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, i.e., 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. <br><br>

中国琼东南盆地东西两区在次级盆地展布、断裂走向及构造样式上均存在显著差异。学界普遍认为，先存构造是造成此类差异的主控因素：盆地西区北东向主干断裂经历了正交拉伸，而东区东西向与北东向断裂则经受斜向拉伸。除先存构造的影响外，本研究提出，由下地壳强度调控的深度依赖性拉伸，以及拉伸方向的转变，同样可能影响琼东南盆地东西两区的构造差异。

本研究采用物理模拟实验，以下地壳强度与拉伸方向转变为核心变量，解析琼东南盆地东西两区的构造差异成因。采用弱化下地壳的模型能够较好地复现该盆地观测到的一级构造特征。该研究结果表明，裂谷作用具有深度依赖性，其受控于速度间断面（velocity discontinuities, VDs）的走向，进而由拉伸方向调控。针对拉伸方向转变与弱化下地壳的模拟实验表明，先存的速度间断面主要控制构造带的展布位置与断裂走向；少量次级构造（晚期特征）会响应拉伸方向的转变，而主干断裂则继承早期变形阶段的特征并持续发育。