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doi:10.14379/iodp.pr.349.2014

Abstract

The South China Sea (SCS) provides an outstanding opportunity to better understand complex patterns of continental margin breakup and basin formation. The sea is situated at the junction of the Eurasian, Pacific, and Indo-Australian plates and is a critical site linking some of the major western Pacific tectonic units. Despite extensive studies, sampling of basement rock and directly overlying basal sediment in the deep basin is lacking. This leaves a large margin of error in estimated ages of the SCS opening and closing, rendering various hypotheses regarding its opening mechanism and history untested. This also hampers understanding of East Asian tectonic and paleoenvironmental evolution.

We drilled five sites in the deep basin of the SCS. Three of these sites (U1431, U1433, and U1434) cored into oceanic basement near the fossil spreading center. The two remaining sites (U1432 and U1435) are located proximal to the northern continent/ocean boundary. We recovered a total of 1524 m of sediment/sedimentary rock and 78 m of oceanic basalt and also carried out downhole geophysical logging (triple combination and Formation MicroScanner-sonic tool strings) at the two deepest sites (U1431 and U1433). These materials and data were extensively examined and discussed during the expedition and allowed us to draw the following principal conclusions on the opening of the SCS:

1. Based on shipboard dating of microfossils in the sediment immediately above the basaltic basement and between the lava flow units, the preliminary cessation age of spreading in both the East and Southwest Subbasins is around early Miocene (16–20 Ma). Further postcruise radiometric dating of basement basalt from these sites plus additional calibration of magnetic anomaly models and paleomagnetic measurements will further refine the age range. Overall, a large difference is not apparent in the terminal ages of seafloor spreading between the two subbasins.
2. At Site U1435, we were able to drill into a structural high standing along the continent/ocean boundary. Coring at this site recovered a sharp unconformity at ~33 Ma, above which is marine sediment and below which are poorly sorted sandstone and black mudstone, interpreted as littoral deposits. Environmental interpretation will require further shore-based studies because the sequence is almost entirely barren of marine microfossils. Nevertheless, we interpret this unconformity to be likely directly related to the continental break-up during the initial opening of the SCS. The onset of seafloor spreading is therefore estimated to be at ~33 Ma.
3. All sites contain deep marine deposits but show significant areal variations in postspreading sedimentary environment and provenance. Site U1431 records evidence for deep-marine turbidite deposition from terrestrial sources. The observed coarser silt turbidites may have a source in Taiwan or other surrounding blocks, whereas interbedded calcareous turbidites at this site could be transported from local sources, such as nearby seamounts topped by carbonate platforms. In contrast, the source for upper Miocene clay and silt turbidites at Site U1433 could be from Borneo or mainland Southeast Asia, with the source of the interbedded carbonate turbidites likely from the Dangerous Grounds or Reed Bank area located south of the site.
4. Sites U1431 and U1434 are close to seamounts developed along the relict spreading center. Occurrences of basaltic clasts and mineral fragments in the volcaniclastic sandstone and breccia may reveal the magmatic history and mantle source of the seamounts and potentially their relationship with the terminal processes of spreading. The volcaniclastic breccia and sandstone at Site U1431 are dated as late middle Miocene to early late Miocene (~8–13 Ma), suggesting a 5 m.y. duration of seamount volcanism starting a few million years after the cessation of seafloor spreading. At Site U1434, volcaniclastic breccia and sandstone are most likely sourced from the adjacent seamount ~15 km to the north. The age of this recovered unit is late Miocene (younger than 9 Ma). Further postcruise sedimentological and geochemical studies, as well as radiometric dating of potassium-bearing mineral fragments, will refine the ages and timing of these seamount activities and reveal how magma sources at the dying spreading center evolved through time.
5. We successfully cored into ocean basement in the SCS for the first time and recovered basalt at three sites (U1431, U1433, and U1434). The cored basalt has variable phase assemblages of plagioclase, olivine, and clinopyroxene and is concluded to be typical mid-ocean-ridge basalt based on petrological and geochemical evidence. Postcruise radiometric dating will determine the absolute ages of the basaltic basement units. Postcruise petrological and geochemical analyses on the basalts will provide information on the mantle sources, melting, and crystallization history of the youngest ocean crust.

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