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doi:10.2204/iodp.sp.349.2013

Introduction

Since the late Mesozoic, the South China Sea (SCS) area (Figs. F1, F2) has been at the center stage of many first-order tectonic and paleoclimatic events. Mesozoic subduction of the paleo-Pacific plate, a fragment of which developed roughly along the present-day northern 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 et al., 2003; Xiao and Zheng, 2004; Zhou et al., 2008; Li et al., 2008a), gradually displaced the paleo-Tethys and built a massive orogen 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 via continental breakup and subsequent seafloor spreading. 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.

Currently, ages of the oceanic crust in the SCS basin are only loosely constrained from magnetic anomaly correlations and empirical relationships between ages and bathymetry and/or heat flow. The many uncertainties in the timing and episodes of the Cenozoic opening of the SCS hampers understanding of other key geological processes in Southeast Asia, including the geodynamic transition from Mesozoic subduction to Cenozoic rifting, the Cenozoic opening mechanism, oceanic crustal accretion and mantle evolution, and paleoceanographic and sedimentary responses. In order to address both regional questions related to East Asian geology and fundamental issues regarding continental breakup and basin formation, it is essential to determine when seafloor spreading initiated, the mechanism through which the SCS basin opened, and when spreading ceased. To do this, we plan to drill into the oceanic basement and retrieve both sedimentary and basaltic rocks from various sub-basins of the SCS basin. Timing of the onset and termination of seafloor spreading of the SCS will be critical to correlating regional tectonic events to the tectonic episodes within the SCS that are inferred almost exclusively from geophysical measurements and inversions. Drilling and coring into basement is the only means of validating various opening mechanisms.

Expedition 349 is 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 (available at www.iodp.org/science-plan-for-2013-2023). These primary objectives are

1. To examine mechanisms, timing, and sequences of Cenozoic seafloor spreading and establish the complex opening history of different sub-basins and styles of oceanic crustal accretion in the SCS and constrain the tectonic controls (such as spreading rate) on distinct magnetic contrasts among the three sub-basins;

2. To examine oceanic crustal accretion and mantle evolution and reveal the crustal nature and affinities of different sub-basins and 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 and develop a more complete 3-D sedimentation and subsidence model and link it to regional climatic processes in response to various tectonic events; and

4. To examine late Mesozoic and early Cenozoic prerifting tectonic transitions and driving forces leading to continental margin breakup and seafloor spreading; to test various hypotheses of dynamic processes controlling the transition from a Mesozoic active continental margin to a Cenozoic passive one and from continental rifting and breakup to seafloor spreading; and to constrain whether the forces driving the opening of the SCS were far-field (triggered by the tectonic extrusion of the Indochina block), near-field (due to backarc spreading or slab pull), or in situ (mantle plume and magmatism driven). This will 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.

During Expedition 349, we aim to drill and core into basement at three sites in two sub-basins within the SCS (Figs. F1, F2) to collect the geological evidence necessary to address these themes.

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