The Zhongyebei (中业北) basin (ZYBB) is an NE-striking, narrow and small sedimentary basin superimposing the southern 1/2 segment of the proposed spreading axes of the SW subbasin of the South China Sea (SCS). More than 4 500 m strata were identified in the Zhongyebei basin, includ-ing the Paleogene lower structure layer and the Neogene upper structure layer. The SW subbasin of the South China Sea has been regarded as an oceanic basin opened by seafloor spreading, as evidenced by the flat and deep (> 4 000 m mostly) seafloor with linear magnetic anomalies, and by the shallow Moho depth of < 12 km as estimated from gravity modeling. The classic model of seafloor spreading predicts that sediments on the oceanic crust are younger and thinner towards the spreading axes. But in the southwestern segment of the SW subbasin, contradictions appear. Firstly, the thick sedimentation in the ZYBB is along the proposed spreading axes. Secondly, the sediments are thinner (500-1 500 m) and younger away from the proposed spreading axes. Thirdly, geological elements of the two sides of spreading axes develop asymmetrically in the southwestern SW subbasin. Two models, the early open-ing model and the limited modeling model, are suggested for resolving this paradox. The former sug-gests that the opening of the SW subbasin was in Late Eocene and earlier than the oldest sediment in the ZYBB. The latter proposes that the opening of the SW subbasin was limited to its northeastern por-tion, and did not extend to the southwest portion. The ZYBB is a rift basin survived from the spreading but subjected to severe syn-spreading magmatic disturbance. The SW subbasin and the ZYBB of the SCS provide a unique opportunity for studying the structural evolution and dynamic mechanism at the tip of a propagating seafloor spreading. Both models have unresolved questions, and further studiesare needed.
