关键词: 多幕裂陷, 水平伸展量, 拆离深度, 构造差异性, 珠江口盆地

Abstract: The Xijiang and Lufeng Sags are located in the Zhu 1 Depression of the Pearl River Mouth Basin in the northern South China Sea; the former displays a nearly symmetric graben controlled by NE-striking listric boundary faults, while the latter has the “two sag sandwiching one high” tectonic framework controlled by six curved major faults. The structure and rifting evolution show obvious spatial and temporal differences. In this paper, based on the three-dimensional seismic data, basin structure and geometry of fault system were descripted, a total of 8 seismic-geological sections in the two sags were selected for balanced section restoration, and quantitative parameters (e.g. horizontal displacement and depth-to-detachment) during the two rifting stages were calculated. The evolution process and their differences of the two rifts were further analyzed and the relationship with regional tectonic evolution was discussed. In the Cenozoic, the study area experienced the first (period of the Wenchang Formation) and second (period of the Enping Formation) phases of rifting and the subsequent Late Oligocene-Quaternary post-rifting stage. Each rifting phase can be subdivided into early and late sub-phases. The rifting evolution of the Xijiang and Lufeng Sags has temporal and spatial differences. From the first to second phase of rifting, the horizontal displacement of the study area has the general characteristics of decreasing with time. The horizontal displacement of the Xijiang Sag continues to decrease with time, and it still contains large displacement in the late sub-phase of the second phase, while it is slightly increased in the early sub-stage of the second phase in the Lufeng Sag. The degree of extension in the late sub-phase is much weaker in the Lufeng Sag than that in the Xijiang Sag. The depth-to-detachment of the two sags has the characteristics of deepening with time. In the early and late sub-phases of the first phase, the depth-to-detachment of the Lufeng Sag is slightly deeper than that of the Xijiang Sag. In the early and late subphases of the second phase, the average depth-to-detachment of the Xijiang Sag increases from ~8 km to ~9 km, while the average detachment depth of the Lufeng Sag jumps from ~8 km to ~35 km. The geometric structure, fault system, horizontal displacement and depth-to-detachment of the study area indicate that the Lufeng Sag was subjected to thermal subsidence in the late sub-phase of the second rifting phase and probably ended the rifting stage earlier, while the rifting of the Xijiang Sag ended after the second rifting phase. The rifting of the study area gradually ending from east to west may be controlled by the seafloor spreading of the Northeast Sub-basin of the South China Sea and the gradual westward seafloor spreading of the South China Sea. The regional plate tectonic evolution also provides the Pearl River Mouth Basin with a differential extension environment with enhanced extension in the SW direction. The differential rifting evolution of the study area is of great significance for understanding the temporal and spatial distribution of the rift time and differential rift evolution in the Pearl River Mouth Basin, and also provides important quantitative constraint for understanding the rifting process in the northern continental margin of the South China Sea. 

Key words: multiphase rifting, horizontal displacement, depth to detachment, differential tectonic evolution, Pearl River Mouth Basin