珠江口盆地珠二坳陷中部“对向拆离型复合洼陷”结构特征及其成因探讨

Structural Characteristics and Genesis of Opposite Detachment Type Composite Sag in Middle of Zhu II Depression,Pearl River Mouth Basin

南海北部陆缘深水区始新世多发育单向拆离断层,而珠二坳陷中部处于珠江口盆地珠二与珠一、珠三坳陷衔接带的南倾单一拆离断裂与北倾多级拆离断裂系交汇区,形成特有的“对向拆离型复合洼陷”,然而其结构特征及成因机制有待深入研究.基于最新高精度全覆盖三维地震数据和始新世全序列钻井数据,恢复始新世关键地质时期(T80/T83)洼陷构造地貌为窄深分隔型至宽浅汇聚型转换格局,沉积中心从两侧近源陡坡带向中央带拆离迁移,对向断裂交汇区隆升断块沿东西向长轴展布;构造演化序列揭示出对向拆离断裂系启动于始新世关键构造变革期(T83~43 Ma),响应远程太平洋板块俯冲方向的变化,区内地壳在伸展应力的作用下拆离减薄,拆离断面之上广泛发生断块翘倾与差异隆升现象,从而接受剥蚀成为动态源区,同时伴生强烈岩浆活动,重塑了对向分布的箕状断陷结构,形成对向拆离区内长轴动态供给模型.厘定对向拆离型复合洼陷长轴动态源区与沉积中心时空配置,可为深水区始新世裂陷期优质烃-储组合预测提供有力支撑.

In deep water area of the northern continental margin of the South China Sea,unidirectional detachment faults were mostly developed in the Eocene,and the middle part of the Zhu II depression was located in the intersection area of the south dip single detachment fault and the north dip multistage detachment fault system in the junction zone of Zhu-2 and Zhu-1 and Zhu-3 depressions of the Pearl River Mouth basin,forming a unique“opposite detachment type composite sag”.However,its structural characteristics and cause of formation need to be further studied.Based on the latest high-precision 3D seismic data and the complete Eocene drilling data,the tectonic geomorphology of the T80/T83 depression in the key geological period of Eocene is restored to be a narrow and deep separated type to a wide and shallow converging type.The depositional center migrated from the near source steep slope to the central zone on both sides,and the uplift fault block in the intersection area of the opposite faults distributed along the east-west long axis.The tectonic evolution sequence reveals that the opposite detachment fault system started in the key tectonic transformation period of Eocene(T83-43 Ma).In response to the change of the subduction direction of the remote Pacific plate,the crust in the region was detached and thinned under the action of extensional stress.The fault block tilting and differential uplift occurred on the detachment section,and thus became a dynamic source region receiving denuding.At the same time,strong magmatic activity was generated.The dustpan-like fault depression with opposite distribution was reconstructed and the long-axis dynamic supply model in the opposite detachment area was formed.The determination of the spatio-temporal configuration of the long-axis dynamic source area and depositional center of the opposite detachment type composite depression is helpful to serve the prediction of high-quality hydrocarbon reservoir assemblage during the Eocene rift in the deepwater area.