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Due to the particular seafloor environment and the special outer mechanical structure of the ocean bottom seismometer (OBS), there are some problems that have not been solved in the lithospheric structure inversion using the OBS teleseismic receiver functions. Based on the analysis of these problems, we combined the Fourier transform and wavelet analysis to suppress the non-stationary noise to obtain a better signal-to-noise ratio and clearer seismic phases recorded by the OBSs at the southwestern subbasin in the South China Sea (SCS). The equipment is a broadband OBS of I-4C type produced by the Institute of Geology and Geophysics, Chinese Academy of Sciences. We then inverted the lithospheric structure by successfully applying the receiver functions to the teleseismic data recorded at the southwestern subbasin in the SCS. The results show that it is feasible to use the receiver functions for the inversion of the lithosphere structure based on the passive observation data collected via the OBS, in which the key is to suppress the non-stationary noise. The Moho depth at the southwestern subbasin in the SCS is 10~12 km, with a crust thickness of 6~8 km. The shallow crust is a low velocity layer with sediment thickness 1~2 km, which consists of sediment and volcanic clastic breccia produced by the magma eruption after the spreading cessation. In the spreading axis center, a low S-wave velocity zone exists at the depths of 6 km to 12 km above the Moho. We interpreted this zone as the result of partial melting of the lower crust or the presence of a magma chamber. In the same area at the depths of 17 km to 30 km, the vertical S-wave velocity gradient is negative. We interpreted it as the result of the hot magma supply moving upward in the mantle.

We present results from a 484 km wide-angle seismic profile acquired in the northwest part of the South China Sea (SCS) during OBS2006 cruise. The line that runs along a previously acquired multi-channel seismic line (SO49–18) crosses the continental slope of the northern margin, the Northwest Subbasin (NWSB) of the South China Sea, the Zhongsha Massif and partly the oceanic basin of the South China Sea. Seismic sections recorded on 13 ocean-bottom seismometers were used to identify refracted phases from the crustal layer and also reflected phases from the crust-mantle boundary (Moho). Inversion of the traveltimes using a simple start model reveals crustal images in the study area. The velocity model shows that crustal thickness below the continental slope is between 14 and 23 km. The continental part of the line is characterized by gentle landward mantle uplift and an abrupt oceanward one. The velocities in the lower crust do not exceed 6.9 km/s. With the new data we can exclude a high-velocity lower crustal body (velocities above 7.0 km/s) at the location of the line. We conclude that this part of the South China Sea margin developed by a magma-poor rifting. Both, the NWSB and the Southwest Sub-basin (SWSB) reveal velocities typical for oceanic crust with crustal thickness between 5 and 7 km. The Zhongsha Massif in between is extremely stretched with only 6–10 km continental crust left. Crustal velocity is below 6.5 km/s; possibly indicating the absence of the lower crust. Multi-channel seismic profile shows that the Yitongansha Uplift in the slope area and the Zhongsha Massif are only mildly deformed. We considered them as rigid continent blocks which acted as rift shoulders of the main rift subsequently resulting in the formation of the Northwest Sub-basin. The extension was mainly accommodated by a ductile lower crustal flows, which might have been extremely attenuated and flow into the oceanic basin during the spreading stage. We compared the crustal structures along the northern margin and found an east-west thicken trend of the crust below the continent slope. This might be contributed by the east-west sea-floor spreading along the continental margin.

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 in3. 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.

International audience; The Southwest Indian Ridge (SWIR) is an ultraslow spreading end-member of mid-ocean ridgesystem. We use air gun shooting data recorded by ocean bottom seismometers (OBS) and multibeambathymetry to obtain a detailed three-dimensional (3-D) P wave tomographic model centered at 49390Enear the active hydrothermal ‘‘Dragon Flag’’ vent. Results are presented in the form of a 3-D seismictraveltime inversion over the center and both ends of a ridge segment. We show that the crustal thickness,defined as the depth to the 7 km/s isovelocity contour, decreases systematically from the center (7.0–8.0km) toward the segment ends (3.0–4.0 km). This variation is dominantly controlled by thicknesschanges in the lower crustal layer. We interpret this variation as due to focusing of the magmatic activityat the segment center. The across-axis velocity model documents a strong asymmetrical structureinvolving oceanic detachment faulting. A locally corrugated oceanic core complex (Dragon Flag OCC)on the southern ridge flank is characterized by high shallow crustal velocities and a strong verticalvelocity gradient. We infer that this OCC may be predominantly made of gabbros. We suggest thatdetachment faulting is a prominent process of slow spreading oceanic crust accretion even inmagmatically robust ridge sections. Hydrothermal activity at the Dragon Flag vents is located next to thedetachment fault termination. We infer that the detachment fault system provides a pathway forhydrothermal convection.