A Wide Angle Ocean Bottom Seismometer Experiment Across Liyue Bank, the Southern Margin of the South China Sea

Abstract

The P wave velocity model of wide angle seismic profile‐OBS973‐2 that extends in NW‐SE direction for 369 km long from the northeastern Liyue Bank to the central sub‐basin is established by the trial‐and‐error 2D ray‐tracing method. This profile model is compared with that in the northern continental margin for the discussion of the conjugate relationship between the northern margin and the southern margin of the South China Sea (SCS). The velocity model consists of six layers, i.e. three sediments layers, upper crust, lower crust and upper mantle. The velocities of the three sediments layers are 1.8~2.0 km/s, 2.0~2.7 km/s and 3.5~4.0 km/s, respectively. The sediment layers are much thinner in general or even absent in some parts. There are some small volcanoes on the top of crust in Liyue Bank and P wave velocities increase downward from 5.5~6.4 km/s in the upper crust (9~10 km thick) to 6.6~7.1 km/s in the lower crust (11 km thick). In the transition zone and sea basin P wave velocities increase downward from 5.9~6.1 km/s in the upper curst (4~5 km thick) to 6.6~6.9 km/s in the lower crust (2~4 km thick). The crust in the transition zone and sea basin shows some tectonic effects of stretching and thinning. The velocity structure of the model indicates that its continental crust is a standard one but its oceanic upper crust is of relatively higher velocity than standard one. The seaward rising of the buried depth of Moho is 23 km in Liyue Bank and 8~12 km in the sea basin. The velocity beneath Moho is 8.0 km/s in the sea basin obviously smaller than 8.2 km/s in Liyue Bank. The comparison of profile OBS973‐2 with a previous profile OBS2006‐1 in the northern margin shows the much more similarity between them. This result and detailed discussion suggest a possibility of conjugate relationship of Liyue Bank with Zhongsha Massif, separated by the central seamount chain of the SCS. Furthermore, the full rates of seafloor spreading of the northwestern sub‐basin and the southwestern sub‐basin are calculated.

Oligocene–Miocene carbonates in the Reed Bank area, South China Sea, and their tectono-sedimentary evolution

“…The crustal structure has been referred to a velocity model indicating a 21 km thick crust of the Reed Bank and a 12 km thick continental crust of the Reed Bank Basin and north Palawan Basin. The Moho depth gap is about 8 km (Ruan et al 2012). Such a gap in the lithospheric thickness was likely to cause steep vertical temperature contrasts, and then induced a mantle convection under the rifted Reed Bank Basin, involving also the North Palawan Basin.…”

“…This detachment system may have developed in the final stage of rifting as indicated by a decollement system close to the continental-oceanic transition area ). Wide-angle refraction/reflection results proved the Dangerous Grounds reflect a magma-poor rift system at the initial stage of mantle unroofing (Ruan et al 2011;Qiu et al 2011).…”

The role of rifting in the development of the continental margins of the southwest subbasin, South China Sea: Insights from an OBS experiment

“…Additionally, as there is no evidence of salt or a Seaward Dipping Reflector, the margins of the SCS have been categorized as magma-poor rifted margins (Nissen et al 1995;Qiu et al 2001;Yan et al 2001;Clift et al 2002;McIntosh et al 2005;Wang et al 2006a;Qiu et al 2011;Lü et al 2011;Ruan et al 2011;Pichot et al 2014). Unlike the classic magma-poor margins of the Iberia-Newfoundland margins (Ranero and Pérez-Gussinyé 2010;Reston and McDermott 2011), the rifting of the SCS was inherently influenced by the composition and the stress field from different continents nearby, such as the Indian Peninsula, the Pacific, and the Indo-Australian Plate.…”

An integrated geophysical study on the Mesozoic strata distribution and hydrocarbon potential in the South China Sea

“…Density structure from gravity fitting of the Liyue Bank profile (CC 0 ) and its crustal structure interpretation. a: gravity anomaly, the mean square error between calculated and observed free-air gravities is 1.05 mGal, b: density structure, c: crustal structure interpretation, P-wave velocities are from Ruan et al (2011) margins. In the Pearl River Mouth Basin and SW Taiwan Basin of the north margin they are relatively thick and continuously distributed, while in the Nansha area of the south continental margin Mesozoic strata are discontinuous and vary considerably in thickness.…”

“…Therefore, we collected available geological and geophysical data in the SCS and its adjacent areas, especially the recent multi-channel seismic and ocean bottom seismometer (OBS) data (blue solid lines in Fig. 1) Ding et al, 2013;Li et al, 2011;McIntosh et al, 2005;Qiu et al, 2001Qiu et al, , 2011Qiu et al, , 2012Ruan et al, 2011;Wang et al, 2006;Wei et al, 2011;Yan et al, 2001;Yao and Wang, 2006), and carried out an integrated geophysical study based mainly on gravity data to obtain the distribution of Mesozoic residual thickness. It helps to acquire the regional knowledge of Mesozoic strata and their hydrocarbon potential in the SCS, which may also provide basis for studying the above mentioned questions.…”