Since the late Mesozoic, the South China Sea (SCS) area (Figs. F1, F2) has been at the center of many first-order tectonic and paleoclimatic events. Mesozoic subduction of the paleo-Pacific plate under the Eurasian plate partially occurred along the present-day northeastern SCS continental margin (Jahn et al., 1976; Hilde et al., 1977; Hamilton, 1979; Holloway, 1982; Taylor and Hayes, 1983; Hayes et al., 1995; Zhou and Li, 2000; Yang and Feng, 2003; Xiao and Zheng, 2004; Zhou et al., 2008; Li et al., 2008a). This subduction resulted in the emplacement of igneous rocks and the formation of a wide orogenic belt in Southeast Asia (Zhou and Li, 2000; Shi and Li, 2012). Subduction is thought to have ceased in the mid-Cretaceous, with a transition to regional extension during the Late Cretaceous. Opening of the SCS began in the Cenozoic through continental breakup and subsequent seafloor spreading. The Ailao Shan-Red River strike-slip fault also displaced the Indochina block by hundreds of kilometers during the Oligocene and Miocene (Tapponnier et al., 1986, 1990; Lacassin et al., 1997; Leloup et al., 2001; Gilley et al., 2003). The early work of Taylor and Hayes (1980, 1983) and Briais et al. (1993) suggested that the SCS opened from ~32 to ~16 Ma during the Oligocene and early Miocene. Recently, Barckhausen and Roeser (2004) and Barckhausen et al. (2014) argued that seafloor spreading was faster in the later stages than at the beginning of the opening and ceased at 20.5 Ma (Anomaly 6a1) over the entire SCS, ~4 m.y. earlier than interpreted in previous studies.

Ages of the oceanic crust in the SCS Basin are only constrained from magnetic anomaly correlations and empirical relationships between ages and bathymetry and/or heat flow. The uncertainties in the timing and episodes of the Cenozoic opening of the SCS hamper understanding of other key geological processes in Southeast Asia, including the geodynamic transition from Mesozoic subduction to Cenozoic rifting, the relationship between the motion on the large strike-slip faults and the extension (extrusion), the Cenozoic opening mechanism, oceanic crustal accretion and mantle evolution, and paleoceanographic and sedimentary responses. In order to address regional questions related to East Asian geology and fundamental issues regarding continental breakup and the mechanism through which the SCS Basin opened, it is essential to determine when seafloor spreading initiated and when it ceased. To do this, during Expedition 349 we drilled into the oceanic basement and retrieved both sedimentary and basaltic rocks from two subbasins of the SCS. Drilling and coring into igneous basement is the only means of validating various opening mechanisms.

Expedition 349 was based on Integrated Ocean Drilling Program Proposal 735-CPP2, developed in part from results of an international workshop held at Tongji University in Shanghai, China, in early 2012 (Li et al., 2012). The primary objectives of the expedition fall under four major categories and address the Earth Connections and Climate and Ocean Change themes in the science plan for the International Ocean Discovery Program (IODP) (available at

  1. To examine the mechanisms, timing, and sequences of Cenozoic seafloor spreading; to establish the complex opening history of different subbasins and styles of oceanic crustal accretion in the SCS; and to constrain the tectonic controls (such as spreading rate) on distinct magnetic contrasts among the subbasins;
  2. To examine oceanic crustal accretion and mantle evolution and reveal the crustal nature and affinities of different subbasins in order to understand oceanic crustal and deep mantle processes associated with tectonic extrusion, magmatism, and magnetization;
  3. To examine paleoceanographic and sedimentary responses to tectonic evolution of the SCS through development of a more complete 3-D sedimentation and subsidence model and linking it to regional climatic processes in response to various tectonic events; and
  4. To examine driving forces leading to continental breakup and seafloor spreading and constrain whether the forces were far-field (triggered by the tectonic extrusion of the Indochina block), near-field (due to back-arc spreading or slab pull), or in situ (mantle plume and magmatism driven) in order to deepen our general understanding of the geodynamic interplay of mantle and lithosphere processes that led to the development of continental margin basins in the geological past and today.