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doi:10.2204/iodp.sp.349.2013

Scientific objectives

Expedition 349 focuses on coring basement at multiple sites around the SCS basin to better understand seafloor spreading, ocean crust accretion, and mantle evolution. In addition, coring the sedimentary sections above basement will allow examination of the sedimentary and paleoceanographic responses to basin opening and eventual subduction beneath the Manila Trench. Drilling in the SCS will allow us to address key problems in tectonics, mantle evolution, and paleoceanography.

1. Date the timing of the opening of different sub-basins of the SCS and correlate the ages from magnetic anomalies to fossil, magnetostratigraphic, and radiometric ages.

Accurate age estimates for the opening history of the sub-basins of the SCS can be correlated with the uplift history of the Tibetan Plateau to further advance our understanding of the possible links between extrusion tectonics (Tapponnier et al., 1982; Briais et al., 1993; Clift et al., 2008) and the proposed continental breakup leading to the formation of the SCS.

Magnetostratigraphy, biostratigraphy, and radiometric dating are the three principal techniques that will be used for chronostratigraphic analysis of the recovered sequences. Age control in the sedimentary section will be made from routine microfossil analyses, paleomagnetism, and isotope analysis. The age sequences can also be constrained by correlating seismic reflections to different drill sites. Because drilling at all proposed sites intends to recover the oldest sediments deposited directly on the top of oceanic basement, paleontological analyses provide a minimum age constraint for the basement. Except for the upper ~900 m at proposed Site SCS-6A in the primary operations plan, we plan to core all intervals within the three primary sites, with micropaleontological analyses conducted on all core catcher material and additional samples from split-core sections to refine the biostratigraphy as time permits. At Site SCS-6A, we expect to recover a shallow to deep marine carbonate succession of Oligocene to middle Miocene age, with a shallow-marine depositional environment expected to have been deposited soon after the rifting–drifting transition. Calcareous microfossils, including nannofossils and foraminifers, should be abundant in the carbonate successions at all sites. Within some intervals, particularly in the Neogene, we expect to find well-preserved and abundant siliceous microfossils (biogenic silica dominated by diatoms and radiolarians) that can provide additional biostratigraphic control.

Basement volcanic rocks will be dated with ⁴⁰Ar/³⁹Ar and other high-resolution zircon dating techniques with uranium-series isotopes (Goldstein et al., 1991, 1994; Goldstein, 1995; Schwartz et al., 2005). As we plan to drill ~100 m into basaltic basement at all three of our primary sites, we hope to retrieve sufficiently unaltered basaltic rocks for these analyses. Oceanic crust rocks are typically very low in K concentrations and therefore more vulnerable to disturbances by submarine alteration. To ensure high-quality ⁴⁰Ar/³⁹Ar dating on submarine samples collected in the SCS, we plan to (1) carefully select and prepare fresh highly crystalline groundmass and plagioclase phenocrysts, which are the most suitable for ⁴⁰Ar/³⁹Ar dating, and (2) apply acid leaching to remove altered portions of the groundmass or mineral separates (Koppers et al., 2011).

The half-spreading rates of the SCS were between 20 and 40 mm/y, as part of a slow-spreading basin (Briais et al., 1993; Song and Li, 2012). Some new geophysical evidence suggests that near the continent/ocean boundary (COB), where proposed Sites SCS-6A and SCS-6B are located, hyperextended crust exhuming possible lower crust and upper mantle could exist (Franke et al., 2011). If so, gabbros with late-stage minerals or felsic lithologies could be available for uranium-lead zircon dating using sensitive high-resolution ion microprobe reverse geometry (SHRIMP-RG), as well as for ⁴⁰Ar/³⁹Ar dating of plagioclase, biotite, and/or hornblende mineral separates. Uranium-bearing minerals, such as zircon, are much more common in oceanic crust than conventionally thought, and a newly developed method that detects tiny amounts of zircon in rock could reliably date the age of ocean crust (Schwartz et al., 2005; Grimes et al., 2007).

2. Measure the magnetization, mineralization, and geochemical compositions of basement rocks to understand the causes of the sharp magnetic contrast between different sub-basins.

Magnetic susceptibilities of extrusive basalts normally decrease with increasing degree of alteration, which reduces their titanomagnetite content (e.g., Bleil and Petersen, 1983). Serpentinization of peridotite at deeper depths is also known to smear surface magnetic anomalies (e.g., Dyment et al., 1997). Detailed mineralogical studies are essential to understand these processes that may be causing the magnetic contrast between the East and the Southwest Sub-basins.

Because of the intriguing patterns of magnetic anomalies in the SCS, careful measurements of magnetic susceptibility are needed to constrain models of, for example, tectonic settings or spreading rates (Dyment and Arkani-Hamed, 1995) that can explain the distinct differences in magnetic patterns between different sub-basins, as well as their crustal affinities. Magnetization measurements from cores are also vital for creation of an initial model for magnetic modeling and inversion in order to better understand the observed magnetic anomalies.

3. Evaluate the origin and source evolution of SCS basement rocks to better understand the formation of SCS oceanic crust and the deep mantle processes driving this formation.

Chemical and isotope compositions of Sr, Pb, and Nd isotopes and other isotopic systems will provide insights into the material influx and deep crustal and mantle processes (Castillo et al., 1991; Tejada et al., 2004). The opening mechanism of the SCS can be constrained by investigating the variation in the Pb-Nd-Sr isotopic composition of cored basement rocks. Variations in εNd values and Pb and Sr isotopic compositions measured at different sites will help us to understand how the SCS mantle evolved through time.

The basement rocks to be cored during this expedition have three potential mantle sources: (1) Indian Ocean/Eurasian lithospheric mantle, (2) Pacific mantle, or (3) a putative mantle plume. Each of these sources has a distinct composition and therefore distinctive geochemical characteristics. For this purpose, we plan to use incompatible trace element and long-lived radiogenic isotope ratios, as these geochemical tracers are not generally affected by variations in degree of partial melting of the mantle and fractional crystallization of the resultant melt. Using our analytical strategy and proposed drilling transect, we should be able to test several geochemical evolution scenarios corresponding to the rifting and spreading models of the SCS, including (1) normal continental rifting leading to seafloor (backarc) spreading, (2) magma-poor rifting due to Indochina extrusion tectonism, and (3) plume-initiated rifting.

These models, as well as others that may be proposed after initial observations are made during the expedition, can be successfully tested with our planned analytical approach. Through this and geochronologic analyses, we seek to understand the relationship between compositional evolution of the lavas and tectonic changes, as such a combination will provide the best tectono-magmatic model for the formation of the SCS.

4. Evaluate the paleoceanographic and climatic responses to the opening of the SCS and develop a 3-D sedimentation and subsidence model.

With three drilling sites within the SCS, we can build a detailed 3-D postspreading model of seismic stratigraphy that will offer invaluable insights into deepwater sedimentary processes and how they evolved through time. This sedimentary model will be coupled with paleoenvironmental and paleoceanographic data from analyses of sediment cores to detect major geological events.

Information on sedimentation rates, provenance, water depths, tectonic subsidence, and facies changes will be determined from coring and will be correlated to known tectonic and climatic events onshore. By correlating drilling and coring information with regional seismic sections, we can build detailed 3-D sedimentation models. Major unconformities revealed in the central basin will be correlated with those in the continental slopes and rifting basins to trace the dynamic transitional process from rifting to drifting. Oligocene shallow-marine sediments deposited at the very early opening stages will also constrain critical paleoceanographic and tectonic changes at the onset of opening of the SCS.

Structural analyses of core samples will focus on brittle deformation features such as fractures, faults, veins, deformation bands, and so on. Statistical analyses on these structures will help reveal the regional stress field and its changes through time. These structures could record major events in the drifting stage and later events, such as magmatism, initiation of subduction along the Manila Trench, and tectonic events in the surrounding blocks.

Mineralogical and geochemical analyses will help identify sediment provenance in the SCS and how it has evolved through time. This is critical for studying sediments deposited before 25 Ma in the SCS, which are potentially shallow-marine sediments that contain information on the rifting–drifting transition. Therefore, mineralogical and geochemical analyses coupled with paleontological and paleoenvironmental studies can reveal early processes associated with continental breakup of the SCS. Analyses of detrital zircon in the sediments, for example, can (1) determine the maximum age of stratigraphic successions and help understand source-to-sink processes, (2) determine provenance characteristics such as age and composition, (3) test regional paleogeographic reconstruction models via provenance analysis, and (4) unravel facets of geological history locked in the mineral chemistry of detrital zircon (Fedo et al., 2003).

5. Obtain downhole geophysical logs to reveal physical properties of the sediments and the top oceanic basement and to provide a record of nonrecovered intervals.

Our proposed deployment of a minimum of two wireline logging tool strings (the standard triple combination [triple combo] and the Formation MicroScanner [FMS]-sonic) will measure a wide spectrum of geophysical properties to provide structural, mineralogical, and geochemical information of the penetrated sequences. These data will be particularly important for nonrecovered intervals that occur when using the extended core barrel (XCB) and rotary core barrel (RCB).

The triple combo tool string records geophysical signals of the penetrated sediments and basement rocks by measuring the total and spectral natural gamma radiation (NGR), density, porosity, and resistivity of the formation. Gamma ray data will be used to infer lithology and provenance. Porosity, sonic, and density logs together will be critical for decompaction and backstripping analyses and for constraining the tectonic subsidence and opening history of the SCS sub-basins. The subsidence and rifting parameters so obtained can offer new insights on the proposed episodic opening history and reveal mantle properties.

Wireline logs will provide a continuous record to aid in the detection of lava flow boundaries, interlayered sediments, and alteration zones in the basement and will enable the dip of lava flows to be evaluated. The number of lava flow units penetrated has implications for how well geomagnetic secular variation has been sampled and hence the extent to which paleolatitudes can be most precisely constrained.

With FMS-sonic logging, we will gain high-resolution quasi-2-D images (electrofacies) of the borehole wall and the structure and orientation of the rocks. These data together will put much needed constraints on volcanostratigraphy and crustal accretion processes (Tominaga et al., 2009). The high-resolution FMS images will help to detect small-scale fractures and lithologic variations, evaluate the dips of lava flows, and reorient core pieces. The General Purpose Inclinometry Tool (GPIT), which includes both a three-axis inclinometer and a three-axis magnetometer, will be used to measure changes in magnetic properties of lithologies and in paleomagnetic direction.

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