Discontinuous Distribution and Tectonic Thinning Characteristics of the High-Velocity Lower Crust in the Northern Chaoshan Depression, South China Sea

The South China Sea (SCS), lying at the junction of the Philippine Sea Plate, Eurasian Plate, and Indo-Australian Plate, has a complex tectonic history, featuring numerous extension and magmatism events. One crucial aspect in the evolution is the formation of the high-velocity lower crust (HVLC, ~7.0–7.5 km/s) through deep magma intrusion or underplating. The presence or absence of the HVLC and its formation time hold crucial importance for understanding its evolution. However, there has long been a persistent and unresolved ambiguity regarding the origin and formation time of the HVLC in the SCS, especially within the Chaoshan Depression. The Chaoshan Depression, located in the northeastern SCS margin, has been identified as a Mesozoic forearc basin associated with the Dongsha Rise's magmatic arc. OBS profiles across the Chaoshan Depression consistently indicate a 2-12 km thick continuous HVLC. The HVLC might have formed from Mesozoic magmatic underplating. Also, it could be the result of Cenozoic magmatic activities related to the Hainan plume.

The foundation for discussing the origin and formation of the HVLC in the Chaoshan Depression lies in its structure and actual presence. The newly acquired OBS data in 2021 in the Chaoshan Depression, with the station spacing reduced from 20 - 80 km to 5 km, significantly enhances the lateral resolution. The results show that the crust of the Chaoshan Depression is thinner than previously reported and the previously identified HVLC in the northern Chaoshan Depression is actually absent, suggesting a potential overestimation of the HVLC in the northern continental margin of the SCS. Based on a more feature - based analysis (the Tc - Th diagram, a scatter plot of crustal thickness vs. HVLC thickness), we tentatively suggest that the HVLC in the northern Chaoshan Depression shows a positive Tc - Th correlation. This indicates a process that can be reconstructed by post - formation tectonic events like extension. Also, the thin HVLC aligns with thinner crust areas and pre-Cenozoic faults. All above suggest that the HVLC in the northern Chaoshan Depression might have formed before continental rifting and was thinned during the continental rifting. In contrast, the HVLC in the southern Chaoshan Depression shows a negative correlation, indicating no post - formation tectonic modification. Therefore, the southern area might be a product of Cenozoic magmatism, and our survey line might represent the northernmost boundary of Cenozoic magmatic activities.