Geological setting

The SCS is a western Pacific marginal sea situated at the junction of the Eurasian, Pacific, and Indo-Australian plates. It developed from Cenozoic continental margin rifting, and its central portion is floored with oceanic crust. Despite its relatively small size and short evolutionary history, the SCS has undergone nearly a complete Wilson cycle from continental breakup to seafloor spreading to subduction. The SCS is a critical site connecting some of the major tectonic units in the western Pacific. It is also well suited for studying various plate boundary activities, such as continental margin rifting (e.g., Hayes and Nissen, 2005), seafloor subduction (the Manila Trench; e.g., Li et al., 2007a), strike-slip faulting (the Red River fault; e.g., Clift and Sun, 2006), and active orogenic processes (Taiwan; e.g., Huang et al., 2001) (Fig. F1).

Hypotheses for the opening mechanism of the SCS differ markedly (Fig. F3) and include (1) India-Eurasia collision and the consequent tectonic extrusion mainly along the Red River fault (Tapponnier et al., 1982; Lallemand and Jolivet, 1986; Schärer et al., 1990; Briais et al., 1993; Leloup et al., 2001; Flower et al., 2001), (2) slab pull and subduction of the proto-SCS under Sabah/Borneo (Taylor and Hayes, 1980, 1983; Holloway, 1982; Hall, 2002), (3) extension related to an upwelling mantle plume (e.g., Fan and Menzies, 1992; Xu et al., 2012), and (4) regional extension related to subduction and retreat of the Pacific plate along the western Pacific margin (Taylor and Hayes, 1980, 1983; Shi and Li, 2012). In addition to these end-member models, hybrid models have been proposed (e.g., Cullen et al., 2010).

The original SCS basin before its subduction along the Manila Trench may have been twice the size of what we see today (Sibuet et al., 2002), so the geodynamic model must be able to explain the formation of a large ocean basin. Motion on the Ailao Shan-Red River fault (RRF) has been dated to 35 to 15 Ma, with displacement of as much as several hundreds of kilometers (e.g., Leloup et al., 2001). Ages obtained from ocean basalts at the proposed drilling sites in the SCS will test the hypothesis that the motion on the RRF is coeval to, and may have driven part of, extension and spreading in the SCS, although only a minor amount of extension associated with the SCS spreading center may have been transferred to the RRF (Rangin et al., 1995; Morley, 2002; Clift et al., 2008). Regional rifting in East Asia occurred long before the India-Eurasia collision (Fig. F3D) and is thought to be associated mainly with subduction of the paleo-Pacific plate (Taylor and Hayes, 1980, 1983; Shi and Li, 2012).

Some hypotheses suggest the existence of a proto-SCS oceanic basin (Haile, 1973; Madon et al., 2000) that was once connected to the Pacific plate and began to close from ~44 Ma (e.g., Hall, 1996, 2002) in order to accommodate the opening of the SCS (Fig. F3B). Supported by the wide occurrence of Mesozoic and/or early Cenozoic marine sediments, a large part of this proto-SCS may have been subducted into, or uplifted as part of, island arcs formed to the south in Borneo/Sabah and Palawan (Hall, 2002; Hutchinson, 1996, 2004), where remnants of the proto-SCS oceanic crust may be present (Hutchison, 2005) and are one possible origin of the ophiolites of South Palawan (Rangin et al., 1990; Tu et al., 1992; Schlüter et al., 1996; Pubellier et al., 2004; Cullen, 2010). Slab-pull force from this subducting proto-SCS plate, along with an in situ mantle plume and massive synrifting volcanism, may also have triggered or contributed to the opening of the SCS, but definitive evidence for these arguments is absent.

The opening of the SCS reveals complex patterns of continental breakup and seafloor spreading. Magnetic and seismic data suggest that the SCS basin can be divided into five magnetically distinct zones (Li et al., 2008b) (Fig. F4). In particular, magnetic amplitudes and orientations in the Southwest Sub-basin (Zone E) differ markedly from those in the East Sub-basin (Zone D). These two sub-basins are divided by a complex boundary called the Zhongnan fault (Figs. F2, F4) according to some authors (Yao, 1995; Jin et al., 2002; Li et al., 2007b, 2008b). This magnetic contrast may support an episodic seafloor-spreading model (Ru and Pigott, 1986) or may be attributed to different crustal types within which the two sub-basins evolved independently. Pautot et al. (1986) suggested that the youngest part of the East Sub-basin developed within an older, preexisting oceanic crust, whereas the Southwest Sub-basin resulted from continental rifting that led to seafloor spreading. Within the East Sub-basin, two distinct negative magnetic anomalies (M1 and M2 in Fig. F4), thought to be the same age, further divide the sub-basin into a central part with high magnetic amplitudes and two separated parts with slightly weaker magnetization (C1 and C1′) near the two conjugate continental margins. The magnetic pattern of the Northwest Sub-basin also differs from its adjacent segment in the East Sub-basin.

Table T1 shows additional contrasts between the East and Southwest Sub-basins, some of which are rather perplexing. For example, the greater water depths of the Southwest Sub-basin may imply relatively older crustal ages (Ru and Pigott, 1986; Yao et al., 1994; Li et al., 2008b), which conflict with younger ages inferred from the higher heat flow and shallower Curie-point depths there (Ru and Pigott, 1986; Li et al., 2010). Recent heating from magmatic activities could have contributed to the high heat flow in the Southwest Sub-basin (Ru and Pigott, 1986; Li and Song, 2012), but this hypothesis needs to be confirmed through drilling.

A number of Cenozoic tectonic models have been proposed, but it remains uncertain whether the SCS basin experienced primarily a single episode or multiple episodes of extension and seafloor spreading and, if multiple episodes, in what sequence the sub-basins evolved (e.g., Taylor and Hayes, 1980; Pautot et al., 1986; Ru and Pigott, 1986; Briais et al., 1993; Yao et al., 1994; Hayes and Nissen, 2005; Li et al., 2007b, 2008b). For example, the opening of the East and Northwest Sub-basins may have predated, or been synchronous with, that of the Southwest Sub-basin (Taylor and Hayes, 1983; Briais et al., 1993; Lee and Lawver, 1995; Tongkul, 1994; Honza, 1995; Zhou et al., 1995; Schlüter et al., 1996; Hall, 2002; Hall and Morley, 2004; Hayes and Nissen, 2005; Braitenberg et al., 2006; Sun et al., 2009). This model contrasts with others in which earlier opening of the Southwest Sub-basin is preferred (Fig. F5) (e.g., Ru and Pigott, 1986; Yao et al., 1994; Li et al., 2007b). This latter group of models considers the sharp contrasts between the East and the Southwest Sub-basins and the important roles of the Zhongnan fault (Figs. F2, F4), which the first group often ignore. There are also two models of propagation of SCS spreading, one propagating northeast toward the Taiwan Strait (Chung et al., 1994) and the other toward the Southwest Sub-basin (Zhou et al., 1995). Furthermore, Barckhausen and Roeser (2004) concluded that seafloor spreading at the southwest rift tip ceased at 20.5 Ma (Anomaly 6a1), ~4 m.y. earlier than interpreted in previous studies.

Previous drilling

Five sites were drilled in the peripheral continental slope of the central SCS basin during Ocean Drilling Program (ODP) Leg 184 (Feb–April 1999) (Wang, Prell, Blum, et al., 2000). That expedition cored hemipelagic sediments in the SCS to determine the evolution and variability of the East Asian monsoon during the late Cenozoic. All Leg 184 sites are located on the continental slope, and none penetrated into basement rocks. The deepest hole cored during the leg reached 861 meters below seafloor (mbsf) at Site 1148 in 3294 m of water (Figs. F2, F4), recovering lower Oligocene sediments. The major objectives of Leg 184 were to improve our knowledge of the variability of monsoonal climates (including millennial- to possibly centennial-scale variability from high-sedimentation rate records), orbital-scale variability from records at all SCS sites, and tectonic-scale variability from late Cenozoic sections. The records from the SCS from both Leg 184 and Expedition 349 will be used to establish links between the East Asian and Indian monsoons and to evaluate mechanisms of internal (climate system feedbacks) and external (orbital and tectonic) climate forcing.

One particularly interesting feature recovered at Site 1148 is a slump zone dated to between 28 and 23 Ma, which spans the Oligocene/Miocene boundary and coincides with a “double seismic reflector” (Wang, Prell, Blum, et al., 2000; Li et al., 2004, 2005; Wang and Li, 2009). The Oligocene/Miocene boundary at Site 1148 also marks sharp uphole decreases in sedimentation rates, organic carbon, and opal contents; changes in Nd; and fluctuations in many logging and elemental records. Subsequent investigations indicated that this slump zone might be related to a major tectonic event synchronizing with a possible ridge jump (Briais et al., 1993; Wang, Prell, Blum, et al., 2000; Li et al., 2005) and/or an episode of regional magmatism associated with the opening of the SCS (Li and Song, 2012). Unfortunately, Leg 184 had very low recovery within this critical zone, and thus the true nature of this major tectonostratigraphic event needs to be further investigated. Expedition 349, targeting the oldest possible oceanic crust and overlying sediments, as well as the oceanic basalts at approximately the Oligocene/Miocene boundary, will help pinpoint the geodynamic cause and paleoceanographic and sedimentary effects of this regional event.

Seismic studies and site survey data

Figure F2 shows the proposed drill sites and available multichannel seismic (MCS) lines used to locate these sites. All proposed primary sites and most alternate sites are located at the intersections of two MCS lines (Table T2). Three of the proposed sites (SCS-6C, SCS-4E, and SCS-4F) are not located on crossing points but require thin sedimentary penetration. All proposed sites were selected from original Society of Exploration Geophysicists (SEGY) data on a SUN workstation. A dense 2-D MCS grid exists in the northern SCS continental margin and the northern part of the central SCS basin. The Chinese National Offshore Oil Corporation (CNOOC) acquired most of these high-quality data recently. The number of MCS lines in the central basin is increasing rapidly, contributed mostly by the South China Sea Institute of Oceanology (SCSIO) of the Chinese Academy of Sciences, Guangzhou Marine Geological Survey (GMGS) of the Land and Resources, the 2nd Institute of Oceanography (SIO) of the State Oceanic Administration, and the Federal Institute for Geosciences and Natural Resources (BGR).

GMGS has undertaken extensive geophysical and geological mapping of a large portion of the central SCS basin in recent years. As a result, multichannel reflection seismic data and shallow sediment cores are regularly added to our existing site survey database. This mapping activity has already started producing 2-D seismic grids around our proposed sites. Although most of these new data are either being processed or in a proprietary state, partial access has been made possible through close collaboration with three co-proponents of Proposal 735-CPP2 from GMGS.

Other MCS and magnetic data were collected near these sites by the R/Vs Vema, Conrad, and Haiyang IV (Taylor and Hayes, 1980, 1983; Yao et al., 1994; Hayes et al., 1995) (Fig. F2). Two stages of Sino-US cooperation in the early 1980s added more dense geophysical data coverage, which includes sonobuoy measurements, two ship expanding spread profiles, and piston cores (Taylor and Hayes, 1983; Yao et al., 1994; Hayes et al., 1995). The German R/V Sonne carried out five cruises in 1987 (SO-49, SO-50B), 1990 (SO-72A), 1994 (SO-95), and 2008 (SO-197) (Franke et al., 2011) and collected >10,000 km of MCS data and high-resolution echograms (Lüdmann and Wong, 1999; Lüdmann et al., 2001).

The area surrounding alternate Site SCS-1C is also well studied and imaged with numerous geophysical surveys during Cruises SCSIO87, 973GMGS, ACT, TAICRUST, ORI645, and ORI689. More recent geophysical studies include the TAiwan Integrated GEodynamics Research (TAIGER) project (McIntosh et al., 2012) and surveys for gas hydrate. There are also various earthquake hypocenter relocations and tomographic studies (Wu et al., 1997; Cheng et al., 1998; Kao et al., 2000).

Swath bathymetry data (Fig. F6) are available for the entire SCS basin (Li et al., 2011), and Generic Mapping Tools (GMT) grids of multibeam bathymetry data around eight of the proposed drill sites have been submitted to the Integrated Ocean Drilling Program Site Survey Data Bank. Digital grid data of swath bathymetry will be available upon request from GMGS and the SIO of the State Oceanic Administration.

Magnetic anomalies covering all proposed drill sites were compiled by the Geological Survey of Japan and Coordinating Committee for Coastal and Offshore Geoscience Programmes in East and Southeast Asia (CCOP) in 1996 (Fig. F4). This compilation offers remarkable coverage and accuracy and yields many new insights into the dynamic opening process of the SCS (Li et al., 2008b, 2010, 2012). 3-D deep crustal and mantle structures in the area have also been imaged with surface wave tomography (Wu et al., 2004).

Previous ocean bottom seismometer (OBS) studies reveal the crustal thickness and velocity structures of the central basin. Yan et al. (2001) conducted an OBS experiment near these sites. In 2006, three OBS profiles were shot by the SIO of the State Oceanic Administration, one on the northern continental margin and the other two crossing the Northwest Sub-basin. In early 2011, the South China Sea Deep (SCSD), a comprehensive 8 y research program, was approved by the National Science Foundation of China (NSFC) (Wang, 2012). With a total budget of ~$23 million US, this program has funded coincident refraction/reflection surveys and deep-tow magnetic surveys near these sites. The two most recent OBS surveys in the SCS in 2012 included an active source 3-D survey around the relict spreading center and a larger scale passive source OBS survey in the central basin. Several of the proposed drill sites (notably Sites SCS-3G and SCS-3H) are located within these survey areas. Preliminary results from these surveys have just been published (Zhang et al., 2013).

Two deep-tow magnetic cruises in the SCS were done in 2012 and 2013. Both deep-towed and conventional surface-towed magnetometers were deployed along four survey lines, which were designed to traverse the four primary sites. High-resolution magnetic anomalies from this survey allow more accurate calibration between magnetic susceptibilities and radiometric ages of core samples and the observed seafloor magnetic anomalies in the vicinity of the drill sites. These site surveys add crucial new data for establishing the best possible model of magnetic anomalies for the whole basin and the age of the ocean crust at the drill sites.

Finally, numerous piston cores were taken in the SCS by the R/V Vema and R/V Conrad (Damuth, 1980). Shallow sediment coring using the French R/V Marion Dufresne near Sites SCS-6A, SCS-6B, SCS-2C, and SCS-2D was also completed in 2012.

The supporting site survey data for Expedition 349 are archived at the Integrated Ocean Drilling Program Site Survey Data Bank.