Crustal extension and magmatism along the northeastern margin of the South China Sea: Further insights from shear waves

• Published in: Tectonophysics 2021-10, Vol.817, p.229073, Article 229073
• Main Authors: Wen, Genggeng, Wan, Kuiyuan, Xia, Shaohong, Fan, Chaoyan, Cao, Jinghe, Xu, Huilong
• Format: Article
• Language: English
• Subjects: Anomalies; Continental crust; Continental margins; Converted S-wave; Crustal extension; Decompression; Diagenesis; Intrusion; Inversion; Isotopes; Lava; Magma; Magmatic activities; Mesozoic; Modelling; Northeastern margin of the South China Sea; P waves; Porosity; Ratios; S waves; Seismic surveys; Seismic velocities; Seismic wave velocities; Travel time; Vp/Vs ratios; Wave velocity
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/j.tecto.2021.229073

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.