Crustal structure of the northwestern sub-basin, South China Sea: Results from a wide-angle seismic experiment

Abstract

The crustal structure of the northwestern sub-basin area of the South China Sea was modeled by inverting a wide-angle seismic survey line across the entire region and on both sides of its bounding continental margins. The survey line extended over 484 km. A total of 14 Ocean Bottom Seismometers (OBS) were deployed at intervals of 30 km to record air-gun array sources with a combined volume of 5160 in 3 . The crustal velocity structure of the northwestern sub-basin area was acquired through the integration of multi-channel seismic data. OBS data were processed and modeled initially using ray tracing inversion techniques. Results indicate that crustal thickness under the continental slope decreases from 21 to 11 km, crustal thickness of the northwestern sub-basin is 7.7 km, and the depth to the Moho ascends from 21 km under the upper continental slope to 11 km under the middle basin. The crust of the northwestern sub-basin is similar to that of the eastern sub-basin in its oceanic crustal structure. This structure has a thicker layer 1 (sedimentary layer) and a thinner layer 2. These characteristics are especially clear in the eastern sub-basin, which differs somewhat from typical oceanic crust. The tectonic geometry and velocity structure of the northwestern sub-basin and its margins comprise a symmetrical conjugate and indicate a pure shear mode with regard to the continental margin rifting mechanism. We did not find clear seismic signals from high velocity layers under the lower crust of the continental margin in the northern part of the northwestern sub-basin, which provides new evidence for the idea that the western part of the northern continental margin of the South China Sea constitutes non-volcanic crust. Because the seafloor spreading period of the northwestern sub-basin was short, layer 2 might have experienced asymmetrical basalt magma flows, which may have blurred the magnetic anomaly lineations of the northwestern sub-basin.wide-angle seismic, crustal structure, South China Sea, tectonic evolution Citation:Wu Z L, Li J B, Ruan A G, et al. Crustal structure of the northwestern sub-basin, South China Sea: Results from a wide-angle seismic experiment.

Crustal structure and extension mode in the northwestern margin of the South China Sea

“…[] and Wu et al . [] also found there is no HVL beneath the lower crust according to the OBS2006‐1 velocity profile across the western portion of the Pearl River Mouth Basin (PRMB), the Northwest Sub‐basin, the Zhongsha Islands (Macclesfield Bank) and extending to the East Sub‐basin (supporting information Figure S1a). Interestingly, recent studies based on the seismic profiles and gravity modeling show that there is about 2–4 km thick HVL beneath the lower crust in the Qiongdongnan Basin (QDNB) [ Qiu et al ., ; Zhao et al ., ].…”

“…However, these seamounts including the fossil ridge are surrounded by the oceanic crust, and so the density at depth would likely be similar to that of oceanic crust (2.90 g/cm 3 ). We note that the OBS2006‐1 refraction profile near MCS profile L1 detected a similar fossil ridge with a P‐wave velocity of 5.8–6.8 km/s in the Northwest Sub‐basin [ Wu et al ., ; Ding et al ., ]. The fossil ridge has two layers including an upper layer with a P‐wave velocity of 5.8–6.6 km/s and a lower layer with a P‐wave velocity of 6.6–6.8 km/s.…”

“…[] and Wu et al . []. PRMB, Pearl River Mouth Basin; QDNB, Qiongdongnan Basin; YGHB, Yinggehai Basin; ZJNB, Zhongjiannan Basin; NFT, Northern Fault Terrace; Zhu II D, Zhu II Depression; Zhu III D, Zhu III Depression; PYS, Panyu Swell; SHASR, Shenhu‐Ansha Rise; SR, Southern Rise; BYS, Baiyun Sag; LWS, Liwan Sag; CCS, Changchang Sag; SNBDS, Songnan‐Baodao Sag; BDH, Baodao High; SDS, Songdong Sag; SXS, Songxi Sag; STH, Songtao High; SNH, Songnan High; BJS, Beijiao Sag; BJH, Beijiao High; YLS, Yongle Sag; LSS, Lingshui Sag; LSH, Lingshui High; YBS, Yabei Sag; YCH, Yacheng High; YNS, Yanan Sag; YNH, Yanan High; LDS, Ledong Sag; LNH, Lingnan High; HGS, Huaguang Sag.…”

Shear wave velocity structure of Reed Bank, southern continental margin of the South China Sea

“…It has shown that the middle and eastern part of the northern margin of SCS is of volcanic type with a high velocity layer in the lower crust, the thick high velocity body beneath Dongsha islands in particular, that was interpreted as magma underplating or intruded mantle Wei et al, 2011aWei et al, , 2011bYan et al, 2001;Zhao et al, 2010). While no high velocity layer in the lower crust has been found in the OBS seismic models in the western part of the northern margin of SCS (Qiu et al, 2001;Wu et al, 2011). On the southern margin of SCS, it has shown that the middle and eastern parts of Nansha Block have similar tectonic characters with crust thickness of 20 km in continent and 5-6 km in basin without high velocity layer in the lower crust (Pichot et al, 2014;Qiu et al, 2011).…”

Passive seismic experiment and ScS wave splitting in the southwestern subbasin of South China Sea

“…More accurate and reliable results of crustal structure were from active wide-angle seismic experiments but only in the northern margin of SCS [10][11][12][13][14][15][16][17]. In recent years several ocean bottom seismometer (OBS) experiments in the southern margin of SCS have been carried out that greatly improved the active seismic data distribution in SCS (Figure 1).…”