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Abstract The northeastern margin of the South China Sea experienced continental rifting followed by magmatism in the Cenozoic. This evolution has strongly influenced the heterogeneity of its crustal structure. To gain insights into the spatial variation in crustal structure, data from 30 ocean bottom seismometers were used to forward model ten P -wave velocity models along both dip and strike directions. The Cenozoic strata (1.7–3.5 km/s), the Mesozoic strata (3.5–5.5 km/s), the crystalline crust (5.5–7.5 km/s) and high velocity lower crust (7.0–7.5 km/s) were identified based on P -wave velocities. Overall, the crust thins from ~25 km on the continental shelf to ~15 km on the slope, with thicker crust underneath the Dongsha Basin (a Mesozoic basin) but thinner crust underneath the pre-Cenozoic NW-trending fault zones. This difference in thickness is possibly controlled by different strength of inherited structures, i.e., the Dongsha Basin and the fault zones. Furthermore, a widespread high velocity lower crust layer, which resulted from the intrusion of a mafic sill complex, was observed over the shelf to slope. Mafic intrusions within the lower crust have uplifted the caprock on the shelf, and magma eruptions have formed many post-rift seamounts on the slope. The evidence suggests that post-rift magmatism overprinted the vertical crustal structure. We therefore propose that the heterogeneous crustal structures in this passive margin are an integrated result of segmental extension followed by magmatic overprinting. Highlights • Ten 2-D crustal models constrained by wide-angle seismic data are presented. • Pre-rift inherited structures controlled the lateral variation of crustal structure. • Post-rift magmatism overprinted the initial crustal structure and rifted features. [ABSTRACT FROM AUTHOR]

In this study, we investigated the extensional tectonics and post-rift magmatism in the southern South China Sea (SCS) using a newly obtained seismic reflection profile, which crosses the Southwest sector of the oceanic basin (SWSOB), West Liyue Trough and Liyue Basin along the southern margin of the SCS. A wide range of magmatic features was identified in the SWSOB, with a late stage marked by a broad Pleistocene unconformity separating horizontal sediment layers from strata deformed by igneous structures. Along the continental slope, most fault activity continued until Horizon T4 (15.5 Ma), implying that crustal extension continued during seafloor spreading in the SW oceanic sector of the SCS. A low-angle listric normal fault with a large offset, marking the southern boundary of the trough, controlled regional crustal extension. Thick syn-rift sedimentary strata and significant crustal thinning in the southern part of the trough reveal relatively strong extension. Post-rift magmatism occurred in this thinned crust and affected sedimentary strata up to the seafloor. Importantly, a series of high-angle normal faults rapidly deepened the top of the basement, forming a sharp contact between the continental slope and oceanic basin. The distal domain is very narrow (~20 km) with no known detachment faults, suggesting rapid continental breakup that may be related to the presence of rigid blocks near the study area. • A low-angle listric normal fault controlled crustal extension of the southern West Liyue Trough. • The continental slope shows a sharp boundary with the ocean basin via a group of high angle normal faults. • The formation of a narrow distal domain is related to the presence of adjacent rigid crustal blocks. [ABSTRACT FROM AUTHOR]

The Lianhuashan fault zone (LHSFZ), which divides the Cathaysia Block (CB) into the coastal volcanic and Cathaysian fold belts, is an important intraplate lineament in the CB. Its fine structure is crucial to understand the Late Mesozoic tectonic evolution of South China. Herein, the fine S-velocity structure of the LHSFZ above 6.0 km and its depth extent were studied by ambient noise tomography and teleseismic relative travel-time residual analysis using a dense (∼1 km station spacing) seismic array spanning 123 km at the southern end of the LHSFZ. Our results indicate that the area within 70 km of the coast has a higher S-wave velocity, which extends to the southeast below 6 km. We propose that the high velocity anomaly is a result of magma intrusion caused by the subduction of the Paleo-Pacific plate in the Late Mesozoic, and the magma upwelling trajectory occurs from the coast to the inland. In addition, a low-velocity anomaly is found beneath the core area of the LHSFZ at a depth of 3–6 km, which is more evident in the S-wave than in the P-wave and corresponds well with the presence of hot springs and earthquakes. We hypothesize that the low-velocity anomaly represents the existing fault system of the Late Mesozoic volcanoes, which now provides channels for fluid migration and facilitates seismicity. Our model suggests that the LHSFZ plays a key role in the evolution of the coastal areas of South China. • We obtained the upper crust structure of the Lianhuashan Fault Zone. • Late Mesozoic magmatism in the Lianhuashan Fault Zone tilts southeast as depth rises. • Upper-crustal fluid migrations facilitate hot springs in the Lianhuashan Fault Zone. [ABSTRACT FROM AUTHOR]

Abstract The origin and evolution of magmatism in rifting margins are a fundamental geological subject, and remain the focus of intense studies. Unlike the classical magma-poor and magma-rich rifting margins, the northern margin of the South China Sea (SCS) exhibits a strong post-rift magmatism, but the evolution and mechanism of this behavior are still poorly understood. In this work we investigate the detailed structural features of the post-rift magmatism in the crust of the SCS northern margin. Our analysis is based on wide-angle and multichannel reflection seismic data, combined with previous seismological results. Our results show a wide distribution of shallow igneous sills and intrusions. These features are spatially and tectonically linked to the lower crustal high-velocity bodies (HVBs) and submarine volcanism, indicating a causative connection between the three features. Considering the existence of an obvious low-velocity branch of the Hainan plume in the northern SCS and uplift of the lithosphere-asthenosphere boundary, we propose that the HVBs reflect consolidated mafic intrusions which formed the lower crustal reservoirs feeding the overlying igneous sills and intrusions. In the SCS northern margin where abundant extensional and detachment faults can act as magmatic channels, such a dynamic process might cause post-rift volcanism. Our results suggest that the crustal magmatic system of post-rift volcanism has a multilevel upward migration mode, and the HVBs in the lower crust could be the product of post-rift magmatism, further indicating that the SCS northern margin had a magma-poor property at the rifting phase but has undergone a strong magmatic rejuvenation by the subsequent mantle plume. Highlights • Revealing seismic features of post-rift magmatism in the SCS northern margin • Lower crustal HVBs link to shallow igneous sills/intrusions and submarine volcanism. • The crustal plumbing system has a multilevel magmatic-transport mode. • Magma-poor SCS northern margin was rejuvenated by the Hainan mantle plume. [ABSTRACT FROM AUTHOR]

The northern continental margin of the South China Sea (SCS) is a unique rifted margin that experienced weak magmatism in the syn -rift stage and intense magmatism in the post-rift stage. This area also exhibits a complicated relationship between crustal extension and magmatism. We present the tomographic inversion of seismic P-wave and S-wave velocity models, as well as the Vp/Vs ratio model, in order to delineate the crustal extension and post-rift magmatic features. The velocity structure is created by forward modelling (RayInvr) and travel-time tomographic inversion (Tomo2D). The results suggest that Mesozoic strata near the Dongsha area has a maximum thickness of ~4.6 km with a Vp of 3.5–5.5 km/s, a Vs of 1.9–3.1 km/s, and Vp/Vs ratios of 1.71–1.76, indicating a low porosity and high degree of diagenesis. High-velocity lower crust (HVLC) is imaged in our model, with a Vp of 7.0–7.5 km/s ± 0.05–0.25 km/s and Vp/Vs ratios of 1.70–1.82 ± 0.05. The composition of the HVLC is mainly mafic, we considered it is related to magmatic underplating due to decompression melting caused by crustal extension. The crustal anomalies with high Vp/Vs ratios of 1.80–1.85 ± 0.04 are identified, which are the product of post-rift magmatic intrusions. The seaward increase in continental lower crustal Vp/Vs ratios, suggests an increasing level of mafic intrusion into the continental crust. We attributed crustal hyperextension to increasing fault density, which caused intense magmatic intrusion and thinner HVLC beneath the continent-ocean transition (COT). • Tomographic inversion of seismic P-wave and S-wave velocity models from OBS data in the northeastern South China Sea are presented. • High-velocity lower crust (HVLC) was mainly mafic composition, which caused by magmatic underplating due to decompression melting. • High Vp/Vs zones are the product of post-rift magmatic intrusion, and crustal extension controlled the magmatic intrusion and distribution of the HVLC. [ABSTRACT FROM AUTHOR]

• A large-scale fault zone was identified on the northern South China Sea. • The fault zone accommodated intra-plate deformation during Cenozoic extension. • The fault zone was inherited from a lithospheric-scale suture zone. • The fault zone played a significant role in segmenting the northern SCS margin. Multibeam bathymetric, seismic and borehole data are used to investigate a large-scale strike-slip structure, the Baiyun-Liwan Fault Zone, in the northern South China Sea. This fault zone comprises NW- to NE-striking faults and negative flower structures that were generated by oblique extensional displacement. Notably, the interpreted data reveals that the Baiyun-Liwan Fault Zone was active during the Cenozoic, recording intense magmatism, and accommodating significant intraplate deformation during progressive continental rifting and ocean spreading. It bounds two distinct crustal segments and played a significant role in segmenting the northern margin of the South China Sea. The geometry of faults and strata within the Baiyun-Liwan Fault Zone also controlled local sediment routing and depocentre evolution during the Cenozoic. As basement and syn-rift structures change markedly across the Baiyun-Liwan Fault Zone, we propose this structure to be inherited from a lithospheric-scale fault zone separating the Mesozoic arc from forearc-related terrains. We therefore stress the importance of pre-existing structures in the development of rifted margins, with the example provided by the Baiyun-Liwan Fault Zone having profound implications for palaeogeographic reconstructions in the South China Sea. At present, the Baiyun-Liwan Fault Zone is incised by the Pearl River Canyon and eroded by recurrent submarine landslides, forming a major area of sediment bypass towards the abyssal plain. [ABSTRACT FROM AUTHOR]