Search

Submarine volcanism is widely developed in the South China Sea (SCS). However, the characteristics, distribution, and genesis of submarine volcanoes in the southern margin of the SCS remain obscure. In this study, we analyzed the characteristics of submarine volcanoes and identified a total of 43 submarine volcanoes in the southern margin of the SCS, based on a newly acquired 310-km seismic reflection profile, along with previous 45 multi-channel seismic (MCS) profiles, petrological results from volcanic rocks sampled by dredging and drilling, nearby ocean bottom seismometer (OBS) wide-angle seismic profiles, and gravity and magnetic data. The study ascertains that most of these volcanoes are located in fault-block belts and graben-horst zones with strong crustal stretching and thinning. These volcanoes exhibit positive high-amplitude external seismic reflections, weak and chaotic internal seismic reflections, and are accompanied by local deformation of the surrounding sedimentary strata. Meanwhile, they have higher positive gravity anomalies and higher magnetic anomalies than the background strata. The petrological dating results show that volcanic ages are primarily in the Pliocene-Pleistocene, with geochemical characteristics indicating dominance of oceanic island basalt (OIB) -type alkali-basalts. Extensional faults have obviously spatial correspondence with post-spreading volcanism, suggesting these faults may provide conduits for submarine volcanism. The high-velocity bodies (HVBs) in the lower crust and magma underplating exist in the southern SCS, which could provide a clue of genesis for submarine volcanism. The inference is that the intensity of post-spreading volcanism in the southern margin might be affected by stretching faults, crustal thinning and magma underplating. [ABSTRACT FROM AUTHOR]

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]