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 and is 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 Ailao Shan-Red River fault; e.g., Leloup et al., 2001; 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 Ailao Shan-Red River fault (Fig. F3A) (Tapponnier et al., 1982; Lallemand and Jolivet, 1986; Schärer et al., 1990; Briais et al., 1993; Flower et al., 2001; Leloup et al., 2001),
  2. Slab pull and subduction of the proto-SCS under Sabah/Borneo (Fig. F3B) (Taylor and Hayes, 1980, 1983; Holloway, 1982; Hall, 2002),
  3. Extension related to an upwelling mantle plume (Fig. F3C) (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 (Fig. F3D) (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 that it is today (Sibuet et al., 2002), so geodynamic models must be able to explain the formation of this larger ocean basin. The Ailao Shan-Red River fault was active from 35 to 15 Ma, with displacement of as much as several hundred kilometers (e.g., Leloup et al., 2001; Gilley et al., 2003). Ages obtained from sites drilled during Expedition 349 will aid in testing the hypothesis that the motion on the Ailao Shan-Red River fault is coeval to and may have driven part of the extension and spreading in the SCS. Others suggested that only a minor amount of extension associated with the SCS spreading center may have been transferred to the Ailao Shan-Red River fault (Rangin et al., 1995; Morley, 2002; Clift et al., 2008). The initiation of regional rifting in East Asia during the Mesozoic occurred before the India-Eurasia collision (Fig. F3D) and is thought to be associated with subduction of the paleo-Pacific plate (Taylor and Hayes, 1980, 1983; Shi and Li, 2012).

Some hypotheses require 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 around 44 Ma (e.g., Hall, 1996, 2002) in order to drive and accommodate the opening of the SCS (Fig. F3B). A large part of this proto-SCS may have been subducted into or uplifted as 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 and a hypothesized in situ mantle plume may also have triggered or contributed to the opening of the SCS.

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) (Zones A–E in Fig. F4). In particular, magnetic amplitudes and orientations in the Southwest Subbasin (Zone E) differ markedly from those in the East Subbasin (Zone D). These two subbasins are divided by a complex set of transform faults forming the Zhongnan fault zone (Figs. F2, F4) (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 the different tectonic contexts within which the two subbasins evolved. Pautot et al. (1986) suggested that the youngest part of the East Subbasin in Zone D developed within an older, preexisting oceanic crust, whereas the Southwest Subbasin in Zone E resulted from continental rifting. Within the East Subbasin, two distinct conjugate magnetic anomalies (M1 and M2 in Fig. F4) are thought to be the same age (anomaly 8 in Taylor and Hayes [1983] and Briais et al. [1993] models) and further divide the subbasin into a central part with high magnetic amplitudes and two separated parts with slightly weaker magnetization (Zones C1 and C1′) near the two conjugate continental margins. The magnetic pattern of the Northwest Subbasin also differs from its adjacent segment in the East Subbasin.

Additional important contrasts exist between the East and Southwest Subbasins. For example, the greater average water depths of the Southwest Subbasin compared to the East Subbasin have been interpreted to 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 of the Southwest Subbasin (Ru and Pigott, 1986; Li et al., 2010). Recent heating from magmatic activity could have contributed to the high heat flow in the Southwest Subbasin (Ru and Pigott, 1986; Li and Song, 2012), but this hypothesis needs to be tested through drilling.

A number of Cenozoic tectonic models have been proposed, but it remains uncertain as to whether the SCS Basin experienced a single episode or multiple episodes of extension and seafloor spreading and, if multiple episodes, in what sequence the subbasins 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 Subbasins may have predated or been synchronous with that of the Southwest Subbasin (Fig. F5A) (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 an earlier opening of the Southwest Subbasin is preferred (Fig. F5B) (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 Southwest Subbasins and the important role of the Zhongnan fault (Figs. F2, F4). There are also two models of slow propagation of the SCS spreading center, one with opening gradually propagating toward the northeast and the Taiwan Strait (Chung et al., 1994) and the other toward the Southwest Subbasin (Zhou et al., 1995).

Previous drilling

Five sites were drilled in the peripheral continental slope of the SCS during Ocean Drilling Program (ODP) Leg 184 (Feb–April 1999; Wang, Prell, Blum, et al., 2000). The major objectives of Leg 184 were to study the variability of East Asian monsoonal climates (including millennial- to possibly centennial-, orbital-, and tectonic-scale variability) from cored late Cenozoic hemipelagic sediment. All Leg 184 sites are located on the continental slope, and none penetrated into igneous basement rock. The deepest hole cored during the leg reached 861 meters below seafloor (mbsf) at Site 1148 in 3294 m of water (Figs. F2, F4), with the oldest sediment recovered of early Oligocene age. The records 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.

Seismic studies and site survey data

Figure F2 shows the sites drilled during Expedition 349 and available multichannel seismic (MCS) lines crossing those sites. Most drill sites are located at the intersection of two MCS lines; however, Sites U1434 and U1435 are not located on crossing points but were interpreted to have only thin sedimentary cover above the igneous basement.

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) recently acquired most of these high-quality data. The northeastern part of the SCS has also been 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 hydrates.

Guangzhou Marine Geological Survey (GMGS) has undertaken extensive geophysical and geological mapping of a large portion of the central SCS Basin in recent years. As a result, MCS 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 drill sites. Other MCS and magnetic data were collected near the drill 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 and 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).

Swath bathymetry data are available for the entire SCS Basin from GMGS and the 2nd Institute of Oceanography of the State Oceanic Administration of China (Li et al., 2011). Magnetic anomalies covering all proposed drill sites were compiled by the Geological Survey of Japan and Coordinating Committee for Coastal and Offshore Geoscience Programs in East and Southeast Asia (CCOP) in 1996 (Ishihara and Kisimoto, 1996) (Fig. F4). This compilation offers remarkable coverage and accuracy and yields new insights into the dynamic opening process of the SCS (Li et al., 2008b, 2010; Li and Song, 2012).

A number of ocean bottom seismometer (OBS) studies have been carried out since 2000 (e.g., Yan et al., 2001; Zhang et al., 2013). The South China Sea Deep (SCSD) major research program of the National Science Foundation of China has funded coincident seismic refraction/reflection surveys, local active source 3-D OBS surveys, the first regional passive source OBS survey, and the first deep-tow magnetic survey (Wang, 2012). Both deep-towed and surface-towed magnetic survey lines were designed to traverse the primary sites, allowing the establishment of the best possible magnetic anomaly model and calibrated age model of the ocean crust of the entire basin. The supporting site survey data for Expedition 349 are archived at the Integrated Ocean Drilling Program Site Survey Data Bank.