Geophysical Characteristics of Breakup Magmatism in the Southern South China Sea Margin

The South China Sea Margin is a good natural laboratory featuring polyphase rifting processes that began in the late Eocene and ended late Miocene. The breakup first occurred in the East Sub-basin, and the expansion direction shifted from a north-south orientation to a northwest-southeast orientation around 23 Ma, with the propagation of new oceanic crust forming the Southwest Sub-basin. Most previous studies have suggested that the heterogeneity of the continental crust, such as the thickness of the lithosphere, primarily controlled the location and direction of ridge propagation. However, the involvement of magmatic activity is still not fully understood, nor is its influence during the breakup process of the Southwest Sub-basin.

This study investigates the crustal structure and the magmatic activity by integrating multichannel seismic (MCS) profiles and shipborne gravity around Taiping Island (Spratly Islands). Five pre-stack time migration profiles further enhanced imaging of lateral stratigraphic variations, providing spatial distribution. The Moho surface in the South China Sea was derived by inverting global Bouguer anomaly data with constraints from Moho depths obtained through OBS and seismic survey data and integrating other stratigraphic interfaces from seismic profiles; these boundaries construct a 2D gravity model.

Gravity simulation results reveal the presence of a high-density igneous body in the continent-ocean transition (COT) east of Taiping Island (Spratly Islands) by comparing seismic interpretations with different densities. The tentative magmatic body corresponds to the area of high-amplitude, high-angle reflectors in seismic profiles overlain by post-rift sediments. Among the five seismic profiles analyzed in this study, high-density bodies were identified in the easternmost profile, absent in profiles located approximately 40 km westward, suggesting a limited western extent. The seismic observation inferred that the magmatic formation occurred during the latest rifting phase in the southwest sub-basin of the South China Sea.

Therefore, this study suggests distinct magmatic intrusions within the South China Sea crust during the spreading of the southwest sub-basin. The location of these high-density bodies is near the boundary between the East Sub-basin and the Southwest Sub-basin and close to the southern segment of the Zhongnan Fault. The formation of these high-density bodies may provide more insights to discover magmatism involving the ridge propagation process.