Submarine sinkholes are unique and important geomorphological features with a typical cavity structure that are of great scientific value. Submarine sinkholes were discovered for the first time in the isolated Ganquan carbonate platform on the Xisha Islands, the northwestern South China Sea. Based on high-resolution multibeam bathymetric data and seismic profile data, we identified 37 submarine sinkholes at water depths ranging from 550 to 1267 m. They are subcircular to circular negative-relief features, and most of them are V- or compound V-shaped in the cross-section. Their average diameters range from 57 to 667 m, and the depth of the depression ranges from 2.5 to 241 m. By comparing submarine sinkholes in the Ganquan platform with those in other carbonate platforms worldwide, we can infer that the Ganquan platform submarine sinkholes are the largest sinkholes developed on an isolated carbonate platform. Remotely operated vehicle (ROV) “Haima 2” images revealed that the inner walls of submarine sinkholes are characterized by stalactite-like structures, possible dikes, flow marks, and corroded holes, which are typical karstic landscape features. The temperature within submarine sinkholes is 2 °C higher than that of the open ocean at the same water depth. Based on the results of the shallow formation profile and multichannel seismic profiles, we propose that the submarine sinkholes in the Ganquan platform probably formed via the dissolution of the carbonate platform via acidic hydrothermal fluids that originated from magmatic activity and migrated along faults.

Seafloor spreading is an important cornerstone of the theory of plate tectonics. Asymmetric seafloor spreading and oceanic ridge jumps are common phenomena in this process and play important roles in controlling oceanic crust accretion, regional tectonics and geological geometric boundaries. As the largest marginal sea in the western Pacific, the South China Sea is an ideal laboratory for dissecting the Wilson cycle of small marginal sea-type ocean basins restricted by surrounding blocks and exploring the deep dynamic processes of confined small ocean basins. In recent years, a lot of research has been conducted on the spreading history of the South China Sea and has achieved fruitful results. However, the detailed dynamic mechanisms of asymmetric seafloor spreading and ridge jumps are still unclear. Therefore, this paper summarizes the basic understanding about the dynamic mechanisms of global asymmetric seafloor spreading and ridge jumps and reviews the related research results of asymmetric seafloor spreading and ridge jumps in the South China Sea. Previous studies have basically confirmed that seafloor spreading in the South China Sea started between ~32 and 34 Ma in the east sub-basin and ended at ~15 Ma in the northwest sub-basin, with at least once oceanic ridge jump in the east sub-basin. The current research mainly focuses on the age of the seafloor spreading in the South China Sea and the location, time and stage of the ridge jumps, but there are relatively few studies on high-resolution lithospheric structure across these ridges and the dynamic mechanism of oceanic ridge jumps. Based on the current research progress, we propose that further studies should focus on the lithosphere–asthenosphere scale in the future, suggesting that marine magnetotelluric and Ocean Bottom Seismometer (OBS) surveys should be conducted across the residual oceanic ridges to perform a detailed analysis of the tectonics magmatism in the east sub-basin to gain insights into the dynamic mechanisms of oceanic ridge jumps and asymmetric seafloor spreading, which can promote understanding of the tectonic evolution of the South China Sea and improve the classical plate tectonics theory that was constructed based on the open ocean basins.

Based on comprehensive interpretation of three-dimensional seismic data and quantitative analysis of basin-boundary fault activity in the Nanpu Sag, this study employs subsidence history backstripping and equilibrium profile techniques to reconstruct the structural evolution of the main profile. The results indicate that the Cenozoic evolution of the Nanpu Sag can be divided into a syn-rift stage and a post-rift stage, with the syn-rift stage further subdivided into Rift I and Rift II episodes. During Rift I, tectonic activity was primarily controlled by the NE- and NEE-trending Xinanzhuang Fault, Shabei Fault, and No. 2 Fault Zone, which formed under a NW–SE extensional stress regime and governed the development of NE- or NEE-trending faults and associated sedimentary subsidence centers. In Rift II, tectonic activity was dominated by a southward-curved normal fault system, composed of the Xinanzhuang, Gaoliu, and Baigezhuang faults, as well as the Shabei Fault, reflecting the influence of a near N–S ex-tensional stress field. The progressive southward migration of the Sag’s subsidence center over time—from the Linque sub-sag in the third section of the Shahe Formation to the Caofeidian sub-sag in the Dongying Formation—and noting, coupled with the pronounced left-lateral strike-slip characteristics of the Baigezhuang Fault and No. 4 Fault, and regional tectonic evolution analysis of the Bohai Bay Basin, support the proposal that a strike-slip extension mechanism—characterized by lateral strike-slip and forward extension—constitutes the fundamental developmental model of the Nanpu Sag. This study deepens the understanding of the tectonic evolution of the Nanpu Sag and provides new insights in-to the dynamic mechanisms governing the formation of similar Sags in the Bohai Bay Basin.