Late Mesozoic tectonic structure and evolution along the present-day northeastern South China Sea continental margin

Late Mesozoic tectonic structure and evolution along the present-day northeastern South China Sea continental margin

Available online at www.sciencedirect.com Journal of Asian Earth Sciences 31 (2008) 546–561 www.elsevier.com/locate/jaes Late Mesozoic tectonic stru...

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Available online at www.sciencedirect.com

Journal of Asian Earth Sciences 31 (2008) 546–561 www.elsevier.com/locate/jaes

Late Mesozoic tectonic structure and evolution along the present-day northeastern South China Sea continental margin Chun-Feng Li

a,b,*

, Zuyi Zhou a,b, Hujun Hao c, Huanjiang Chen a, Jialin Wang Bing Chen a,b, Jiansheng Wu a,b

a,b

,

a

School of Ocean and Earth Sciences, Tongji University, Shanghai 200092, China State Laboratory of Marine Geology, Tongji University, Shanghai 200092, China Nanhai East Institute of CNOOC Research Center, Guangzhou 510240, Guangdong, China b

c

Received 15 April 2007; received in revised form 8 August 2007; accepted 10 September 2007

Abstract The northeastern South China Sea continental margin holds the key to understanding Late Mesozoic tectonics and evaluating hydrocarbon potentials in Mesozoic tectonic and stratigraphic structures offshore southeast China. With newly obtained and processed seismic data, and new drilling and logging data, we correlate regional Mesozoic stratigraphy and analyze major Mesozoic tectonic events and structures. In particular, we focus our study on the three major tectonic units in the area, the Chaoshan Depression, the Tainan Basin, and the Dongsha–Penghu Uplift, which are separated by basement high, thrust fold, and (or) faults. Stratigraphic correlations suggest a major phase of southeastward regression, spanning in time from the late Early Jurassic (180 Ma) to the Early Cretaceous (120 Ma). Seismic data reveal two major tectonic events, with the first one in the Late Jurassic to the Early Cretaceous, contemporary with the regression, and the second one in the Late Cretaceous. Regional magnetic anomaly map after the reduction to the pole clearly reveals the boundary between the Dongsha–Penghu Uplift and the Chaoshan–Tainan depositional system. The seismic and magnetic data also suggest that, while the Dongsha–Penghu Uplift has highly magnetized sources buried mostly in the upper crust at depths from about 2 km to about 20 km, the Chaoshan–Tainan depositional system has thick Mesozoic sediments of low magnetization. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: South China Sea; Mesozoic tectonics; Seismic interpretation; Euler deconvolution; Reduction to the North Pole; Magnetic anomaly

1. Introduction Geological studies and hydrocarbon explorations in the northeast South China Sea continental margin have been traditionally focused on structures and terrestrial sequences within Cenozoic rifting basins, such as the Pearl River Mouth Basin (Figs. 1 and 2). Recently, a necessity has been established for strategic research on the hydrocarbon potential within Mesozoic structures and marine sequences (Zhu and Wang, 2000; Cai et al., 2000, 2004; Liu, 2003). Earlier studies, both onshore and offshore, suggested that *

Corresponding author. Address: School of Ocean and Earth Sciences, Tongji University, Shanghai 200092, China. Tel.: +86 21 65988582. E-mail address: cfl@mail.tongji.edu.cn (C.-F. Li). 1367-9120/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2007.09.004

Upper Mesozoic marine facies are existent in the northeast South China Sea continental margin (Su et al., 1995; Hao et al., 2001). One school of scientists argue that these marine facies are affiliated with the Meso-Tethys, which was suggested to reach the northeast South China Sea area in the Jurassic (i.e., Cai et al., 2000; Xia and Huang, 2000; Zhou et al., 2005), while some others emphasize the influence of the Paleo-Pacific (i.e., Taylor and Hayes, 1983; Holloway, 1982; Hayes et al., 1995; Yang et al., 2003; Xiao and Zhen, 2004). Today, more tend to favor that the northeast South China Sea area was once situated in a transition zone between the Meso-Tethys and Paleo-Pacific (Chen et al., 1998; Liu et al., 1996; Sun et al., 1989; Zamoras and Matsuoka, 2001; Xia et al., 2004; Zhou et al., 2005). Paleontologic studies in the area indicate that some late

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Fig. 1. Topographical and bathymetrical map of the South China Sea and its adjacent region. Three isobaths shown on the map are 1000, 2000, and 3000 m, respectively. T, Taiwan. Bold lines are seismic sections used in this study. The black open box shows the study area.

Mesozoic macrofossils show closer affiliation to Tethyan biota, whereas some others show more characteristics of the Pacific biotic province (Chen et al., 1998; Liu et al., 1996; Hutchison, 1989; Fontaine et al., 1983; Kudrass et al., 1986). These studies suggest complicated geological evolutions of the region, and more efforts are required in discerning the tectonic affiliations of Mesozoic sedimentary basins. However, detailed studies on the late Mesozoic tectonics in the region are hampered by at least three facts. First, Cenozoic continental margin rifting and oceanic floor spreading largely transformed the Mesozoic regime. Despite some different views (Darbyshire and Sewell, 1997; Sibuet et al., 2002), it has been suggested that there once existed a late Mesozoic Paleo-Pacific subduction zone of Andes type along the southeastern China continental margin (Jahn et al., 1976; Hilde et al., 1977; Hamilton, 1979; Zhou and Li, 2000). However, the present-day north-

ern continental margin of the South China Sea is a passive one. Continental margin rifting and subsidence formed large basins, such as the Pearl River Mouth Basin (Figs. 1 and 2), with thick Cenozoic sediments overlying the basement of Mesozoic age. Secondly, poor seismic reflection qualities at depth from earlier explorations have restrained possible identifications of deep structures and seismic sequences. Thirdly, there had been no well drillings targeted specifically for the Mesozoic in the area until 2003 when the first well (MZ-1-1, Fig. 2) was drilled into the Mesozoic in the Chaoshan Depression (Shao et al., 2007; Wu et al., 2007; Li et al., 2007), despite that a dozen of wells within the Tainan Basin and the Peikang basement high reached Lower Cretaceous and Jurassic strata. Due to theses difficulties, limited discussions on the Late Mesozoic structures and evolutions in the area are mostly based on indirect evidences from land or speculations from seismic reflections.

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Fig. 2. Shaded relief map showing major Mesozoic and Cenozoic tectonic units. Two isobaths shown on the map are 500 and 3000 m, respectively. TNB, Tainan Basin; RTF, relict transform fault; NW Basin, northwestern South China Sea sub-basin; Ph, Penghu; Pk, Peikang; D-PU, Dongsha–Penghu Uplift; A, B, C, D, E, and 973G are seismic sections or geological traverses; *, drilling site of well MZ-1-1; * labeled by 1, 2, and 3 in the Peikang area are wells PK-2, PK-3, and WH-1, respectively.

This study is promoted by the availability of new drilling and logging data of well MZ-1-1, as well as petrologic analyses on sidewall coring samples (Shao et al., 2007). In addition, deep seismic reflection and potential field data are newly acquired (Figs. 1 and 2), many with refined acquisition parameters (Yan and Liu, 2004; Huang et al., 2005; Yan et al., 2006; Zhou et al., 2006; Li et al., 2007), and many earlier seismic lines are reprocessed to improve the qualities of deeper reflections (Chen et al., 2002). With these new data, we focus our study on three major tectonic units in the area, which are the Chaoshan Depression, the Tainan Basin, and the Dongsha–Penghu Uplift (Fig. 2). Such a study not only will offer new insights on the Late Mesozoic tectonophysical processes fostering the transition from a Mesozoic active continental margin to a Cenozoic passive one, but will also help guide future potential hydrocarbon exploration in Mesozoic basins. 2. Geological setting In the northeast South China Sea continental margin, the Chaoshan Depression, the Tainan Basin, and the Dongsha–Penghu Uplift (Fig. 2) are three major geological units adjacent one to another. They are also the major targets for studying Late Mesozoic geology in the area, the

reason being that these Mesozoic structures are relatively thin overlain by Cenozoic sediments. The Chaoshan Depression is regarded as a relict Mesozoic basin (Wang et al., 2003; Ren and Jiang, 2004) (Fig. 2). Reflection seismic data show that the Cenozoic sediments over the Tainan Basin are averaged at about only 1.0 km in thickness, but the pre-Cenozoic sequence is very thick (Fig. 3) and does not show a clear basement on seismic sections (Hao and Zhang, 2003). A sharp angular unconformity characterizes the transition from the Mesozoic sequences to the Cenozoic sequences (Fig. 3). This angular unconformity dates back to the Late Cretaceous (100 Ma), and represents a depositional hiatus of a duration of about 65 Ma. Developed on a Mesozoic depositional system, the present-day Tainan Basin (Fig. 2) is a Cenozoic rifting basin, filled with sediments from the Upper Oligocene to the Quaternary (Lee et al., 1993; Tzeng, 1994; Lin et al., 2003). Seismic and well data suggest that the top of the Mesozoic there is also marked by an unconformity representing a stratigraphic hiatus from the Upper Cretaceous to Lower Oligocene, which indicates a long episode of uplift and erosion during the Late Cretaceous and Early Tertiary (Lee et al., 1993; Tzeng, 1994; Lin et al., 2003; Li et al., 2007). Numerous wells have been drilled

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Fig. 3. (a) Observed free-air gravity anomaly and the total field magnetic anomaly along line A. (b) Magnetic depth estimation using Euler deconvolution with five different structural indices (SI). (c) Seismic section A over the Chaoshan Depression and the Dongsha Uplift. No data are acquired near the Dongsha Island. TWTT, two way travel time. Tg is the basement of the Cenozoic sequence. See Figs. 4 and 5 for approximate ages of horizons Tm1, Tm3, Tm4, and Tm5. SI = 1, 2, and 3 correspond to thin prism (or line of poles), point pole (or line of dipoles), and point dipole, respectively (Thomson, 1982; Durrheim and Cooper, 1998; Cooper, 2002).

within the Tainan Basin, some of which uncovered Lower Cretaceous and Jurassic rocks (Shaw and Huang, 1996; Lee et al., 1993; Tzeng, 1994; Lin et al., 2003; Zhou, 2002), and discovered petroliferous structures in Lower Cretaceous sandstones. The Dongsha–Penghu Uplift is a basement high located to the north of the Tainan Basin and the Chaoshan Depression (collectively called Chaoshan–Tainan depositional system). It elongates from the southwest to the northeast in a strike subparallel to the continental margin, and connects with the Peikang basement high further north onshore Taiwan Island (Fig. 2). A conspicuous feature associated to this uplift is an outstanding positive magnetic anomaly belt (Fig. 4), which we christen offshore southeast China magnetic anomaly (SCMA) and will be further examined later in this paper. Late Mesozoic biostratigraphy on the Peikang basement high are well studied (Matsumoto, 1979; Huang, 1978; Huang and Chi, 1979; Yuan et al., 1985; Shaw and Huang, 1996).

Knowing these three tectonic structures, it will be important to study how they are related with each other, and examine whether they followed similar or different deformational patterns, during their Late Mesozoic evolutions. 3. Geophysical data and methods Seismic data in SEGY format come from three sources, Guangzhou Marine Geological Survey (GMGS), South China Sea Institute of Oceanology (SCSIO), and China National Offshore Oil Corporation (CNOOC). Both GMGS and SCSIO data were acquired in 2001, with their acquisition parameters shown in Table 1. CNOOC data used in this study were acquired in multiple years, from 1980 to 2000. Some of these lines were reprocessed in 1998 and 2000 by various contracting companies to increase the signal-to-noise ratio. Line E (Fig. 2) is the result of truncation and catenation of four different lines reprocessed in 1998. All 2D seismic data used in this study,

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Fig. 4. Regional total magnetic anomaly map, overlapped with isobath contours of 1000, 2000, and 3000 m. Magnetic data from Geological Survey of Japan (1996). A, B, C, D, E, and F are seismic sections or geological traverses. The outstanding positive magnetic anomaly subparallel to the coastline of China is the offshore South China magnetic anomaly (SCMA).

Table 1 Acquisition parameters for seismic data used in this study

Streamer channel Record length (s) Sampling rate (ms) Receiver interval (m) Shot interval (m) Airgun volume (in.3)

GMGSa

SCSIOb

CNOOCc

240 10.024 2 12.5 50 3000

48 12.0 2 25.0 50 1370 or 1520

240 5.07.0 2 12.5 25 mostly P3000

a GMGS: Guangzhou Marine Geological Survey, Chinese Ministry of Land and Resources. b SCSIO: South China Sea Institute of Oceanology, Chinese Academy of Sciences. c CNOOC: China National Offshore Oil Corporation.

which amount to about 2280 km in total length, were loaded on a SUN workstation for interactive interpretation. Seismic reflections are correlated with well logging data from well MZ-1-1. Free-air gravity anomalies are extracted along seismic lines from satellite-derived free-air gravity data of 2-min grid (Sandwell and Smith, 1997). Total field magnetic data are from the compilation by Geological Survey of Japan (1996) based on various sources. Since our working area

is at low latitudes, we performed reduction to the North Pole on the total magnetic field. We then applied 2D Euler deconvolution (Thomson, 1982; Reid et al., 1990; Durrheim and Cooper, 1998; Cooper, 2002) onto the magnetic data to estimate magnetic positions and depths, primarily that of the SCMA. Drilling and logging data of well MZ1-1 were acquired by industrial standards and are provided by CNOOC. Based on these new and other previously published data, we first make a regional correlation on stratigraphy and sedimentary environments. 4. Stratigraphic correlations In eastern Guangdong Province (Fig. 2), regional geological investigations found shallow marine clastic sedimentary rocks from the Triassic to the Lower Jurassic (Hao et al., 2001; Qiu and Wen, 2004). Started from the Middle Jurassic, volcanic activities, due possibly to the subduction of the Paleo-Pacific plate, dominated the area, and terrestrial sedimentary rocks are often interbedded with volcanic rocks such as volcanic tuff and breccia (Fig. 5). After the late Early Cretaceous, volcanic activities subsided and the area received lacustrine and pluvial sediments (Guangdong Geological Bureau, 1988; Chen et al., 2003).

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Fig. 5. Regional Late Mesozoic stratigraphic correlations.

There are very few wells drilled on the Dongsha–Penghu Uplift, however many were drilled on its northeastern extension, the Peikang basement high onshore Taiwan. We take for the moment the stratigraphy of the Peikang basement high as an analogy to that of the Dongsha–Penghu Uplift. Although this analogy might be remotely factual, the two structures seem to be spatially connected and one appears to be the natural extension of the other. They both are basement highs covered with very thin Cenozoic sediments and they both show high positive magnetic anomalies (Fig. 4). Many wells on the Peikang basement high reached Mesozoic rocks, and thick Lower Cretaceous clastic sediments (about 1539 m thick) were discovered by well WH-1 (Fig. 2) (Yuan et al., 1985). The majority of the Cretaceous rocks are shale, siltstone, and sandstones, with occasional occurrence of oolitic limestone. Some wells, such as PK-2, PK-3, and WH-1 (Fig. 2), also unraveled Jurassic sedimentary rocks, including tuffaceous sandshale, sandstones, black shale, and crystalline limestone (Matsumoto, 1979; Zhou, 2002). Readers are referred to Matsumoto (1979) for a figure showing Mesozoic sediment lithologies and ages of the sequences drilled at these wells. Upper Cretaceous strata are found to be absent in both the Tainan basin and the Peikang basement high. Lower

Cretaceous rocks in the Tainan Basin are mainly shale, siltstone, and sandstones, which can be classified as littoral and lacustrine facies based on rock textures and fossils (Shaw and Huang, 1996; Lee et al., 1993; Tzeng, 1994; Lin et al., 2003; Zhou, 2002). The Lower Cretaceous strata are underlain unconformably by a sequence of black shale, which is interpreted as of late Jurassic age. Well MZ-1-1 drilled on the northern slope of the Chaoshan Depression revealed rich information on the Mesozoic rock types and sedimentary environments (Shao et al., 2007; Wu et al., 2007). An angular unconformity Tm4 in the Lower Cretaceous strata divides the Mesozoic rocks revealed by well MZ-1-1 into two sequences (Fig. 6). The top of the upper sequence are mainly terrestrial clastic rocks. Gypseous cementation does not appear in the lower part, but appears in the upper part of the top upper sequence, showing a change from an early humid to a late arid inland environment (Shao et al., 2007). The bottom of the upper sequence is characterized by a thick suite of mafic volcanic rocks interbedded with mudstone and sandstones. The lower sequence consists mainly of mudstone, siltstone, and sandstones, with thin oolitic limestone and volcanic rocks. Based on rock textures and fossils, it is suggested that water depth increased from the

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Fig. 6. Seismic Line F passing through well MZ-1-1. TWTT, two way travel time.

Middle Jurassic to the Early Cretaceous (Shao et al., 2007; Wu et al., 2007). The area near well MZ-1-1 then experienced a depositional hiatus, represented by the unconformity Tm4 on the seismic section, and later massive basaltic eruptions of about 300 m thick in the Late Cretaceous (Figs. 5 and 6). This pattern of environmental change is conformable with the overall tectonic setting and evolution of the southeastern China continental margin. Fig. 5 summaries and compares the lithology and facies from inland Guangdong, the Peikang basement high, the Tainan Basin, and the Chaoshan Depression. Despite a time lag (50 Ma determined from fossil records) from Guangdong to the Chaoshan Depression, an overall pattern of change from early marine to later terrestrial sedimentary environment is observed in the Chaoshan–Tainan depositional system and in Guangdong. The Peikang basement high seems to have kept a shallow marine environment from the Late Jurassic to the Early Cretaceous, but also with clear evidences for Early Cretaceous volcanic activities. All offshore areas in this study underwent severe Late Cretaceous and Early Paleogene erosion. 5. Major tectonic structures 5.1. Seismic horizon definitions Line F (Figs. 2 and 6) is a seismic section passing through well MZ-1-1, which was drilled on the northern slope of the Chaoshan Depression where a fold was developed (Figs. 2, 4, and 6). On this section, horizon Tm4 is an

Early Cretaceous unconformity separating two different seismic facies, a weak reflection sequence above and a strong one below Tm4. These two sequences correlate with the two sedimentary sequences interpreted from well MZ1-1, respectively (Fig. 5). It is noted that volcanic rocks revealed by well MZ-1-1 are often associated with weak reflections. Horizon Tm5 is interpreted as representing the base of a strong reflection sequence of Jurassic age. Horizon Tm3 exists mainly within the Chaoshan Depression, and forms an unconformity near the thrustfold. Horizons Tm3 and Tm4 form a wedge tapering out northwards near the fold hinge. Late Cretaceous strata near well MZ-1-1 were strongly eroded, causing the basement angular unconformity Tg to be overlain by late Cenozoic sediments. Tm1 is a horizon that appears to be only present in the central Chaoshan Depression. 5.2. Mesozoic structures and their transitions From the network of 2D seismic lines (Figs. 1 and 2), we are able to extrapolate horizons interpreted from Line F (Figs. 2 and 6) to other seismic lines. Line E (Figs. 2 and 7) is a NE-striking seismic section passing from the Chaoshan Depression to the Tainan Basin. Within the Chaoshan Depression, seismic data show a thick suite of strata with strong and laterally continuous reflections beneath Tg (Fig. 7). Other than faulted by some Cenozoic faults, many of which reach the seafloor, the Mesozoic strata show little deformation in the central part of the depression when viewed along line E (Figs. 2 and 7). Strong deformation

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Fig. 7. (a) NE-striking seismic line E traversing from the Chaoshan Depression to the western Tainan Basin. (b) Seismic interpretation of (a). CDP, common depth point (interval = 12.5 m). TWTT, two way travel time. Tg is the basement of the Cenozoic sequence. See Figs. 4 and 5 for approximate ages of horizons Tm1, Tm3, Tm4, and Tm5.

do occur to the northeast near the transition zone between the Chaoshan Depression and the Tainan Basin. First there is a fault-bounded trough structure at the northeastern margin of the depression (Fig. 7). Within the trough, the whole suite of Mesozoic strata as well as the bottom section of the Cenozoic strata are folded. This trough is therefore likely to be a Cenozoic structure. The most noticeable feature in the transition zone is a basement high, which is also fault-bounded and shows chaotic reflections. Mesozoic sequences above Tm4 appear to be missing on the basement high. Further to the northeast is the Tainan Basin. Line E (Figs. 2 and 7) shows the part of the Tainan Basin on the continental slope where Cenozoic deformation is severe. However, Mesozoic sequences can still be traced laterally, though not so continuous as those in the Chaoshan Depression. Within the Tainan Basin, Mesozoic strata above Tm3 appear to vary considerably in thickness, due to erosion and deformation. Compared to the Chaoshan Depression, Mesozoic strata in the Tainan Basin appear to be thinner, and Upper Cretaceous sequences are more severely eroded.

From stratigraphic correlation (Fig. 5) and seismic interpretation (Fig. 7), we interpret that before the deposition of horizon Tm3, both the Tainan Basin and the Chaoshan Depression areas are in a shallow marine to deep sea environment. From the late Early Cretaceous to Late Cretaceous, differential movement between the Tainan Basin and the Chaoshan Depression occurred. The Chaoshan Depression kept receiving Upper Cretaceous sediments, while Upper Cretaceous sediments in the Tainan Basin are strongly eroded or non-existent. Erosion occurred in both areas until the Late Oligocene when the Tainan Basin started to subside and receive Cenozoic sediments, but the Chaoshan Depression kept a shallow water environment. Line A (Figs. 2 and 3) also passes through the Chaoshan Depression but is perpendicular to Line E (Figs. 3 and 7). Same as on the nearby line F (Figs. 2 and 6), a folding structure on line A (Figs. 2 and 3) marks the northern margin of the Chaoshan Depression. This fold also serves as the transition between the Dongsha Uplift and the Chaoshan Depression. The Dongsha Uplift shows weak and chaotic seismic reflections. To the south of the Dongsha

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Island (Fig. 3), deformation appears to be the strongest in the depression, characterized by both Cenozoic and Mesozoic faulting, folding, and magmatic activities. The southern margin of the depression appears to be further south of the southern end of line A (Figs. 2 and 3). Line B (Figs. 2 and 8) is parallel to line A (Figs. 2 and 3), but does not pass through the Chaoshan Depression. Line B is located at the proximity of the Chaoshan–Tainan transition zone (Fig. 2), and it actually shows the transition from the Dongsha Uplift to the western part of the Tainan Basin (Figs. 2 and 8). On line B, the Dongsha–Tainan tran-

sition is marked by a group of southeast dipping faults. Mesozoic strata above Tm3 appear to be largely missing in both the Dongsha Uplift and the Tainan Basin area. For structures and stratigraphy within the Tainan Basin, readers are referred to Li et al. (2007). 6. The offshore southeast China magnetic anomaly (SCMA) The SCMA, being a positive magnetic belt (Fig. 4), is developed immediately to the north of the Chaoshan–Tainan depositional system. Toward the causes of this anom-

Fig. 8. Seismic section B over the Tainan Basin and the Dongsha Uplift (c). (a) Observed free-air gravity anomaly and the total field magnetic anomaly along line B. (b) Magnetic depth estimation using Euler deconvolution with five different structural indices. TWTT, two way travel time. Tg is the basement of the Cenozoic sequence. See Figs. 4 and 5 for approximate ages of horizons Tm4 and Tm5. SI = 1, 2, and 3 correspond to thin prism (or line of poles), point pole (or line of dipoles), and point dipole, respectively (Thomson, 1982; Durrheim and Cooper, 1998; Cooper, 2002).

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aly, there have been many discussions (Taylor and Hayes, 1983; Xia et al., 1994; Yao et al., 1994; Kido et al., 2001; Zhang, 2002; Cheng, 2004). Xia et al. (1994) noticed that the northern flank of the anomaly is sharper than the southern flank, and argued that magnetic source bodies are thin horizontal finite plates that are shallow buried (1–3 km in depth), possibly being interplate, extrusive mafic rocks. Kido et al. (2001) suggested that the SCMA might be rooted in the uppermost sediment and upper part of the lower crust based on the tertiary volcanism. Zhang (2002) and Cheng (2004) noticed that the SCMA is strongly correlated with the width and shape of a high-velocity (>6.5 km/s) uplift zone in the lower crust. Cheng (2004) further suggested that the uplifted high-velocity zone is from magmatic underplating during the final rifting and early spreading stage. Apparently, most of these authors prefer a Cenozoic age for the magnetic sources causing the SCMA. The SCMA shares some similarities with the North American east coast magnetic anomaly (ECMA) (Holbrook and Kelemen, 1993), both lining up along the continental margins and trending parallel to the coastline and tectonic trends. Different from ECMA, however, the SCMA does not extend over the continent–ocean transition zone based on the studies of numerous seismic lines in the region, but primarily on the continental shelf. Regional studies indicate that the early opening of the South China Sea did not accompany strong magmatic activities (Wu et al., 2001; Yan et al., 2006), or the South China margin may at most represent an intermediary form of continental extension between the end member extremes of the Iberia-type non-volcanic and the Greenland-type volcanic margin (Clift et al., 2001). The SCMA is thus unlikely to be associated with Cenozoic magmatism concomitant with continental rifting and oceanic floor spreading, while the ECMA is typically regarded as resulting from synrifting magmatism. As shown earlier, well MZ-1-1 discovered a thick layer of mafic extrusive rocks buried at about 1.4 km below the seafloor. Also the well reached a thick layer of granodiorite at a depth of about 2.4 km. These mafic to intermediate rocks, often shown as dim zones on reflection seismic data in this area (Fig. 6), could be among the causative magnetic sources for the SCMA. 6.1. Reduction to the North Pole The SCMA is located at low latitudes (about 22°N), where induced polarization has a small inclination. This small-angle polarization could severely bias the observed magnetic field, making it difficult for us to identify the true locations of subsurface magnetic sources. To facilitate interpretation, it is necessary to reduce the data as if their causative sources were vertically polarized at the North Pole. After the data reduction, theoretically the magnetic maxima or minima should be located directly above the magnetic sources, allowing us to make more accurate regional geological interpretations.

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In data reduction, we use the program FFTFIL of Hildenbrand (1983), which performs various two-dimensional filtering operations using fast Fourier transforms, and can handle large input grids. Data gridding is done using the minimum curvature algorithm (Briggs, 1974). Our input grid has 1300 points along the longitude and 1000 points along the latitude, which are all evenly divided in degrees. We choose an initial working area slightly larger than our final mapping area (Figs. 4 and 9) to remove edge effects. Fig. 9 shows the result of data reduction, from which many important new observations can be immediately drawn. First, one can notice that the peak of the SCMA is shifted to the north by approximately 20 km; secondly, the SCMA on the new map (Fig. 9) becomes less continuous along its strike than shown on the original map of the total magnetic field (Fig. 4); and thirdly, a new sharp boundary now occurs on the southern flank of the positive anomaly belt, and the original one-peaked SCMA seems to bisected along this sharp boundary into a positive belt to the north and a quiet belt to the south (Fig. 9). Even more striking is that, when examined carefully, the sharp boundary corresponds nicely with the northern margin of the Chaoshan–Tainan depositional system. In other words, on the reduction-to-the-pole map, both the Tainan Basin and the Chaoshan Depression show moderately negative and quiet magnetic anomalies. This is an important discovery because it confirms that the Tainan–Chaoshan depositional system indeed has thick Mesozoic sediments of relatively low magnetization. This observation would not have been obtainable without the reduction to the pole. Additionally, to the southwestern Taiwan there is a triangular area of quiet magnetic anomaly, bounded to the southwest by the relic transform fault (Hsu et al., 1998; Sibuet et al., 2002; Li et al., 2007). Between the transform fault and the eastern sub-basin of the South China Sea, there is a large area of positive magnetic anomaly, indicating strong presences of magnetic sources. Indeed, large igneous bodies have been observed on the seismic line 973G passing that area (Fig. 2) (Li et al., 2007). While the Chaoshan–Tainan depositional system shows moderately negative and quiet magnetic anomalies on the reduction-to-the-pole map, the Dongsha–Penghu Uplift to the north shows in general positive, albeit discontinuous, magnetic anomalies (Fig. 9). The Penghu area is characterized by the clustering of many isolated magnetic anomalies of small wavelengths (Fig. 9). To the southwest, the magnetic anomaly pattern of the SCMA changes, showing longer wavelengths. This indicates that the SCMA, though appears to be a fairly uniform structure on the total magnetic field before the reduction to the pole, may be in fact caused by some small and isolated magnetic sources. Its appearance as a large uniform structure before the reduction may be simply due to the oblique polarization. The significant difference in magnetization between the Dongsha–Penghu Uplift and the Chaoshan–Tainan system suggests that they have fundamentally different basements beneath the Cenozoic sediments. The Chaoshan–Tainan

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Fig. 9. Regional total magnetic anomaly after the reduction to the North Pole, overlapped with isobath contours of 0, 1000, 2000, and 3000 m. Magnetic data from Geological Survey of Japan (1996). Bold straight lines are seismic sections or geological traverses. See Figs. 2 and 4 for line labels. D-PU, Dongsha–Penghu Uplift. The low magnetic zone immediately to the south of D-PU corresponds with the Chaoshan–Tainan depositional system. The SCMA corresponds roughly to the D-PU (also see Fig. 4 for the location of the SCMA).

depositional system is underlain by thick Mesozoic sediments of low magnetization, whereas the Dongsha–Penghu Uplift is underlain largely by volcanic rocks of high magnetization. This observation from magnetic data is consistent with results from seismic interpretations made earlier in this paper. One may speculate whether the basement depth plays a major role in affecting magnetic anomaly patterns; however, we found no consistency between the basement depths and the magnetic anomalies. For example, on line A the Cenozoic basement of the Chaoshan Depression is shallower than that of the Dongsha–Penghu Uplift (Figs. 2 and 3), yet the shallower basement of the Chaoshan Depression does not give a higher magnetic anomaly (Fig. 3). 6.2. Euler deconvolution To determine the positions and depths of the magnetic anomalies, we performed Euler deconvolution along four traverses, two of which coincide with seismic lines A and B, respectively (Fig. 2). No seismic data are available for traverses C and D, but traverse C is very close and almost parallel to seismic line 973G and correlations between the seismic section and depth estimates can be made thereof.

Traverse D are made passing through the Penghu area and the Tainan Basin, southwest Taiwan. All traverses are nearly perpendicular to the strike of the SCMA. The Euler deconvolution has been widely applied in estimating magnetic depth (Thomson, 1982; Reid et al., 1990; Durrheim and Cooper, 1998; Cooper, 2002; Li, 2003). In the 2D case, Euler deconvolution can be stated as (Thomson, 1982; Hsu, 2002) ðx  x0 Þ

oT oT þ ðz  z0 Þ ¼ nðB  T Þ; ox oz

ð1Þ

where T is the observed magnetic field at location (x, y) caused by a magnetic source at location (x0, y0), B is the field base level, and n is called the structural index or attenuation rate (Blakely, 1996). The structural index n is a parameter of source geometry and measures the rate of decay of the field strength with distance from the source (Reid et al., 1990). If a constant n is assumed, n can be simultaneously calculated with (x0, y0) by least square inversion. Based on higher order derivatives of magnetic field data, Hsu (2002) generalized the conventional Euler deconvolution to give the estimate of magnetic depth and structural index jointly without knowing the base level of the magnetic field. The method of Hsu (2002) is certainly an inter-

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esting and important generalization over the conventional Euler deconvolution. However, Hsu’s method requires higher order (2) derivatives of the magnetic data, which can enhance noise in the data and bias the final results. Very often n may not be constant for complicated sources (Steenland, 1968; Ravat, 1994, 1996), and thus Thomson (1982) recommended trying various values of n rather than solving for it simultaneously. Since we are only interested in the approximate depths or depth ranges, or relative depths, of the magnetic sources, we adopt Thomson’s scheme and calculated magnetic depths for each traverse using five different structural indices (n = 1.0, 1.5, 2.0, 2.5, 3.0) and show the depths together on a single plot for each traverse. Although we made calculations based on 2D lines only, it is pointed out by Li (2003) that 2D Euler deconvolution can be just as accurate as 3D Euler deconvolution. One of the many advantages of Euler deconvolution is that it does not require any assumptions for the source geometries (Thomson, 1982). The lengths of moving windows for Euler deconvolution are 30 km for traverses A and B, and 25 km for traverses C and D. These numbers correspond roughly to half wavelength of the SCMA as shown on these traverses. Different structural indices give different depth estimates. In general, the larger the index the deeper the estimated depths. The correct structural index is the one that theoretically gives the smallest solution scattering (Thomson, 1982), but in practice this may not always be true due to noises in data and anomaly interferences (Silva et al., 2001). Clustering in the depth estimations often falls on magnetic top or contrast, or falls within geometrical prolongations of the source bodies (Thomson, 1982; Silva et al., 2001; Hsu, 2002). Often independent evidences are needed to pinpoint the exact geological nature of the clustering. Despite the fact that the Euler deconvolution is more suitable for estimating magnetic depths and locations, by examining the point clustering calculated with various structural indices, it is possible to visualize the depth prolongations of magnetic sources or contrasts. Fig. 3b shows the depth estimation along seismic line A. When we study the results of Euler deconvolution, we look at the clustering of data points, which give the estimated depths and locations magnetic sources. It is noted from the point clustering of Fig. 3b that the magnetic sources of the SCMA are mainly located within the Dongsha Uplift but close to the northern margin of the Chaoshan Depression. As already indicated earlier by the magnetic data with reduction to the pole, the magnetic sources are located not directly beneath, but to the north, of the SCMA before the reduction to the pole. The estimated depth points appear to be converging downwards with larger structural indices. We estimate that there is a major southeastward dipping magnetic source or contrast buried mainly in the upper crust. The upward bifurcation of the clustered points might be reflecting branching magnetic sources or contrasts. Within the Chaoshan Depression, magnetic sources appear to be only coarsely distributed but deeply buried, and they

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collectively form a concave-upward magnetic ensemble mimicking the structure of the depression. Further to the south, a shallow magnetic source is imaged on the central fold zone in the Chaoshan Depression (Fig. 3). Along seismic line B (Figs. 2 and 8), there also appears to be a southeastward dipping magnetic body or contrast within the middle to lower crust of the Dongsha Uplift. Across this magnetic body or contrast, magnetic depths change abruptly. To the north, magnetic sources appear to be near the surface, while to the south magnetic sources are likely to be deeply buried, ranging in depths from about 5 to 18 km. Similar to line A (Figs. 2 and 3)., there is also a shallow magnetic source on the continental slope at distance 95 km (Fig. 8). Traverse C (Fig. 2) is designed to pass through the SCMA and the Tainan Basin. The calculated depth points for magnetic sources of SCMA mostly cluster at a depth of about 8 km, while shallow magnetic bodies may exist near the southern edge of the SCMA (Fig. 10). Within the Tainan Basin, magnetic sources are scarcely distributed. Traverse D (Fig. 2) passes through the Penghu area and the part of the Tainan Basin where the present-day Manila accretionary prism is developed. Beneath the SCMA, shallow magnetic sources or contrasts are present (Fig. 11). To the southeast multiple magnetic sources or contrasts appear and their depths increase southeastwards, from several kilometers to more than 10 km. The existence of multiple magnetic sources in the Penghu area from Euler deconvolution is consistent with observations from the magnetic anomaly map after the reduction to the pole (Fig. 9). At the point near the horizontal distance of about 100 km, there appears to be a boundary separating two different magnetic domains. Across the boundary, further to the southeast within the Tainan Basin, scarcely distributed shallow magnetic sources appear near or on the continental slope. 7. Conclusions and discussions Previous studies suggest that there were two episodes of transgression in the area, one occurred from the Late Triassic to the Early Jurassic and a second one during the Early Cretaceous (Hao et al., 2001; Qiu and Wen, 2004). However, the Middle Jurassic to Early Cretaceous deep sea facies found in the Chaoshan–Tainan depositional system contradicts with the two-episode model. Because the Triassic and Early Jurassic rocks are not exposed or drilled in the offshore area, it is unclear whether there had been just one episode of transgression from the Triassic to the Early Cretaceous, or indeed two separated ones. On the other hand, based on Fig. 5, one can be sure that there was a major episode of regression occurred in both onshore Guangdong and the Chaoshan Depression. Despite the difference in onset time, the regression in both places is characterized by a depositional hiatus, followed by severe volcanic activities. Less characteristic, but similar depositional features can also be noticed in the Peikang basement high and the Tainan Basin area. We suggest that the area

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Water depth

1 2 Tainan Basin

3

a

4 0

20

Total magnetic anomaly (nT)

250

40

60

80

100

120

140

160

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60

SCMA

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200

40

150 100

20 50 0

0

-50 -100 0

b 20

40

60

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100

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-20

SI = 1.0 SI = 1.5 SI = 2.0 SI = 2.5 SI = 3.0

10 Depth (km)

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0

20

30

40

c 0

20

40

60

80

100 120 Distance (km)

140

160

180

200

Fig. 10. (a) Water depth profile along traverse C. (b) Observed free-air gravity anomaly and the total field magnetic anomaly along traverse C. (c) Magnetic depth estimation using Euler deconvolution with five different structural indices. SI = 1, 2, and 3 correspond to thin prism (or line of poles), point pole (or line of dipoles), and point dipole, respectively (Thomson, 1982; Durrheim and Cooper, 1998; Cooper, 2002).

experienced one local episode of southeastward regression from the inland area to the offshore area, spanning temporally from the late Early Jurassic to the Early Cretaceous. This regression and associated volcanic activities are contemporary with, and similar in the migration patterns to, the presumed subduction of the Paleo-Pacific plate in the area (Jahn et al., 1976; Hilde et al., 1977; Hamilton, 1979; Holloway, 1982; Zhou and Li, 2000). One possible scenario explaining the link between the regression and the subduction is that the eastward and southeastward retreat of the Paleo-Pacific subduction zone induced orogenic and volcanic activities to migrate seaward, triggering the local regression. The Chaoshan Depression and the Tainan Basin area experienced two major tectonic events. One occurred from the Late Jurassic to the Early Cretaceous, causing the regional unconformity Tm4 and the boundary fold between

the Chaoshan Depression and the Dongsha–Penghu Uplift (Figs. 3 and 6). From the late Early Cretaceous to Late Cretaceous, differential vertical movement occurred between the Tainan Basin and the Chaoshan Depression. The Chaoshan Depression kept receiving Upper Cretaceous sediments, while Upper Cretaceous sediments in the Tainan Basin were strongly eroded. The second major tectonic event occurred in the Late Cretaceous, causing the whole offshore area to uplift and undergo erosion. Despite the two major tectonic events in the area, the Mesozoic strata in the central Chaoshan Depression appear to be only weakly deformed and faulted. The transition between the Chaoshan Depression and the Tainan Basin is marked by a basement high and faults, between the Chaoshan Depression and the Dongsha–Penghu Uplift by a thrust fold, and between the Tainan Basin and the Dongsha–Penghu Uplift by faults.

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559

Water depth

0.5

Manila acretionary prism

1 Tainan Basin

1.5 2 2.5 0

a 50

100

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200

50 gravity anomaly magnetic anomaly

SCMA

150

25

100 50

0 0 -50

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-100 -150

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Water depth (km)

0

0

Depth (km)

10

20 SI SI SI SI SI

30

40

c 0

50

100 Distance (km)

150

= 1.0 = 1.5 = 2.0 = 2.5 = 3.0

200

Fig. 11. (a) Water depth profile along traverse D. (b) Observed free-air gravity anomaly and the total field magnetic anomaly along traverse D. (c) Magnetic depth estimation using Euler deconvolution with five different structural indices. SI = 1, 2, and 3 correspond to thin prism (or line of poles), point pole (or line of dipoles), and point dipole, respectively (Thomson, 1982; Durrheim and Cooper, 1998; Cooper, 2002).

The northern South China Sea continental margin experienced a transition from a Mesozoic active continental margin to a Cenozoic passive one. This transition is contemporary with a southeastward marine regression and the retreat of the Paleo-Pacific subduction zone (Zhou and Li, 2000). This transition in tectonic regimes is likely characterized by first a massive volcanic eruption phase in a littoral arc environment, then a terrestrial or littoral depositional phase, and finally a long erosional phase. Studies on the magnetic data after the reduction to the pole show that the Chaoshan Depression and the Tainan Basin have relatively quiet magnetic anomalies, indicating low magnetization in their Mesozoic rocks. On the other hand, the Dongsha–Penghu Uplift corresponds with high magnetic anomalies. The boundary between the Dongsha–Penghu Uplift and the Chaoshan–Tainan depositional system can be clearly identified from the magnetic data

after the reduction to the pole. These magnetic observations are in line with those from seismic data interpretation, suggesting that the Chaoshan–Tainan depositional system is underlain by thick Mesozoic sedimentary rocks of low magnetization, whereas the Dongsha–Penghu Uplift has quite a different basement with high magnetization. Magnetic depth estimations using Euler deconvolution imply that the sources of the SCMA are mostly buried in the upper crust, with a possible depth ranging from a few kilometers to about 20 km. The magnetic sources may have variable shapes along the strike of the SCMA, and some may extend to a depth of 40 km. The SCMA becomes less continuous on the magnetic map after the reduction to the pole, an observation that also confirms the variable sources of the SCMA. Magnetic sources in the Chaoshan–Tainan depositional system are scarce and are normally deep buried.

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