Extensional tectonics and post-rift magmatism in the southern South China Sea: New constraints from multi-channel seismic data

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 (similar to 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.