trough Basin, the North Jiangfen Basin, and the South Dongsha Basin. The South Xinhua and the South Qunfen basins are on the continental rise. According to the gravity data, most of the depositional basins lie on crust of continental or intermediate thickness, except for the South Xinhua and South Qunfen basins, which lie on oceanic crust. The structural setting of the continental margin in the northern South China Sea The continental margin in the northern part of South China Sea is the underwater extension of the South China mainland. Since Mesozoic time, a series of northeast-eastnortheast structural lines have developed on the South China mainland. They continued to be active in Cenozoic time, controlling the development of the Mesozoic-Cenozoic time, controlling the development of the Mesozoic-Cenozoic depositional basins. The northeast-trending structures may extend into the South China Sea. From west to east, 359

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Fig. 1. Distribution of Tertiary basins in the northern part of the South China Sea.

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these trends include the Weizhou Island fracture belt in the Beibu Gulf, forming the northwestern boundary of the Beibu Gulf Basin; the Qixi-Haibei uplift separating the southwest Weizhou Island segment from the Wushi segment; the Wanshan-Hainan Island Uplift: then Zhu-3-Southeast Hainan Depression; the central Uplift Belt and the Shenhu Uplift in the Zhujiangkou Basin; the Zhu-1-Zhu-2 Depression; and the Taiwan ShoalDongsha Uplift. The outer Zhujiangkou Central Fracture located in the central part of Zhujiangkou Basin is reflected in the aeromagnetic data. West of the fracture is a low or moderate-to-higher anomaly area, but to the east of it, there is a moderate magnetic gradient. Survey results suggest a major marginal subsidence zone, formed on the continental margin or the northern South China Sea and extending from the Beibu Gulf and Yinggehai basins in the west through the Zhujiangkou Basin, to the Southwest Taiwan Basin in the east. with a general east-northeast trend similar to the trend of shoreline of the northern South China Sea, and forming an angle with the northeast-north-northeast trends. The fault zone on the northern margin of the Zhujiangkou Basin is a series of eastnortheast fault terrace belts recognizable on seismic reflection records. The fault planes dip southward with a maximum fault displacement of 2000 m. Cenozoic rocks are less than 1 km north of the fault zone, but more than 5 km south of the zone. The formation of the Zhujiangkou Basin, especially the Zhu-1 Depression, was controlled by this fault zone. This zone is also reflected in the gravity and magnetic data, in particular a gravity terrace with a +lO to -10 mgal gradient in Bouguer gravity anomalies. The thickness of the crust north of the fault zone is 34 km, as calculated from the gravity data, but thins to 30 to 31 km south of the zone. The magnetic survey data suggest that a significantly varying negative magnetic anomaly belt extends east-west for more than 400 km off the coast of Hongkong-Shantou. Its maximum magnetic amplitude is -200 gamma. This belt is thought to be a deep basement fracture. A series of structures on the South China mainland trending north-northeast ends north of this fracture belt. The trends of the structural lines south of the belt are predominantly east-northeast, east-west or northwest. The structural framework south of the belt is apparently different from that on land. The fault zone on the edge of the continental slope is another structural line trending in an east-northeast direction and forming the boundary between the continental slope and the deep-sea basin. The free air-gravity anomaly is -2O--60 mgal and it is connected with a negative anomaly in the axis of the Xisha Trough to the west. This anomaly belt is 30-90 km wide. South of the zone, the magnetic anomalies of the deep-sea basin are strips trending west-easterly, but to the north is a magnetic quiet area. The west-northwest structures are also well developed, including the Hainan-Shenhu Shoal-Yitong Shoal Uplift Belt, the Southeast Hainan Island-Xisha Trough Depression Belt on the southern flank of the uplift belt, the Zhu-3-Zhu-2 Depression Belt. and the North Weitan Shoal-Dongsha Uplift Belt in the Zhujiangkou Basin on the northern flank of the Hainan-Shenhu-Yitong Shoal Uplift Belt. The Taiwan Shoal-Dongsha-Zhongsha Uplift Belt and the outer Zhujiangkou Central Uplift-Shenhu Uplift-Xisha Uplift Belt are two important structural lines extending northeast across the continental margin in the northern part of the South China Sea. The southern portion of the former forms the western boundary of the deep central basin. and the northern portion is reflected by the 500-m isobath, which is the boundary between the east-northeast-trending structural line and the northeast-trending structural line. The northeast structural lines are dominant west of this boundary. The HongkongShantou Fracture Belt and the fracture on the margin of the continental slope cease here and according to the magnetic data do not extend to the west. Thus the superimposed split assemblage of northeast-east-northeast and west-northwest structural lines forms the major structural framework of the continental margin in the northern South China Sea. The fact that the Outer Zhujiangkou Central Fracture and the Outer Zhujiangkou Central uplift belts are not reflected on the seismic profile but are defined by the aeromagnetic data indicates that the northeast structural trends are the EGY

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expression of Mesozoic basement structural trends, or early Paleogene sedimentary layers. However, the east-northeast and west-northwest structural lines developed in EoceneOligocene sedimentary layers as a result of reconstruction and superimposition on the northeast structures in the later stage. The Xisha Trough trending approximately eastwest dissects the northeast structural line and then merges with it without extending very far toward the west, suggesting that the superimposition of the later stage did not change the northeast trending framework. The north-northeast or northeast and east-northeast or west-northwest structural lines form block structures. The Tertiary sedimentary basins on the continental margin in the northern part of South China Sea were produced by this structural framework. Stratigraphic iayers and sedimentary cycles qf the Tertiary basins in the continentat’ margin of the northern South China Sea (Fig. 2) Tertiary deposits on the continental margin of the northern South China Sea are very much developed, with maximum thickness reaching 10 km. Analysis of the drilling data and seismic sections suggests that Lower Tertiary sequences are incompletely developed, as they are usually absent on the rises and well distributed in the split segments, with very large variations in thickness and domination of continental deposits. Upper Tertiary sediments are well developed and extensively overlap the rises and split segments. Their thickness is constant and they are dominated by marine deposits. With the exception of bioclastic limestone and reef limestone developed on Yongxin Island on Xisha Rise, most of the Upper Tertiary sequences of the basins are composed of a suite of monotonous elastic rock series, with frequent alternations of coarse and fine particles. The age of the stratigraphic sequences in the major Tertiary basins and simple lithologic descriptions are shown in Figs. 3-6. The Beibu Gurf Basin. This basin was split in its early stage and inherited the northeast-north-northeast trend of the Mesozoic red-bed basins (Fig. 3). The faulted depression trending east-northeast was superimposed in a later stage. A complete Tertiary sequence is present. The Lower Tertiary is continental, composed of three groups of formations. The oldest is a red molasse that includes the upper Cretaceous and the Chang Liu formation of Paleocene-Lower Eocene age. Paleocene sediments are in conformable contact with upper Cretaceous sediments. They are situated in the bottom of the basin, extending northeast. The central group is composed of, e.g., the dark rocks, Liu Sha Gang formation, deposited during the maximum subsidence of the basin, in Mid-Eocene-Lower Oligocene time. The climate at that time was warm and humid. A thick sequence of oil shale and mudstone formed an important oil-source rock system in the basin. Turbidite sandstones are the major oil-bearing beds in the Beibu Gulf Basin. The upper group is represented by the Weizhou formation, which is widely distributed over the basement, or the Liu Sha Gang formation, but is absent from the rises. Upper Tertiary sediments are neritic elastic cyclothems of fairly constant thickness, and extensively overlap the rises and split segments. Zhujiangkou Basin. The deepest deposits in this basin are unknown (Fig. 4). Possibly they are red molasse and dark rock, similar to those in the Beibu Gulf Basin. The oldest sequence revealed by drilling is of kaolinitic, coal-bearing sediments of Oligocene age deposited in fluviolacustrine and swamp environments, intercalated with multilayers of lignite and bituminous shale. The large quantities of kaolinite were derived from the erosion of Mesozoic granite. Shallow-water dolomitic sandstone is extensive. Toward the deeper and southern part of the basin, there may be fine lacustrine or neritic elastic rocks. With disconformities above and below, this is a very important oil-bearing sequence in the Zhujiangkou Basin. The Upper Tertiary system is a suite of neritic elastic formations characterized by frequent and alternating coarse and fine sediments. At the bottom of that formation, there is possible a kaolinitic coal-bearing formation on the north slope of the basin and neritic carbonates to the south. Southeast Hainan Basin (Fig. 5). The Lower Tertiary Linshui formation was deposited in a littoral beach sedimentary environment. Although it is intercalated with neritic

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JIN-MIN Wu

Description

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Fig. 3. Generalized Cenozoic stratigraphy of the Beibu Gulf Basin.

carbonate rocks, there are also large amounts of terrestrial detrital accumulations. Upper Tertiary sediments are a suite of neritic elastic rock formations rich with limestones. As the basin inclines steeply to the south, the sediment particles tend to be finer in that direction. Oil has been found in the Lower Tertiary sandstone sequence. Upper Miocene sediments are absent in the Yinggehai No. 9 well and near the edge of the basin. Yinggehai Basin (Fig. 6). Due to the great thicknesses, drilling has not reached lower Tertiary sediments. Upper Tertiary sediments are neritic elastic formations rich in

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Fig. 4. Generalized Cenozoic stratigraphy of the Pearl River Mouth Basin.

limestones and chalks. Bioreef limestone is developed in the Mid-Miocene Meishan formation. Upper Miocene and Pliocene sediments are dominated by limy mudstone, with possible development of mud domes. The lower part of the Upper Miocene is sometimes absent. Yongxin Island Basin. Only Upper Tertiary sediments are present and they consist of about 1 km of formations deposited in a neritic platform environment. Xisha Trough Basin (Fig. 7). No drilling data are available from this basin. It is assumed by seismic sections that Oligocene sediments are continental molasse that is a I- to 2-km fill in the panlike and graben-like segments. Miocene sediments lie between the bioreef limestone of the Yongxin Island Basin and the neritic elastic rocks of the Southeast Hainan Basin. They are possibly a suite of neritic fine elastic rocks with intercalations of biolimestone formations, 3 km or more in thickness. During upper Pliocene-Pleistocene time, the basin subsided rapidly due to faulting on both margins. The basin subsided and filled to a thickness of 1 to 2 km with abyssal elastic formations and sediments from eruptive volcanics.

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Fig. 5. Generalized Cenozoic stratigraphy of the Southeast Hainan Basin.

Sedimentary cycles (Fig. 8) Tertiary sediments in the basins on the continental margin in the northern part of the South China Sea contain a complete sedimentary cycle, from gradual transgressions to final regressions. The Beibu Gulf Basin is typical, containing transgressive sedimentary cycles from inland fluviolacustrine to littoral abnormal marine and to neritic continental shelf facies (Figs. 3-6). Four subcycles can be determined based on the different transgressive stages: 1. The Paleocene-Early Oligocene subcycle is mainly continental. It is observable in the Beibu Gulf Basin, with the largest subsidence stage evidenced by the middle member of the Mid-Eocene-Early Oligocene Liu Sha Gang formation. Frequent sea incursions occurred in the later part of the stage.

Characteristics of geological structures of tertiary basins

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Fig. 6. Generalized Cenozoic stratigraphy of the Yinggehai Basin.

2. The Late Oligocene-Early Miocene subcycle is relatively minor in scale. It was the first transgression in the north part of the South China Sea. The earliest transgressions were in the Southeast Hainan and Xisha Trough Basins, evidenced by sediment and biolimestones. The north slope of the Yinggehai Basin was still land, with possible marine sedimentation to the south. The northern slope of the Zhujiangkou Basin was coastal marsh and estuary delta facies. The Beibu Gulf Basin was a neritic near-shore environment as evidenced by shallow-water benthonic foraminifera. Regressions occurred later. There are no marine fossils in the upper member of the Xiayang formation and thus its deposition was possibly in a continental or transitional environment. 3. Another major transgression is evidenced by the Mid-Miocene-Late Miocene subcycle, when water bodies deepened, sediments became finer, and a plankton foraminifera fauna thrived. But on the north slope of the Zhujiankou Basin the transgression was smaller. Regression took place during Late-Mid-Miocene time and reached its peak during the Early-Late Miocene time. All or part of the Upper Miocene is absent in many basins. including the Southeast Hainan and Yinggehai basins.

JIN-MIN Wu

368

Fig. 7. Integrated cross-section of the Xisha Trough Basin.

4. In the Late Miocene-Pliocene subcycle, the largest transgression in the north of the South China Sea occurred in early Pliocene time. It was not only wide in range, but also generally deep and biologically rich including planktonic foraminifera. For example, in the Yinggehai Basin, there is a foraminifera mudstone thousands of meters thick. During Mid- and Late Pliocene time, the sea became shallower, and then the greatest regression took place in early Pleistocene time. Continental sediments were deposited during the Pleistocene time in the Beibu Gulf Basin and in the north of the Zhujiangkou Basin. The sea intruded from south to north. These four subcycles can generally be matched in time to the four stages of Tertiary transgression in Southeast Asia. The known productive formations (Eocene-Lower Oli-

15000(y)

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10 20 30 40KM

Fig. 8. Cross-sections of the Pearl River Mouth Basin.

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gocene, Upper Oligocene-Lower Miocene and Mid-Miocene) in the north of the South China Sea are all closely related to the middle stages of the sedimentary subcycles. Thus, the oil-bearing formations are either in the sandstone intercalations of the oil-generating sequence, or nearby. This does not suggest any long-distance migration. Structural.ftiatures in the Tertiary basins in the northern continental margin qf the South China Sea Dizerent basement .features in the basin. Data from drilling, geophysical exploration, and regional geological studies indicate that basement features vary in different basins. There is different basin structure to the north and south of the line connecting the Red River Fault with the fault along the southwest edge of Hainan Island and the fault at the northern side of the Xisha Trough. The northern portion is separated east-west by the outer Zhujiangkou Central Fault and the Shenhu Fault. The intensity of the magnetic field in the west is weak, with a peak value of +50 gamma. It is inferred that the basement of this region is composed of Lower Paleozoic metamorphic rocks, Upper Paleozoic sedimentary rocks, and medium-to-acidic Mesozoic magmatic rocks. This has been confirmed by drilling in the Beibu Gulf Basin and in the Southeast Hainan Basin as well as by the Yinggehai No. 1 and No. 6 wells at the northern edge of the Yinggehai Basin. It is assumed that similar basement is present in the Zhu-3 Depression of the Zhujiangkou Basin (Fig. 8). The east part of the outer Zhujiangkou Central Fault is represented by a negative magnetic field striking generally northeast, with a local anomaly trending east-northeast with an intensity of -50 to -160 gammas. It is inferred that the basement of the Zhu-1 Depression is dominated by Mesozoic volcanic and sedimentary rocks. A gentle but large positive anomaly running east-west in the Zhu-2 Depression in the south may reflect magnetic rocks with strong intensities. The southern side of the fault along the southwest edge of Hainan Island on the northern margin of the Xisha Trough is dominated by broad gentle positive anomalies, with intensities reaching +80 to +160 gammas. Proterozoic basement of 627 m.y. Rb-Sr isotopic age was revealed in Xiyong No. 1 well. Strongly metamorphosed Proterozoic gneiss has been found in the Red River Graben in Vietnam. It is suggested, therefore, that the main body of the Yinggehai Basin, south of the Southeast Hainan Basin. and the west of the Xisha Trough Basin have Proterozoic basement. The southwestern part of the Yinggehai Basin might be underlain by Indo-Sinian folded basement. The eastern part of the Xisha Trough Basin is represented by a broad gentle positive anomaly extending east-northeast, with the amplitude increasing from west to east to +200 gammas. These anomalies may reflect ultrabasic rock bodies intruded into the basement of the basin. The massive-type, east-west-trending, +300-+400-gamma anomaly in the eastern Xisha Trough Basin, reflects oceanic crust. In the Southwest Taiwan Basin to the east of the Dongsha Islands, the metamorphosed Cretaceous basement revealed by drillings in Tongliang No. 1 well at Penghu and F-l well has large-scale wavy folds and is interpreted as Yenshanian folded basement. Its magnetic field is characterized by mixed positive and negative anomalies of broad and gentle variations. Splitting qf the basins. The superimposition of the north-northeast and east-northeast trends resulted in multiple faulting, which split the basin. Figure 9 shows that these features are grabens and pan-type depressions. Grabens include the Zhu-1 Depression and the Xisha Trough Basin. whereas pan-type depressions include the Zhu-2 and Zhu3 depressions and the Southeast Hainan Basin. The Beibu Gulf Basin is a double pantype basin. These fault depressions or segments form cutoff basins that are favorable to the rapid burying of organic matter. Local structure in the basins. Normal faults abound; compressive folding is absent. Three major local structural types have been developed, i.e., inherited anticlines formed on basement rises, rolling anticlines caused by differential compaction or gravity slump related to syngenetic faults, and trap structures related to faults. The first and the third types are the major occurrences; however, the nonstructural traps and bioreef traps are worthy of attention.

2

Se

33

2

'1

L-4412

Fig. 9. Cross-sections of the Beibu Gulf Basin (L-5499), Yinggehai Basin (L-3 I 112), and Southeast Hainan Basin (L-4412).

L-31112

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Development stages of the basin. The structural background of the depressions, combined with the transgressive sedimentary cycles, defines three stages of basin evolution: initial stage of fault depressions, mid-stage depressions, and a late stage of overlapping and coverage. The first-order positive structural units within the basin were usually covered with sediments only at the last stage, and thus the basin was developed gradually from the splitting state into a uniform basin. Geothermal flow of the basin. The structural environment of extension and the intrusion of ultrabasic and basic rocks have caused geothermal flows of higher values. For example, the average geothermal value measured at the southern edge of the Zhu-2 Depression is 1.95 HFU, 30% higher than the average world value. High heat flow is favorable to the transformation of organic matter to oil and gas. The geothermal gradients in the Zhujiangkou and Beibu Gulf basins are, respectively, 4.2”C/lOO m and 3.9-4.5”C/ 100 m higher than that in the productive Tertiary basins on land. It is therefore estimated that an ideal depth for oil generation can be found just below 2000 m. Thus, very favorable conditions exist for the formation and accumulation of oil and gas. The mechanism qf the.formation of Tertiary Basins in the northern continental margin qf the South China Sea The South China Sea is situated at the southeast end of the Eurasian plate. It is bordered by the western Pacific plate to the east and the Indian Ocean plate to the west via the Zhongnan Peninsula microplate. The formation and evolution of the South China Sea is the result of interaction between these lithospheric plates. The formation of Tertiary basins at the northern continental margin of the South China Sea is closely related to the spreading and subduction of the central basin in the South China Sea. During the Mesozoic Yenshanian Movement, which was due to the downthrust and compression of the Pacific Ocean plate against the Asian continent, the continent east of Asia was uplifted, with the accompanying eruption of a large amount of medium-toacidic magmatic rocks. The volcanic rocks in Fujian and Zhejiang provinces were part of this outwelling. Soon afterward, due to the rising and splitting of the earth’s crust, a series of Mesozoic and Cenozoic basins trending northeast-north-northeast formed behind the volcanic arcs and accumulated sediments from upper Cretaceous to Eocene time. The Chang Liu formation in the Beibu Gulf Basin was produced then and similar sediments may occur in the Zhu-3 Depression. Since Mesozoic time four important plate movements have taken place. Mid-Late Eocene event (40 m.y. BP). Due to the change in direction of the movement of the Pacific Ocean plate and the collision of the Indian Ocean plate with the Eurasian continent, the Paleo-Sarawak oceanic crust between Liyue Bank and Borneo was downthrust and subducted under Borneo. This even caused the uplift of the edge of the South China continent and the breakup and subsidence of its seaward edge, with most basins receiving fill-type sediments. Late Oligocene-Early Miocene event (32-l 7 m.y. BP). This event is called the Nanhai Movement. The South China Sea basin spread twice, forming deep-sea basins, and giving rise to large-scale transgressions in those sedimentary basins. Influenced by the spreading of the deep-sea basins, the east-northeast structural line was especially developed. Mid-Miocene-Late Miocene event (16-12 m.y. BP). The relative speed of west Pacific plate motion increased 16 million years ago from 4 cm/y to 15 cm/y. Such a rapid downthrust of the west Pacific plate relative to the Asian continent caused the general uplift of the southeast end of the Eurasian plate and the total or partial absence of the Upper Miocene from many basins. Volcanic sediments were deposited in the west part of Taiwan. Pliocene event (5 m.y. BP). With the cooling and subsidence of the basins and an increase of east-west compressional stress, spreading in the central deep basin ceased. The Philippine Sea plate moved horizontallly along the Taiwan longitudinal valley and sinistral Philippine Fault, and overthrust the central basin in the South China Sea. The oceanic crust of the South China Sea basin was downthrust along the Manila Trench,

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toward Luzon, causing the “warping back” of the Taiwan-Luzon thrust zone and the largest transgression in the South China Sea. As a result, Pliocene sediments were generally overlapped and covered. This event caused the rapid uplift of Taiwan and Luzon Island and Late Tertiary regression in the South China Sea. Since then, the deep South China Sea basin has been inactive. New tensional splitting east of the Xisha Islands, following the structural orientation of earlier stages, gave rise to new oceanic crust 4.93km thick, which is thought to be the thinnest in the South China Sea. CONCLUSION

The Tertiary basins in the north of the South China Sea are related to the gradual disintegration of the continental margin in the north of South China Sea in space, and to the frequent activities of plates enclosing and spreading the South China Sea basin in time. Different types of oil- and gas-bearing basins were formed with different structural attributes but similar sedimentary cycles. Further studies on these basins will surely help in the exploration for oil and gas in the north of the South China Sea.