OBS Survey and Crustal Structure of the SW Sub‐Basin and Nansha Block, South China Sea

Abstract

OBS973-1 is a wide angle reflection and refraction deep seismic line using Ocean Bottom Seismometers (OBS), collected for the first time in the southern part of South China Sea. The line crosses the southern margin of the Southwest Sub-basin and the middle part of Nansha Block. By seismic phase analysing and travel time fitting, a 2D P-wave velocity model along the line is constructed. The modeling results show that the surface sedimentary layer has a velocity of 2.5∼4.5 km/s and a thickness of 1000∼3000 m, with locally rough basement interface. The crystalline basement has a velocity of 4.5∼5.5 km/s on the top, increasing to 6.8∼6.9 km/s near the bottom of the crust. In the middle crust there is a small velocity discontinuity (0.1∼0.2 km/s), while the Moho on the bottom crust has a large velocity contrast (1.2 km/s). In the top mantle the velocity is 8.0∼8.1 km/s. The Moho depth and the crustal thickness have much difference between the northern and southern segments along the line. The northern segment is in the deep sea basin, where the Moho is 11 km deep and the crystalline crust is only 5∼6 km thick, which are typical characteristics of oceanic crust. In contrast, the southern segment is on the continental block, where the Moho has a maximum depth of 24 km and the crust has a corresponding thickness of 20 km, which are representative characteristics of thinned continental crust. From the deep sea basin to the continental block, the Moho depth and the crustal thickness increase rapidly. In the continental area, the upper crust and lower crust have similar thickness and variation. No High Velocity Layer (HVL) is seen in the lower crust. These may imply that pure-shear extension and uniform thinning are dominating inside the crust, and magma underplating is not developed in the lower crust. Comparing with the structure models of OBS973-2 and OBS973-3, it is inferred that the middle and eastern parts of Nansha Block have similar tectonic characters, and the two sides of Southwest Sub-basin are a pair of non-volcanic asymmetric conjugate continental margins.

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

“…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 inversion of the Nansha profile (BB 0 ) and its crustal structure interpretation. (a) gravity anomaly, the mean square error between calculated and observed free-air gravities is 0.48 mGal, (b) density structure, (c) crustal structure interpretation, P-wave velocities are from Qiu et al (2011). developed rather early, started in Indosinian period, formed in Yanshanian period, and were most active in Late Jurassic. Under control of NE faults group, Mesozoic strata in the north and south continental shelf and slope were mostly distributed in NE direction.…”

“…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.…”