Neotectonic regime on the passive continental margin of the northern South China Sea

Neotectonic regime on the passive continental margin of the northern South China Sea

ELSEVIER Tectonophysics 311 (1999) 113–138 www.elsevier.com/locate/tecto Neotectonic regime on the passive continental margin of the northern South ...

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ELSEVIER

Tectonophysics 311 (1999) 113–138 www.elsevier.com/locate/tecto

Neotectonic regime on the passive continental margin of the northern South China Sea Thomas Lu¨dmann Ł , How Kin Wong Institute of Biogeochemistry and Marine Chemistry, University of Hamburg, Bundesstrasse 55, D-20146 Hamburg, Germany Received 14 July 1998; accepted 15 April 1999

Abstract Between 1989 and 1994, more than 6600 km of reflection seismic profiles were obtained in the South China Sea off Hong Kong with the German research vessel Sonne during cruises SO-50B, SO-72A and SO-95. A seismo-stratigraphic interpretation of this data set leads to a new age assignment of the unconformity T0 which we place within the Pleistocene. Both Neogene unconformities T1 and T0 are generated by uplift of the Dongsha Rise and truncation of their overlying strata. This uplift is caused by intrusion of magma into the upper crust. Our seismic profiles show plutons which have penetrated the sedimentary cover, whereby their original stratification in the contact zone is eliminated. These magmato– tectonic events may be correlated to the two main collision phases between Taiwan and the continental margin of East China 5–3 and 3–0 ma ago. The collisional events subsequent to the NNW to WNW drift of Taiwan transformed the compression into strike–slip movements along the continental margin of Southeastern China. The accompanying stress regime is transtensional, with subsidence of the cooling oceanic crust since the cessation of rifting and its consumption beneath the Manila Trench providing the extensional stress. The strike–slip movements remobilized many of the rift and drift faults providing pathways for magma ascent. The tectonic framework of the northern South China Sea is characterized by Miocene faults trending NE–SW. These faults are scarce but are distributed throughout the study area. Pliocene faults striking ENE–WSW to NE–SW are concentrated west of the Dongsha Islands and are mostly strike–slip in character. Recent faults are generally oriented NE–SW subparallel to the synrift faults. They result in part from local uplifts where they are normal in character, but strike–slip motion also occurs. Most of the faults involve the basement and represent reactivated zones of weakness of the rift and drift phases.  1999 Elsevier Science B.V. All rights reserved. Keywords: neotectonics; magmatic intrusions; unconformities; South China Sea; Pearl River Mouth Basin

1. Introduction The study area on the northern continental margin of the South China Sea is located off Hong Kong between the Dongsha Islands and Taiwan (Fig. 1). Ł Corresponding

author. Tel.: C49 (0)40 42838 6335; Fax: C49 (0)40 42838 6347; E-mail: [email protected]

Three cruises (SO-50B, 1987; SO-72A, 1990; and SO-95, 1994; Fig. 2) have been carried out with the German research vessel Sonne in this area, during which more than 6600 km of multi-channel reflection seismic profiles and high resolution 3.5-kHz pinger or Parasound echograms as well as a number of box core samples have been obtained. The seismic equipment consisted of a Geco-Prakla mini-streamer

0040-1951/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 9 9 ) 0 0 1 5 5 - 9

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Fig. 1. Map of the South China Sea. The study area is framed and the outline of the Pearl River Mouth Basin is shadowed.

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Fig. 2. Location of the profiles obtained with the R=V Sonne in 1987 (SO-50B), 1990 (SO-72A) and 1994 (SO-95). Profiles presented in this paper are shadowed and numbered. ESP D expanding spread profile shot by Spangler-Nissen et al. (1995). Sonobuoy stations (dots) after Ludwig et al. (1979), exploration well LH 11-1-1A (star) after Tyrrell and Christian (1992).

(active length 100 m), three Geco-Prakla air guns and one S.S.I. GI-gun (total volume 7 l). This article is focused on the characterization of the geologic evolution of the northern continental margin of the South China Sea from the cessation of the drift phase to Recent times. Since the

Mesozoic, this continental margin went through a succession of geotectonic regimes: from an active Andean type margin during Cretaceous to a divergent rift to a passive continental margin since Paleogene times. The last regime persisted since the end of the drift stage of the South China Sea in the

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upper lower Miocene and exhibits deviations from the classical model of passive margin development (see below). The most important structural feature within the study area is the Pearl River Mouth Basin, which is elongated NE–SW between Hainan Island and the Strait of Taiwan (Fig. 1). It is an epicontinental rift basin developed mainly on continental crust and in the vicinity of the ocean=continent transition zone (Li, 1984; Guong et al., 1989).

2. Geological evolution of the South China Sea 2.1. Geology of the South China Sea Here only a brief overview of the complex geological evolution of the South China Sea will be given, with emphasis on its postdrift phase. The proto-China margin originated in the Middle Jurassic as a convergent Andean type continental margin, with the central part of Taiwan, the North Palawan Block and the Reed Bank (Liyue Bank) as paleoforearcs (Hilde et al., 1977; Hamilton, 1979; Taylor and Hayes, 1983; Zheng, 1985; Letouzey et al., 1988; Williams et al., 1988). The rift phase with uplift of the rift shoulders and its erosion lasted from the Upper Cretaceous to the Middle Oligocene (Taylor and Hayes, 1980; Chen et al., 1987). The beginning of the subsequent drift phase with a north– south direction of opening is marked by a major breakup unconformity (T7 ). Reinterpreting the magnetic anomalies mapped (11-5d according to Taylor and Hayes, 1980, 1983; Holloway, 1982), Briais and Pautot (1990) placed this drift phase from the end of the lower Oligocene through to the early Middle Miocene (32–15.5 ma, anomalies 11-5c). With the cessation of seafloor spreading, the deep South China Sea Basin and its adjacent continental margins started to subside as a consequence of thermal relaxation of the oceanic crust (Taylor and Hayes, 1983). During the upper Miocene, the North Palawan Block collided with the West Philippine Archipelago in the vicinity of Mindoro and Panay (Holloway, 1982; McCabe et al., 1983; Rangin et al., 1985). This led to a decoupling of the Philippine plate from the resulting island arc and a change in subduction polarity. Eastward subduction along the Manila Trench

between Mindoro and Panay stopped, giving way to westward subduction east of the Philippines at the Philippine Trench (Uyeda and McCabe, 1983; McCabe and Cole, 1989). During the late Upper Mio– Pliocene about 5–3 ma BP (Teng, 1990), the North Luzon Arc collided with the East China continental margin, resulting in the consumption of the South China Sea along the northern Manila Trench. The North Luzon Arc was overthrust onto the continental margin in a NNW to WNW direction (Suppe, 1981; Stephan et al., 1986; Angelier et al., 1990; Teng, 1990) with an accompanying crustal shortening of 200–300 km in northern Taiwan (see Teng, 1990, p. 69, fig. 9; Suppe, 1981). From the Upper Pliocene (3 ma) to Recent times, a second major collision phase, which led to uplift of the mountain ranges in northern Taiwan, took place (Su, 1985; Angelier et al., 1990; Teng, 1990). Earthquakes in the vicinity of the Manila Trench demonstrate that subduction of the oceanic crust of the South China Sea is still active (Cardwell et al., 1980; Hamburger et al., 1983; Yang et al., 1996). The Pliocene and Quaternary have also been a time of extremely active basaltic volcanism in Indochina, Hainan, the Leizhou Peninsula as well as in Taiwan (Late Pliocene–Quaternary), with an activity maximum during the Pleistocene and the Holocene (Zheng, 1985). 2.2. Structural framework and sedimentary history The northern South China Sea is dominated by three major fault systems primarily normal in character. The first, which strikes NE–SW, has been active during the rift phase from the late Cretaceous through Oligocene time (Chen et al., 1987; Yu, 1990; Ru et al., 1994). Its NW–SE-directed extensional movements associated with rifting exhibit increasing intensity towards the south and west. A second, east–west to ENE–WSW trending system was developed between the Late Eocene and the Early Miocene, with minor activities in the Middle to Late Miocene. The associated north–south extensional movements intensify towards the eastern and northern part of the margin (Yu, 1990, 1994; Lee and Lawver, 1992, 1995). A third system, to which the Red River Fault belongs, is characterized by NW–SE sinistral strike–slip movements (Hinz and Schlu¨ter, 1985; Pautot et al., 1986; Briais et al., 1989).

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The geological evolution of the Pearl River Mouth Basin can be divided into three main phases (Feng and Zheng, 1983; Jin et al., 1984; Su and He, 1987; Guong et al., 1989; Yu, 1994): basement rifting and basin subsidence from the Upper Cretaceous to the Early Oligocene; faulting, subsidence and deposition within the subbasins from the Late Oligocene to the Early Miocene; and subsidence and filling of the entire basin since the Middle Miocene. The sedimentary history of the northern South China Sea was dominated by the deposition of the terrestrial Shenhu, Wenchang and Enping formations during the rift phase (Table 1). Later, with the advent of seafloor spreading, the sea encroached onto the continental margin and a marine paleo-environment was established (Li, 1984; Wu, 1988; Guong et al., 1989; Feng et al., 1992). Meanwhile, the continental margins surrounding the South China Sea were a site of outbuilding of giant carbonate platforms and local reef growth (Wu, 1988, 1994; Chen and Hu, 1989; Chen et al., 1994). Subsequent to the drift phase, thermal subsidence of the continental margins terminated reef building abruptly (Aquitanian to Lower Burdigalian; Erlich et al., 1990); only locally has it persisted into Recent times. This reef development has already been discussed in detail, especially in the Pearl River Mouth Basin (Cai, 1987; Hu and Xie, 1987; Hao, 1988; Chen and Hu, 1989; Fulthrope and Schlanger, 1989; Erlich et al., 1990, 1991; Christian and Tyrrell, 1991; Turner and Hu, 1991; Tyrrell and Christian, 1992; Wang et al., 1992). The lithostratigraphy of the well LH 11-1-1A (Fig. 2; Tyrrell and Christian, 1992) and p-wave velocities from sonobuoy stations (Table 2; Fig. 2) form the basis of our seismo-stratigraphic correlations. On the Dongsha Rise, 87 m of Plio–Pleistocene marine siltstone and mudstone with thin carbonate interbeds and 798 m of Middle to Upper Miocene marine mudstone and siltstone have been drilled. This is underlain by 476 m of porous, Lower Miocene carbonate (Zhujiang carbonate), which is made up of an upper unit of bank and platform facies and a lower unit of fine-grained chalky limestone of low energy platform facies. In turn, this unit is underlain by 149 m of porous Upper Oligocene to Lower Miocene quartz sandstone (Zhuhai sandstone), which overlies the Mesozoic granitic basement.

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3. Seismo-stratigraphy of the northern continental margin of the South China Sea 3.1. Bathymetry A bathymetric map of the study area compiled from Hydrosweep and Seabeam data collected by the R=V Sonne as well as from nautical charts (British Hydrographic Office, 1986; SCSIO, 1986) shows a wide continental shelf (Fig. 3), with the 200 m isobath extending more than 200 km from the Chinese coast. In the vicinity of the Dongsha Islands between 200 and 600 m water depth, there is a plateau elongated in the northeasterly direction adjoining the shelf edge. This plateau is rhombohedral in shape, with the Dongsha Islands located to the southeast where the plateau is the broadest. These islands form an atoll which emerges only a few tens of meters above the sea surface. To the southwest, the upper slope is smooth and gently dipping, whereas to the northeast it is dominated by a more-or-less rugged relief. 3.2. Seismic sequences and facies In the northern South China Sea, the Cenozoic section can be divided into seven seismic sequences (Table 3). A subdivision in parasequences has not been attempted because of the lack of adequate sediment samples and well logs. The nomenclature of the sequences and unconformities within the Pearl River Mouth Basin is not unambiguous in the literature. For example, the Middle Oligocene regional breakup unconformity has been named T 7 as well as T6 . For the sake of simplicity, we shall follow the nomenclature of Guong et al. (1989) for both the sequences and the unconformities. There are two types of acoustic basement within the study area (Table 3). The first (Bsed ) is characterized by strong hyperbolic surface reflections. Parallel reflectors can be recognized sporadically beneath the basement, suggesting that Bsed is of sedimentary origin. The second type (Bign ) consists exclusively of strong, prolonged hyperbolic reflections. Examples are the oceanic crust and magmatic intrusions within the continental crust. Twenty-seven wells in the Pearl River Mouth Basin penetrate into the pre-Tertiary basement. They show that the dominant rock types

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Table 1 Stratigraphic table with sequences, unconformities and tectonic events of the Pearl River Mouth Basin

T. Lu¨dmann, H.K. Wong / Tectonophysics 311 (1999) 113–138 Table 2 Sonobuoy velocities in km=s in the northern South China Sea (after Ludwig et al., 1979) Sonobuoy

128C17 131C17

Sonobuoy velocities (km=s) V1

V2

V3

V4

V5

V6

2.00 2.20

2.60 4.40

3.05

5.10

6.10

7.20

are probably granite, diorite and quartz–porphyrite of Mesozoic (Late Yenshanian movement) or Cenozoic age (rift phase) (Guong et al., 1989). The Shenhu, Wenchang and Enping formations (sequences I–III) are characterized by subparallel, discontinuous reflectors of varying amplitude (Table 3), which are typical of non-marine deposits (Sangree and Widmier, 1977). They occur as rift depression fill and comprise sandstones and siltstones or mudstones. Sandy conglomerates also occur occasionally. The Zhuhai Formation (sequence IV1 ) is characterized by chaotic, hyperbolic reflections with high amplitudes (Table 3). This seismic facies may be attributed to coarse, clastic, fluvial-marine to coastal sediments deposited by erosion of the elevated rift shoulders. Sequence IV2 (a Lower Miocene sandstone) is seismically masked on the Dongsha Rise by carbonate rocks of the overlying sequence IV3 . The latter is marked by strong surface reflections of high continuity. The reef and platform carbonates are bounded at the top by a prominent unconformity (T4 ). Sequences IV2 –IV3 are correlatable to the Zhujiang Formation. Sequences V and VI, the seismic equivalent of the Middle to Upper Miocene Hanjiang and Yuehai formations (Table 3), are bounded above by the major erosional unconformity T1 . The unconformity T2 , which separates the Hanjiang and Yuehei formations (Chen et al., 1987), has not been observed in our profiles (see below). The lithologies involved are interbedded siltstones or mudstones and sandstones with intercalations of carbonates and dolomites. These sequences exhibit continuous parallel reflectors of medium amplitudes. The Pliocene Wanshan Formation and the Quaternary (sequences VII) (Table 3), in which clay dominates over silt and sand, are characterized by

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continuous parallel reflections of high to medium amplitude (Table 3). A seismo-stratigraphic interpretation of our profiles leads to a subdivision of the Plio–Quaternary period into ten seismic sequences off Hong Kong (Lu¨dmann and Wong, submitted).

4. Cenozoic geotectonic evolution of the northern South China Sea 4.1. Fault systems Fig. 4 shows the distribution of fault systems of different ages within the study area together with the location of the continent=ocean boundary (modified after Taylor and Hayes, 1983) and that of the Dongsha Rise. Because of the limited penetration of our seismic profiles, we can neither confirm nor refute the subcrustal, landward-dipping fault postulated by Yao et al. (1994) and Hayes et al. (1995). The majority of the faults mapped are still active. Most of them have deep roots and possibly represent reactivated zones of weakness of the rift and drift phases modified by postdrift tectonic movements. The oldest faults are demonstrably pre-Miocene in age; they strike NW–SE and have been detected only within the vicinity of the continent=ocean boundary. Miocene faults are sparse and widely spaced; they strike NE–SW. Pliocene faults are particularly abundant west of the Dongsha Rise. They strike ENE– WSW and subordinately NE–SW and are generally strike–slip in character (see Section 4.3). The Recent faults, some of which flank basement uplifts, trend generally NE–SW, following the direction of the synrift faults. Other Recent faults are detachment surfaces of mass movements at the continental slope, where giant sediment packages have been torn from their original sites of deposition and transported downslope. On the continental margin around the Dongsha Islands, there are areas up to 100 km in dimension which are extremely structurally deformed. They are marked by a succession of faults only a few kilometers apart, and are possibly attributable to basement uplifts (Fig. 5). A system of deep, NW–SE-trending faults dissects the continental slope (Fig. 5). Except for the fault near 118ºE, these faults are dextral west of 117ºE while they are sinistral to the east. The re-

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Fig. 3. Bathymetric map of the study area compiled from Hydrosweep and SeaBeam data collected by the R=V Sonne as well as from nautical charts (British Hydrographic Office, 1986; SCSIO, 1986).

sulting offsets are also reflected in the magnetic anomalies, the ocean=continent boundary as well as in the deepsea basin boundary faults. We suggest that these Oligo–Miocene strike–slip faults are transforms linked to faults onshore China. Thus, the change in the sense of fault motion as observed on different transforms, e.g., along the Mid-Atlantic

Ridge, is not surprising. These faults might have developed as conjugate faults of the Tancheng–Lujiang wrench fault system in the Mesozoic and were reactivated during continental breakup and the subsequent drift phase. The Tancheng–Lujiang wrench fault system is a major, NE–NNE-trending strike–slip system that straddles Taiwan and East China including our

Table 3 Seismic sequences and facies at the northern margin of the South China Sea

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Fig. 4. Maps of the study area showing: pre-Miocene faults; Miocene faults; Pliocene faults; Recent faults as well as important sites of sediment mass transport (slumps).

study area (Fig. 6). It was active during the Mesozoic as a left lateral fault zone, but the motion became right lateral in Neogene times (Xu et al., 1987). The inverted structures in Figs. 9, 13 and 14 are younger in age and have a different orientation. They exhibit pronounced strike–slip motions which may be the result of a transfer of NNW–WNW compres-

sion into continental margin-parallel shear due to the impingement of the Luzon Island Arc with East China. We do not exclude the possibility that the WSW–SSW-trending strike–slip faults in part may represent reactivated zones of weakness created by the Tancheng–Lujiang wrench fault system during the Mesozoic.

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Fig. 5. Schematic tectonic map of the northeastern South China Sea. The line of circles gives the position of the ocean=continent boundary (modified after Taylor and Hayes, 1983). Thin, continuous straight lines with numbers mark magnetic anomalies (after Briais et al., 1993). n.d. D no data.

4.2. Reconstruction of the Neogene–Quaternary tectonic regime During the postdrift tectonic evolution of the Pearl River Mouth Basin, two major uplift events occurred. According to Chen et al. (1987), these events

are marked by erosional unconformities with ages within the Middle=Upper Miocene (T2 ) and at the Miocene=Pliocene boundary (T1 ), respectively (Table 1). Our seismic profiles (see below), however, clearly demonstrate that the first Dongsha movement took place at the Miocene=Pliocene boundary and

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Fig. 6. Structural map of the South China Sea. Modified after Tapponnier et al. (1986), Ru and Pigott (1986), Xu et al. (1987), Zhou et al. (1989), Hsu¨ et al. (1990), Yu (1990) and Yang (1992). The Mesozoic Tancheng–Lujiang wrench fault system is dot–dashed. Focal mechanisms after Lin et al. (1980): 1 D Dongsha event (1966, Ms D 5:3, strike D 243º, dip D 55º); 2 D Nano event (1918, Ms D 7:3, strike D 216º, dip D 80º). Motion vectors (with rates in mm=a) for the Philippine Sea plate and South China Sea plate are relative to the Eurasian plate and based on earthquake analyses (Ma, 1988) (open arrows). The motion vectors for Taiwan are based on geodetic (GPS) data (Hu et al., 1997) (solid arrows). Insert shows geotectonic framework: HB D Huanan Block; SCSB D South China Sea Block; TLAP D Taiwan–Luzon Accretionary Prism; LA D Luzon Arc; PSP D Philippine Sea Plate; OT D Okinawa Trough.

should therefore be correlated with T1 , while the second occurred within the lower Middle Pleistocene (ca. 460 ka BP) and gave rise to the unconformity T0 of Guong et al. (1989). Our seismic profile 18 over the flank of the Dongsha Rise shows only one distinct type 1 sequence boundary, which forms the upper boundary of the Lower Miocene carbonates (IV3 ) (Fig. 7). Therefore, this sequence boundary has to be interpreted as the regional unconformity T4 . Assuming that the

younger unconformity of profile 18 is T2 , then, by using the age assignment of T2 by Chen et al. (1987) and Guong et al. (1989) and the p-wave velocities from sonobuoy 128C17 of Ludwig et al. (1979; Fig. 2 and Table 2), the thickness of the Middle Miocene (sediments bounded by the two unconformities) would be 1495 m over the flank of the Dongsha Rise. However, even within the Pearl River Mouth Basin, drill hole data show that the thickness of the Middle Miocene ranges only between 500 and

T. Lu¨dmann, H.K. Wong / Tectonophysics 311 (1999) 113–138 Fig. 7. (a) Part of seismic profile 18 (SO-72A) and (b) interpretation showing Lower Miocene carbonates overlain by ca. 1500 m of Middle and Upper Miocene mudstones and siltstones. See Fig. 2 for location. Roman numerals are sequence designations. 125

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1100 m, whereby the maximum thickness is reached only in the subbasin Zhu 1 (Fig. 6; Chen et al., 1987; Guong et al., 1989). Since it is most unlikely that the Middle Miocene is thicker over the flank of a rise than in an adjoining basin, we conclude that the younger unconformity of profile 18 cannot be T2 , and that it must be younger. In addition, the lithostratigraphy of the exploration well LH 11-1-1A drilled on top of the Dongsha Rise 20 km east of our profile shows that both the Middle and the Upper Miocene are represented by a homogeneous succession of marine mudstones and siltstones; a major unconformity does not exist within this section (Tyrrell and Christian, 1992). Thus, we expect that T2 cannot be observed seismically and that the younger unconformity is T1 , which marks the upper boundary of the Upper and Middle Miocene sequence. With this interpretation, there is still a 700-m discrepancy between the thickness of the Middle and Upper Miocene drilled in well LH 11-1-1A on the Dongsha Rise (798 m, Tyrrell and Christian, 1992) and the thickness (1492 m) deduced from our seismic profile 18 on the rise flank. We attribute this discrepancy primarily to a rapid increase in sediment thickness from the top of the rise (550 m) to its margin, as indicated by our profiles, and secondarily to a possible slight overestimate of the p-wave velocity. Profile 4 shows a section across the Dongsha Rise (Fig. 8) which is capped by a Lower Miocene carbonate platform bounded at its top by the unconformity T4 formed during the drowning of the platform. Its southwestern rim is dominated by a reef complex. The Upper to Middle Miocene strata (sequences V– VI) terminate onlap at the reef apron and at the northeastern end, but downlap in the small depression near the center of our profile. Their thickness on top of the rise reaches only 550 m (using p-wave velocities from sonobuoy 131C17 of Ludwig et al., 1979; Fig. 2; Table 2). Pliocene and Quaternary layers (sequence VII) unconformably overlie this Miocene section, forming a basin fill sequence (near the center of the profile). Thus, we interpret that the swell was uplifted at the end of the Miocene, with concomitant tilting of its northeastern part which has led to an apparent downlap termination. During this first Dongsha movement, parts of the Miocene have been eroded. To the west of the Dongsha Rise, the Panyu Low–High (Figs. 4, 6 and 9), a buried anticline,

exhibits extensive WNW–ESE wrench faulting after folding and truncation of the Miocene sediments. A second post-Pliocene uplift must also have occurred because over most parts of the Dongsha Rise, except in local depressions, Pliocene sediments are absent. Another piece of evidence for a post-Pliocene uplift of the Dongsha Rise is provided by profile 16 (Fig. 10), which shows that NNW of the Dongsha Islands, the northwestward dipping Middle and Upper Miocene section is uplifted. It is underlain by a basement block possibly of Lower Miocene carbonates. The Miocene sediments terminate offlap against a thin veneer of Pleistocene deposits which was sampled by a box grab at stations 2, 8, 9 and 22 (Fig. 11, black squares). Radiocarbon analyses of their fossil content (mostly mollusc shells) yielded an age of 23; 553 š 985 years (cessation of the last glaciation) (Wiesner, 1993). The Pliocene sediments have been extensively eroded, only in the north-northeast do they unconformably overlie the Miocene. A subsurface map with the Quaternary backstripped shows extensive outcropping of the Miocene strata around the Dongsha Islands and on top of the Dongsha Rise (Fig. 11). Box grab samples from this area (Fig. 11, stations 7 and 10, black triangles) consist of light brown, massive carbonate fragments commonly with a black outer rim of apatite and goethite. They document the exposure of the reef carbonates on the ocean bottom as well as their subsequent hydrothermal alteration. The foraminiferal population of the biomicrites attests to a Middle Miocene to Middle Pliocene age (Wiesner, 1993). We associate the two major postdrift uplift events within the Dongsha region with the intrusion of magma into the upper crust. Evidence for this can be found both in the rock samples described above and in our seismic profile 10 across the southwestern margin of the Dongsha Rise (Fig. 12). This profile shows an igneous body (a pluton) penetrating the Middle to Upper Miocene deposits. Within the contact zone, contact metamorphism has erased the original stratification of the Miocene sediments. The uplifted Miocene sequence is unconformably overlain by Pliocene sediments which terminate onlap against it. In the eastern part of the seismic section (Fig. 12a, arrow), the Pliocene strata are tilted; they terminate offlap against the relict Pleistocene sands at the seafloor as the result of a post-Pliocene uplift.

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Fig. 8. Reflection seismic profile 4 (SO-95) and interpretation. See Fig. 2 for location. Roman numerals are sequence designations.

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Fig. 9. (a) Part of seismic profile 9 (SO-72A) and (b) interpretation. See Fig. 2 for location. Roman numerals are sequence designations.

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Fig. 10. (a) Part of seismic profile 16 (SO-72A) and (b) interpretation. See Fig. 2 for location. Roman numerals are sequence designations.

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Fig. 11. Geological map of the study area with the Quaternary section backstripped. Included are box grab stations of cruise SO-95 where Pleistocene deposits (squares) and hydrothermally-altered Miocene reef carbonates (triangles) are sampled. Stations are numbered.

4.3. Postdrift uplift events The Dongsha movements at the Mio=Pliocene boundary and in the post-Pliocene, respectively, together with the accompanying magmato–tectonic events may be correlated to the complex geotectonic framework within the northern South China Sea. Fig. 6 shows the present tectonic regime of this area:

the west is characterized by the stable South China block; the southeast by thermal subsidence of the cooling oceanic crust; the east by consumption of the South China Sea along the Manila Trench; the north by the impingement of the Luzon Arc with East China at Taiwan and the south by a transform margin between Indochina and the South China Sea. The insert in Fig. 6 shows the geotectonic regime at

Fig. 12. (a) Reflection seismic profile 10 (SO-72A) and (b) interpretation. See Fig. 2 for location. Roman numerals are sequence designations.

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the continental margin of the northern South China Sea. Starting in the Upper Miocene, the Philippine Sea plate (PSP), particularly its western continuation (viz., the Luzon Arc, LA), collides with part of the Asian continental margin (the Huanan Block, HB) or, as proposed by Sibuet and Hsu (1997), with the former Ryukyu arc and forearc. The collision front is marked by a large, transpressional accretionary prism, the Taiwan–Luzon Accretionary Prism (TLAP) (Liu et al., 1997). Near Taiwan, the Huanan Block is a collision-related foreland basin with elastic deformations of its continental crust (Teng, 1990). It is bounded to the north by a major, NW–SE-trending strike–slip fault (Zhou et al., 1989; Yang, 1992) which transforms the collision-related NNW–WNW compression into dextral strike–slip subparallel to the continental margin. To the south, a wedge-shaped block (the South China Sea Block (SCSB); see insert of Fig. 6) is squeezed out in a south-westward direction. This block has a strike– slip boundary to the north and a convergent boundary to the east. Its western limit is as yet unknown because of the lack of data. Within the South China Sea Block, the tectonic stress is largely transtensional, being a combined effect of the dextral strike–slip motion just described and crustal stretching due to subsidence and subduction of the oceanic crust. The transtension leads to upwelling of mantle material, so that magma can intrude into the upper crust to produce an uplift of the caprocks (centered round the Dongsha Islands). Seismic indications for volcanism are, however, lacking. We correlate these two postdrift magma intrusion and uplift events in the northern South China Sea (marked by the erosional unconformities T1 and T0 ) with the two major collision phases between Taiwan and East China (5–3 ma and 3 ma to Recent times, respectively, Teng, 1990; Table 1). The age of T1 is 5.2 ma BP (at the Mio=Pliocene boundary; Haq et al., 1988) and marks the onset of the first collision phase. Unconformity T0 is at least Late Pleistocene in age, because it separates the Miocene sequence from relict sands of the last glaciation. Our seismo-stratigraphic interpretation in conjunction with a regional relative sea level curve for the past 700,000 yr (Xue et al., 1996) places the uplift leading to T0 at the lower Middle Pleistocene (Lu¨dmann and Wong, submitted), suggesting that it may correlate with the culmination of

folding of the Taiwan Ranges within the second collisional phase (Teng, 1990). In Table 1, the original age assignment of 1.85 ma to T0 of Guong et al. (1989) is shown dotted. Published geophysical data provide additional evidence for the tectonic events we have postulated. Satellite-derived marine free-air gravity anomalies around Dongsha Islands reach amplitudes of 200 mGal (Sandwell and Smith, 1994). A deep-sounding expanded spread profile recorded east of the Dongsha Islands (see Fig. 2; Spangler-Nissen and Hayes, 1995; Spangler-Nissen et al., 1995) documents the existence of high-density lower crust beneath the Dongsha Rise (3:1 ð 103 kg=m3 , 7.0 km=s; Spangler-Nissen et al., 1995). We suggest that this high density, the high heat flow of 144 mW m 2 and the associated magnetic anomaly (300 nT peak to peak) may be indicators of Plio=Pleistocene magmatism within the Dongsha region rather than a result of preexisting crustal material or of a composite of prerift lower crustal material and intrusions of mantle material (Spangler-Nissen et al., 1995). A high heat flow of 83.57 mW m 2 for the Dongsha Rise has also been measured by Li and Rao (1994). Yao et al. (1994) as well as Xia et al. (1994) detected a zone of positive magnetic anomalies (>100 nT) elongated NE–SW, extending from western Taiwan to the area just north of the Dongsha Islands. The location of these anomalies may delineate one of the predicted zones where magma has risen into the crust. The occurrence of strike–slip movements along the northern continental margin of the South China Sea is well-documented in earthquake focal mechanisms (Wei and Chung, 1995) and in our reflection seismic data. Fig. 6 shows two events in the vicinity of the Dongsha Islands characterized by NE–SW dextral strike–slips (NE of Dongsha, 1966, Ms D 5:3; Nanao, 1918, Ms D 7:3; Lin et al., 1980). These motions may be compared with the dextral Neogene movements of the Tancheng–Lujiang wrench fault system. Our seismic profile 13 across the top of the Dongsha Rise reveals a strike–slip fault in the south-southeast whereby the top and base of the fault zone exhibit different directions of motion (Fig. 13). At the southeastern rim of the Dongsha Rise, strike–slip faults show both normal and reverse displacements and the dip of the individual horizons varies abruptly across the fault (Fig. 14).

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Fig. 13. (a) Part of reflection seismic profile 13 (SO-95) and (b) interpretation. See Fig. 2a for location. Roman numerals are sequence designations.

The schematic tectonic map (Fig. 5) shows a lower continental slope which is divided into several narrow basins separated by basement highs made up of prerift crustal blocks as well as younger magmatic bodies. The basins developed during the rift phase are indicated by the occurrence of rift sediments. The upper slope is dominated by the Dongsha Rise, which is in the process of disintegrating into

a number of tectonic blocks under the influence of the transtensional tectonic stress in the northern South China Sea. The Panyu Low–High might have been a part of the Dongsha Rise which has broken away during the post-drift uplift events. As we have already mentioned, the lower slope exhibits an obvious segmentation into NW–SE-trending blocks which cannot be traced onto the shelf. The system of

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Fig. 14. Part of reflection seismic profile 13 (SO-95). See Fig. 2b for location.

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NW–SE-striking transform faults responsible for the segmentation might have given rise to the magnetic anomaly offsets; this would imply a post-drift age for these faults.

5. Conclusions Two post-drift uplift events are postulated within the Dongsha region. The first took place at the Mio=Pliocene boundary and the second in the lower Middle Pleistocene. Both are marked by major unconformities (T1 and T0 , respectively). These uplifts are attributed to magmato–tectonic events correlatable to the main collision phases between East China and Taiwan 5–3 and 3–0 ma ago. GPS studies indicate that the present maximum compressional stress axis trends WNW–ESE (100º–110ºN) (Deffontaines et al., 1997; Hu et al., 1997). This is in good agreement with results from studies on focal mechanisms (Yeh et al., 1991), borehole breakouts (Suppe et al., 1985) as well as Quaternary fault analyses (Angelier et al., 1986; Lacombe et al., 1993; Lee, 1994). Starting in the Middle Miocene, the Luzon Arc changed its motion from NNW to WNW (Teng, 1990). This WNW compression is converted to WSW–SSW strike–slip movements along the northern margin of the South China Sea, giving rise to a transtensional neotectonic regime. Extension induced by cooling of the oceanic crust and its subduction along the Manila Trench might have created or reactivated faults along which magma ascended into the upper crust. The resulting plutons uplifted the caprocks and induced local hydrothermal alterations. Most of the older faults involve the basement. They correspond to zones of weakness during the rift and drift phases, and are superposed by postdrift tectonic movements. Miocene faults trending NE– SW are scarce and are distributed throughout the study area. Pliocene faults striking ENE–NE are concentrated west of the Dongsha Islands and are generally strike–slip in character. Recent faults, also dominantly strike–slip, trend NE–SW parallel to the synrift faults. They are in part associated with local uplifts. Schematic structural profiles across the northern South China Sea show an obvious division in swells and basins with a dominance of landward rotation

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of basement blocks (Feng and Zheng, 1983; Chen et al., 1987; Letouzey et al., 1988; Guong et al., 1989; Wang et al., 1992). Antithetic basinward-dipping normal faults as well as crustal blocks rotated basinward along listric faults as predicted by the pure (McKenzie, 1978) and simple shear (Wernike, 1981) rift basin models are rare. Instead, landwarddipping detachment faults are common (Hayes et al., 1995). Thus, not only crustal extension, but also horizontal crustal displacements along the Mesozoic Tancheng–Lujiang wrench fault system as well as the neotectonic collision-related movements must have been responsible for the disintegration of the northern continental margin of the South China Sea. Accordingly, the Pearl River Mouth subbasins may also have a pull-apart component.

Acknowledgements We thank T.-Y. Lee, M. Pubellier and one anonymous reviewer for their encouragement and helpful suggestions which led to a substantial improvement of the manuscript. We gratefully acknowledge the unfailing help of the captain, officers and crew of the R=V Sonne during the three South China Sea cruises reported here. Special thanks are due to Dr. H.E. Christian Jr. (consultant of Amoco Orient), who has kindly made some literature available to us. We thank Dr. Shiguo Wu from the South China Sea Institute of Oceanology, Academia Sinica, for constructive discussions. This work was funded by the German Federal Ministry for Education, Science, Research and Technology (Projects MFG0052, 03G0072A and 03G0095A).

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