珠江口盆地裂后阶段性差异沉降及其成因机制*

位于南海北部陆缘的珠江口盆地裂后沉降特征不同于陆内典型断陷盆地。研究表明,盆地裂后期发生了阶段性有序差异沉降,可分为4个阶段: (1)渐新世早期(~33.9~27.2 Ma),以盆地整体缓慢沉降,大规模海侵为主要特征;(2)渐新世晚期(~27.2~23.0 Ma),以邻近西北次海盆的珠四坳陷强烈沉降为主要特征,差异沉降控制了陆架坡折带的发育和该时期陆架浅水和陆坡深水沉积环境的分布;(3)中新世早—中期(~23.0~10.0 Ma),陆缘强烈沉降区向北扩展至珠二坳陷,尤其是白云凹陷,导致陆架坡折带向北跃迁,并奠定了现今陆架浅水和陆坡深水的沉积格局;(4)中新世晚期—现今(~10.0~0 Ma),陆缘构造沉降逐渐减弱,陆坡由沉积区转变为沉积过路区,沉积物得以大量进入西北次海盆。渐新世2期快速沉降的初始时间,分别对应于南海扩张脊的跃迁,陆缘裂后沉降随扩张脊向南跃迁而向北扩展,并伴有岩浆作用的早强晚弱特点,而沉降量的大小则与裂陷期地壳的薄化程度正相关,反映了陆缘岩石圈经历了早期挠曲回弹的均衡调整和扩张脊跃迁导致地幔物质有序向南撤离而沉降的演化过程。珠江口盆地裂后有序差异沉降控制了陆架坡折带的发育,进而控制了浅水与深水两大沉积体系的展布。

Staged differential subsidence and its genetic mechanism in post-rifting stage of Pearl River Mouth Basin

The post-rifting subsidence characteristics of the Pearl River Mouth Basin,located in the northern margin of the South China Sea,are different from those of typical intracontinental rift basins,and have distinguishing feature of the South China Sea. The results show that there are four stages of orderly differential subsidence in post-rifting stage: (1)During the early Oligocene(~ 33~27 Ma),the basin was dominated by slow subsidence and characterized by large-scale transgression;(2)During the late Oligocene(~ 27~23 Ma),the basin characterized by the strong subsidence of Zhu IV depression,adjacent to the northwest sub-basin of the South China Sea. Differential subsidence controls the development of shelf-slope break zone and the distribution of shallow-water and deep-water sedimentary environment respectively in shelf and slope during this period;(3)During the early and middle Miocene(~23~10 Ma),the strong subsidence area extended northward to Zhu II depression,especially the Baiyun sag,which resulted in the northward transition of continental shelf slope break and established the present sedimentary environment pattern of shallow-water and deep-water respectively in shelf and slope;(4)From late Miocene to present(~10~0 Ma),the tectonic subsidence of the continental margin gradually weakened. The continental slope changed from a depositional area to a bypass area, and a large number of sediments entered the northwest sub-basin. The initial time of rapid subsidence in Oligocene corresponds to the transition of the spreading ridge in the South China Sea. Post-rift subsidence of continental margin expands northward with the southward transition of spreading ridge,accompanied by early strength and late weakness of magmatism. The amount of subsidence is positively related to the thinning degree of the crust during the rift stage. It reflects that the continental margin lithosphere experienced the evolution process of early balanced adjustment of flexure and the orderly evacuation of mantle materials to the south,because of the transition of spreading ridge in the South China Sea. The orderly differential settlement of the Pearl River Mouth Basin controls the development of the shelf slope break zone,and then controls the distribution of the shallow water and deep water sedimentary systems. Different depression zones in the basin have different tectonic subsidence history,which shows that their thermal evolution history must have the same zoning differences,thus having different hydrocarbon generation and diagenesis laws. It provides reference for the exploration and research of continental margin rift basins.