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doi:10.14379/iodp.pr.349.2014

Scientific objectives

Expedition 349 focuses on coring into igneous basement at multiple sites in 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 along the Manila Trench.

1. Date the timing of the opening of different subbasins of the SCS and correlate the ages from magnetic anomalies to biostratigraphic, magnetostratigraphic, and radiometric ages.

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 expedition sites intends to recover the oldest sediment deposited directly on the top of oceanic basement, paleontological analyses will provide a minimum age constraint for the basement. Except for the upper ~900 m at Site U1432 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. 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 rock will be dated with ⁴⁰Ar/³⁹Ar (Koppers et al., 2011) and possibly other high-resolution zircon dating techniques with uranium-series isotopes (Goldstein et al., 1991, 1994; Goldstein, 1995; Schwartz et al., 2005). Ocean crust rock typically is very low in K concentrations and therefore more vulnerable to disturbances by submarine alteration. To ensure high-quality ⁴⁰Ar/³⁹Ar dating on basement samples collected in the SCS, we will (1) carefully select fresh highly crystalline groundmass and plagioclase phenocrysts, which are the most suitable for ⁴⁰Ar/³⁹Ar dating and (2) apply extended acid leaching procedures to remove altered portions of the groundmass or mineral separates (Koppers et al., 2011).

The half-spreading rates of the SCS are slow to intermediate, between 20 and 40 mm/y (Briais et al., 1993; Song and Li, 2012). Near the continent/ocean boundary, where Sites U1432 and U1435 are located, hyperextended crust exhuming possible lower crust and upper mantle could exist (Franke et al., 2011). If so, gabbro 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. A newly developed method that detects tiny amounts of uranium-bearing minerals, such as zircon, in rocks 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 subbasins.

Magnetic susceptibilities of extrusive basalt 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 cause the magnetic contrast between the East and the Southwest Subbasins. Because of the complex 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 subbasins, as well as their crustal affinities. Magnetization measurements from cores are also vital for creation of an initial model for predicting magnetic anomaly strength, in order to better understand the observed magnetic anomalies.

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

Trace element chemical analyses and measurement of Sr, Nd, and Pb isotopic ratios 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 these geochemical tracers in the igneous basement rock. The basaltic rock cored during this expedition could have three potential mantle sources: (1) Indian Ocean/Eurasian lithospheric mantle, (2) Pacific mantle, or (3) a putative mantle plume.

All of these sources have distinctive geochemical characteristics in their incompatible trace element and long-lived radiogenic isotope ratios, which are not affected by variations in degree of partial melting of the mantle and fractional crystallization of the resultant melt. Therefore, based on the Expedition 349 drilling transect, we should be able to test several geochemical evolution scenarios corresponding to the rifting and spreading models of the SCS, including (1) continental rifting leading to seafloor spreading (due to Indochina extrusion tectonism or slab pull with southward subduction under Borneo), (2) subduction-induced back-arc spreading, and (3) plume-initiated rifting.

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

Because our drill sites are located in different parts of 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 and will be correlated to known tectonic and climatic events. By core-log-seismic integration, we can build detailed 3-D sedimentation models. Major unconformities and boundaries found in the different subbasins will be correlated with those in the continental slopes and rifting basins to trace the dynamic transitional process from rifting to spreading, and also constrain critical paleoceanographic and tectonic changes during opening of the SCS.

Mineralogical and geochemical analyses will help identify sediment provenance in the SCS and how it has evolved through time. Analyses of detrital zircon in the sediment, for example, can (1) determine the maximum age of stratigraphic successions and lead to a better understanding of the source-to-sink processes, (2) determine provenance characteristics such as age and composition, (3) test regional paleogeographic models via provenance analysis, and (4) unravel facets of geological history locked in the mineral chemistry of detrital zircon (Fedo et al., 2003).

Structural analyses of core samples will focus on deformation features such as fractures, faults, veins, deformation bands, etc. Postcruise statistical analyses on these structural features will help reveal the regional stress field and its changes through time. Together with geophysical downhole logging and other data, the structural features recorded in cores could reveal regional SCS magmatic and tectonic events, as well as local rock deformation and stress heterogeneities.

5. Obtain downhole geophysical logs to reveal physical properties of the sediments and the top oceanic basement and to provide a record of unrecovered 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 and will provide structural, mineralogical, and geochemical information of the penetrated sequences. These data will be particularly important for unrecovered intervals that typically occur when using the extended core barrel (XCB) and rotary core barrel (RCB).

The triple combo tool string records geophysical signals of the penetrated sediment and basement rock 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 will be critical for decompaction and backstripping analyses and for constraining tectonic subsidence. The subsidence and rifting parameters so obtained can offer new insights on the episodic opening history of the SCS and reveal mantle properties.

Wireline logs will provide a continuous record to aid in the detection of lava flow boundaries, interlayered sediment, and alteration zones in the basement to evaluate the dip of lava flows. 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 obtain high-resolution quasi-2-D images (electrofacies) of the borehole wall to reveal the structure and orientation of the rock. These data will provide constraints on volcanostratigraphy and crustal accretion processes (e.g., 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 Inclinometer Tool, 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.

Drilling and coring strategy

Our proposed operations plan for this expedition consisted of drilling three sites into basement (proposed Sites SCS-3G [U1431], SCS-6A [U1432], and SCS-4B [U1433]). Because of the predicted depth to basement at these sites (865–1830 mbsf) and to maximize our operational time, we requested and received approval from the Environmental Protection and Safety Panel and the Texas A&M Safety Panel to drill down through the uppermost ~900 m at the second site (U1432), provided that we encountered nothing unexpected at the first site (U1431), which was tied seismically to the second one. Even with this drill-down approval, it was unlikely that we would have sufficient time in the schedule to reach basement at all three of our primary sites unless operations proceeded better than predicted in the operations plan. With these issues in mind, we identified 10 alternate sites in the vicinity of our primary sites that required shallower penetration depths to reach basement. Three additional alternate sites in other locations targeted secondary objectives that could be addressed if for any reason we were unable to drill in the vicinity of one of our primary sites (Li et al., 2013).

Alternate sites with shallower penetration depths to basement were also important because coring at deeper depths is challenging. Hole stability is always a risk during coring and logging operations, and the longer the open-hole section, the higher the risk. Hole cleaning is also more difficult in deeper sections, particularly when coring dense basement material. We planned a reentry system to 900 mbsf for the deepest site (U1432; predicted total depth of 1930 mbsf) to help mitigate these issues. Given the relatively shallower total depths planned at the other two primary sites (U1431 and U1433), we felt that we could achieve the objectives without the aid of a reentry system. The reentry system at the deep site would serve two main purposes. First it would stabilize the upper portion of the hole, where unconsolidated sediment is more likely to cause hole stability problems. It also would provide a smaller annulus for hole cleaning, which increases annular velocity without having to significantly increase pump rates to remove dense basement cuttings from the hole. Higher flow rates generally result in washed-out sections in parts of the hole and can lead to stability issues. For the other primary sites, we planned to deploy a free-fall funnel to decrease the amount of time required to reach the basement objective.

This strategy of having multiple alternate sites approved for drilling prior to the start of the expedition proved to be valuable during this expedition. While installing the reentry system in Hole U1432B, we discovered that two of the three fiber optic cables within the subsea camera wireline had failed. During a subsequent deployment, the camera system failed and we initially suspected that the final fiber optic cable had broken. Fortunately, the failure was related to the pan and tilt unit of the camera, so that after the problematic unit was removed the system worked again. This failure prompted discussions of alternate plans to meet the expedition objectives without the ability to perform reentries. During cementing of the final (10¾ inch) casing string in Hole U1432B, the pipe became stuck in the cement and ultimately forced us to abandon the hole. There was not enough time left in the expedition schedule to try another approach to reach the basement objective at that site; however, we were able to use the remaining time to reach basement at our primary site in the Southwest Subbasin (Site U1433). Additionally we were able to core at two alternate sites, proposed Sites SCS-4E (U1434) and SCS-6C (U1435). The latter site was thought to be a basement high very near the continent/ocean boundary (Fig. F6) with very thin (~10 m) sediment cover, whereas Site U1434 formed a short sampling transect with Site U1433, with Site U1434 located closer to the relict spreading center and also adjacent to a large seamount (Fig. F7).

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