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The deep crustal structure of the Northwest Sub-basin (NWSB) of the South China Sea (SCS) is of great importance for understanding the tectonic nature of the continent-ocean transition (COT) and magmatism in this oceanic basin. The 2-D wide-angle reflection/refraction seismic profile OBS2006-2 is almost parallel to the extinct spreading ridge (ESR) of the NWSB. In addition to the original data, we added the data of two reprocessed OBS stations, and carried out seismic phase re-picking and travel-time imaging to obtain the crustal velocity structure along this profile. Resolution tests demonstrate that the newly acquired velocity structure is more reliable than the prior interpretation. The depth of the Moho (23.5–11.8 km) and crustal thickness (20.5–6.5 km) systematically changes from continental crust of the Xisha Block to the oceanic crust within the NWSB. The COT zone has a width of ∼20 km and the depth of the Moho decreases from 15.0 to 11.0 km, corresponding to a ∼4 km decrease in crustal thickness (6–10 km). A high velocity layer (HVL, 7.2–7.4 km s−1) exists at the bottom of the crust at the location where the sharp lateral transition of the continental crust to the oceanic crust occurs. Age dating shows that the Double-peak Seamount was formed at ∼23 Ma, after the cessation of the NWSB seafloor spreading (∼32-25 Ma). The crust beneath the Double-peak Seamount is oceanic with a thickness of 9 km. We infer that this oceanic crust was formed by magmatic upwelling and decompression melting along a pre-existing zone of weakness.

In the late 90’s, some faults identified within oceanic crust were demonstrated to be artifacts arising from out-of-plane scattering along linear sediment-buried fault scarps. Symmetrical mantle reflections observed southwest northern Sumatra on seismic reflection profiles have been identified as faults cutting through the upper mantle down to unprecedented depths reaching ~45 km. Seawater being conveyed along sub-vertical re-activated fracture zones (FZs) to the upper mantle, the mantle portions of FZs are serpentinized and act as mirrors for seismic rays. We suggest that the mantle features are not faults but artifacts resulting from out-of-plane reflections on these mirrors. Two perpendicular seismic profiles crossing the same FZ display two dipping features down to 30 km, which cannot be explained as faults from recent tectonic and structural constraints but merely as out-of-plane reflections on this FZ. This result confirms that most of mantle reflections observed southwest northern Sumatra are fakes rather than faults.

In order to ensure that the gravity data processing and inversion are consistent with the actual geology, we propose that, in integrated geophysical research, 3D gravity anomaly separation (GAS) should take into account the gravity response of the inhomogeneous mantle. This is an effective method by which the residual gravity anomaly (RGA) may be obtained, as well as an important basis for the inversion and interpretation of gravity data. Aimed at the petroliferous basin structure, evolution of continental margin, ocean continental transition zone and other widely discussed geological problems, we extensively investigate the upper mantle structure in many regions around the world, and the results show that the lateral inhomogeneity of the mantle is widespread. In many comprehensive studies based on gravity and seismic data, the gravity anomaly modeling and separation are typically performed within the crust (above the Moho), which actually relies on an assumption of homogeneous mantle. Although this assumption simplifies the difficulty of the gravity modeling and separation, it ignores gravity anomalies caused by inhomogeneities of the deep mantle. The RGA, along with the inverted density structure and other interpretations, involves the risk of deviating from the actual geology. On the basis of the 2D forward calculation of several typical interface models, we implement the quantitative analysis of the effect that the inhomogeneous mantle has on the Bouguer gravity anomaly. In case studies of the southeastern United States and the Sichuan Basin, taking the shear wave velocity (Vs) structure of the upper mantle as the constraint condition, we calculate the gravity anomaly caused by the inhomogeneous mantle by means of the forward method. It is found that the gravity response of the inhomogeneous mantle cannot be neglected. Therefore, we conduct 2.5D joint simulation combining gravity and seismic data in the Sichuan Basin. The fitting results after separating the gravity response of the inhomogeneous mantle are clearly superior to those of the homogeneous mantle hypothesis. On the basis of the 3D depth data of several sedimentary strata, the Moho and the mantle high-velocity bodies, we implement 3D GAS, obtain the RGA of the basement in the Sichuan Basin, and discuss the geological origin of the “Dazu high gravity”.

P-wave velocity models were obtained by forward and inverse modeling from 38 ocean bottom seismometers deployed in the central East sub-basin of the South China Sea (SCS). Four types of crust have been defined; a) thin oceanic crust (6 km) with significant intrusive roots, and d) thick oceanic crust with enhanced spreading features (>6 km) but without significant roots. Within the central East sub-basin, the thin oceanic crust, only identified inside a 80-km wide zone, is located within an overall 150-km wide domain characterized by N055° seafloor spreading trends. The post-spreading volcanoes were formed during a N-S tensional episode around 6-10 Ma, several millions of years after seafloor spreading ceased in the SCS. Seafloor spreading (N055° and N145°) and post-spreading (N000° and N090°) features are observed in the morphology of some of these volcanoes. The rupture of the brittle thin oceanic crust was focused where the crust was the weakest, i.e. at the intersection of the extinct spreading ridge with former fracture zones. From geological and geophysical arguments, we suggest that the post-spreading volcanism might have been influenced by the Hainan plume activity through a buoyancy-driven partial melting mechanism.

The marginal sea and back-arc basins in the Western Pacific Ocean have become the focus of tectonics due to their unique tectonic location. To understand the deep crustal structure in the back-arc region, we present a 545-km-long active-source ocean bottom seismometer (OBS) wide-angle reflection/refraction profile in the East China Sea. The P wave velocity model shows that the Moho depth rises significantly, from approximately 30 km in the East China Sea shelf to approximately 16 km in the axis of the Okinawa Trough. The lower crustal high-velocity zone (HVZ) in the southern Okinawa Trough, with Vp of 6.8–7.3 km/s, is a remarkable manifestation of the mantle material upwelling and accretion to the lower crust. This confirms that the lower crustal high-velocity mantle accretion is developed in the southern Okinawa Trough. During the process of back-arc extension, the crustal structure of the southern Okinawa Trough is completely invaded and penetrated by the upper mantle material in the axis region. In some areas of the southern central graben, the crust may has broken up and entered the initial stage of seafloor spreading. The discontinuous HVZs in the lower crust in the back-arc region also indicate the migration of spreading centers in the back-arc region since the Cenozoic. The asthenosphere material upwelling in the continent-ocean transition zone is constantly driving the lithosphere eastward for episodic extension, and is causing evident tectonic migration in the Western Pacific back-arc region.

The 140-km wide last phase of opening of the South China Sea (SCS) corresponds to a N145° direction of spreading with rift features identified on swath bathymetric data trending N055° (Sibuet et al., 2016). These N055° seafloor spreading features of the East Sub-basin are cut across by a post-spreading volcanic ridge oriented approximately E–W in its western part (Zhenbei–Huangyan seamounts chain). The knowledge of the deep crustal structure beneath this volcanic ridge is essential to elucidate not only the formation and tectonic evolution of the SCS, but also the mechanism of emplacement of the post-spreading magmatism. We use air-gun shots recorded by ocean bottom seismometers to image the deep crustal structure along the N–S oriented G8G0 seismic profile, which is perpendicular to the Zhenbei–Huangyan seamounts chain but located in between the Zhenbei and Huangyan seamounts, where topographic changes are minimum. The velocity structure presents obvious lateral variations. The crust north and south of the Zhenbei–Huangyan seamounts chain is ca. 4–6 km in thickness and velocities are largely comparable with those of normal oceanic crust of Atlantic type. To the south, the Jixiang seamount with a 7.2-km thick crust, seems to be a tiny post-spreading volcanic seamount intruded along the former extinct spreading ridge axis. In the central part, a 1.5-km thick low velocity zone (3.3–3.7 km/s) in the uppermost crust is explained by the presence of extrusive rocks intercalated with thin sedimentary layers as those drilled at IODP Site U1431. Both the Jixiang seamount and the Zhenbei–Huangyan seamounts chain started to form by the intrusion of decompressive melt resulting from the N–S post-spreading phase of extension and intruded through the already formed oceanic crust. The Jixiang seamount probably formed before the emplacement of the E–W post-spreading seamounts chain.

The probable N055°-oriented extinct spreading ridge of the East Sub-basin is cut across by the post-spreading volcanic ridge oriented approximately E–W in its western part and called the Zhenbei–Huangyan seamounts chain. A three-dimensional ocean bottom seismometer survey was conducted in 2011 and covered both the extinct spreading ridge and the Zhenbei–Huangyan seamounts chain in the East Sub-basin. The comprehensive seismic record sections of 39 ocean bottom seismometers underline the high-quality data with clear and reliable P-wave seismic first arrivals. The three-dimensional tomographic results show that the crust is oceanic on each side of the Zhenbei–Huangyan seamounts chain. The seafloor spreading lineaments and associated fracture zones pattern provide channels for magma migration and eruption. Based on the ratio of volcanic extrusive and intrusive materials and the shape of the upward convex 6.4 km/s iso-velocity curve beneath the Zhenbei and Huangyan seamounts, we conclude that the Zhenbei and Huangyan seamounts appear to be dominantly formed by intra-crustal intrusion. Combining previous geochemical studies of post-spreading volcanic ridge outcropping samples, the formation mechanism of the seamounts chain might be explained by a buoyancy-driven decompression melting mechanism occurring as separately evolving partial melting bodies beneath each seamount. Copyright © 2016 John Wiley & Sons, Ltd.