Explaining Structural Difference Between the Eastern and Western Zones of the Qiongdongnan Basin, Northern South China Sea: Insights From Scaled Physical Models

The western and eastern zones of the Qiongdongnan Basin (China) show clear contrasts in sub‐basin distribution, fault orientation, and structural styles. Generally, pre‐existing structures are considered to control such differences, that is, NE‐striking main faults in the western zone underwent orthogonal stretching, while E–W‐ and NE‐striking faults in the eastern zone underwent oblique stretching. In addition to the influence of the pre‐existing structures, we suggest that depth‐dependent stretching controlled by the strength of the lower crust together with a change in extension direction may also affect the structural differences of the western and eastern Qiongdongnan Basin. In this study, we use physical analog modeling to explain the structural differences between the eastern and western zones of the Qiongdongnan Basin with the main variables being the strength of the lower crust and a change in extension direction. The model with weak lower crust is successful in accounting for the observed first‐order structural characteristics of this basin. This finding indicates that the rift formation is depth‐dependent and controlled by the orientation of the velocity discontinuities (VDs) and thus the extension direction. Experiment with a change of the extensional direction and weak lower crust reveal that the pre‐existing VDs mainly control the structural belts’ location and fault strikes; some minor structures (late features) respond to the extension change, whereas the main faults are inherited and developed from the previous stage of deformation.