Expedition 367 Preliminary Report: South China Sea Rifted Margin

Abstract

International Ocean Discovery Program Expedition 367 is the first of two consecutive cruises that form the South China Sea Rifted Margin program. Expeditions 367 and 368 share the common key objectives of testing scientific hypotheses of breakup of the northern South China Sea (SCS) margin and comparing its rifting style and history to other nonvolcanic or magma-poor rifted margins. Four primary sites were selected for the overall program: one in the outer margin high (OMH) and three seaward of the OMH on distinct, margin-parallel basement ridges. These ridges are informally labeled A, B, and C within the continent-ocean transition (COT) zone going from the OMH to the steady-state oceanic crust of the SCS. The main scientific objectives include 1. Determining the nature of the basement within critical crustal units across the COT of the SCS that are critical to constrain style of rifting, 2. Constraining the time interval from initial crustal extension and plate rupture to the initial generation of igneous ocean crust, 3. Constraining vertical crustal movements during breakup, and 4. Examining the nature of igneous activity from rifting to seafloor spreading.In addition, sediment cores from the drill sites will provide information on the Cenozoic regional tectonic and environmental development of the Southeast Asia margin.Expedition 367 successfully completed operations at two of the four primary sites (Site U1499 on Ridge A and Site U1500 on Ridge B). At Site U1499, we cored to 1081.8 m in 22.1 days, with 52% recovery, and then logged downhole data from 655 to 1020 m. In 31 days at Site U1500, we penetrated to 1529 m, cored a total of 1012.8 m with 37% recovery, and collected log data from 842 to 1133 m. At each site we drilled to reach the depth of the main seismic reflector (acoustic basement), which prior to the expedition had been interpreted to be crystalline basement. Our objective was to determine which lithospheric layer constitutes the basement of the COT and whether there was middle or lower continental crust or subcontinental lithospheric mantle exhumed in the COT before the final lithospheric breakup. At Site U1499, coring ~200 m into the acoustic basement sampled sedimentary rocks, including early Miocene chalks underlain by pre-Miocene polymict breccias and poorly cemented gravels composed of sandstone pebbles and cobbles. Preliminary structural and lithologic analysis suggested that the gravels might be early synrift to prerift sediment. At Site U1500, the main seismic reflector corresponds to the top of a basalt sequence at ~1379.1 m. We cored 149.90 m into this volcanic package, recovering 114.92 m (77%) of sparsely to moderately plagioclase-phyric basalt comprising numerous lava flows including pillow lavas with glass, chilled margins, altered veins, hyaloclastites, and minor sediment. Preliminary geochemical analyses show that the basalt is tholeiitic. We speculate that the basalt might belong to the very early stage of magmatism prior to steady-state seafloor spreading (known as an "...

Seismic Imaging of an Intracrustal Deformation in the Northwestern Margin of the South China Sea: The Role of a Ductile Layer in the Crust

“…Seismic travel times were modeled to image the rift‐related structures and to provide hints on the brittle/ductile behavior of the continental crust, and thus further insights into crustal deformation during rifting. The crusts show a regional deformation pattern in the failed rifts, and hyperthinned continental crust in the ocean‐continent transition zone (OCT) is overprinted by early stage magmatism, which is in good agreement with the shallow lithology measurements from IODP drillings (Jian et al., 2018; Larsen et al., 2018; Sun et al., 2018).…”

“…However, the rupture mode of the continental crust and the formation mechanism of the oceanic crust are still highly controversial. In 2017, the 367/368 International Ocean Discovery Program (IODP) drill legs were conducted in deep water (>3,000 m; Figure 1) at the edge of the northern margin of the SCS to reveal the nature of the structure from continent to ocean (Jian et al., 2018; Sun et al., 2018). The coring data suggest that the SCS has experienced fast lithospheric extension without mantle exhumation (Larsen et al., 2018), different from the well‐established margins along the Atlantic Ocean (Dean et al., 2000; Whitmarsh et al., 1993).…”

Deep seismic structure of the northeastern South China Sea: Origin of a high‐velocity layer in the lower crust

“…As described above, we consider that the HVL within the COT in the northeastern SCS is most likely related to underplating or magma emplacement rather than the serpentinized upper mantle in the lower crust. In future work, this inference could be confirmed or invalidated by serpentinization samples retrieved by deep‐sea drilling [ Sun et al ., ]. However, based on the activity young volcanisms over the COT, we boldly consider that even though serpentinized mantle emplaced initially, it has been replaced by the subsequent magma underplating to a large extent.…”

Distribution, Seismic Characteristics and Controlling Factors of Carbonate Platforms on the Northwestern Margin of the South China Sea

“…1). The South China Sea has undergone multi-phased continental rifting since the Paleogene, most likely from early Eocene to early Oligocene, ultimately facilitating the opening of the South China Sea during the early Oligocene to middle Miocene (32-15 Ma) (Taylor and Hayes, 1983;Li et al, 2014;Sun et al, 2016). Underlying the northwestern South China Sea continental margin are a number of grabens and half-grabens separated by structural highs (Fyhn et al, 2009a;Franke, 2013;Savva et al, 2014;Cameselle et al, 2017).…”