渤海湾盆地古近纪构造变革的穿时特征及成因探讨:以渤海海域西南部和陆上济阳坳陷为例

Diachronous characteristics and genetic mechanism of the Paleogene tectonic transition in Bohai Bay basin:a case study of the southwest Bohai Sea and Jiyang Depression

基于“海-陆”不同矿区地震解释资料的统层处理,对渤海海域西南部古近纪幕式裂陷的构造变革特征开展系统研究,通过与陆上济阳坳陷对比分析,识别出对“海-陆”不同坳陷“穿时”的构造变革特征,并对其成因进行探讨。研究结果表明:盆地的发育演化体现多幕式的伸展过程与走滑变形的叠加效应。受控于始新世中—晚期全球板块的重组及其诱导的郯庐断裂带右旋走滑变形的影响,渤海海域经历由N—S向伸展向NW—SE向伸展的转变,沙三段沉积期末短暂的挤压抬升将两幕具有显著构造差异变形特征的裂陷过程分隔开来;与此同时,NW向先存断裂在新的应力场下发生张扭性变形,NEE向新生断裂体系对盆地构造格局进行强烈改造,盆地沉降中心随之发生迁移。相比之下,陆上的济阳坳陷在沙四段沉积期末就已完成向NE向构造格局的转型,渤海海域具有明显的非同步性,体现了“海-陆”不同坳陷穿时的构造变革特征。认为深部地幔物质上涌产生的水平拉张与深大断裂走滑活动控制的岩石圈优势伸展方向的叠加导致了渤海湾盆地中部构造的差异性发育演化,渤海海域处于渤海湾盆地的裂陷中心,在沙三段沉积期伸展作用强于走滑作用,先存的NW向构造格局占据主导;而济阳坳陷远离裂陷中心,同时期走滑作用相对增强,其派生的NW向拉张应力导致了盆地构造格局优先向NEE向转型。同一时期伸展与走滑应力的配比关系造成构造格局在空间上的不均一性,可能是渤海湾盆地古近纪“穿时”构造变革事件的主要动力学成因。

Based on the systematical interpretation of the seismic data collected from the different oilfields,an integrated study of the Paleogene tectonic features in Southwest Bohai Sea was carried out.More importantly,based on the comparative analyses with Jiyang Depression,the characteristics and mechanism of the diachronous tectonic transition in different sub-basins were discussed.The results show that the evolutionary history of the basin implies the episodic rifting process and the overprint of the strike-slip deformation.Controlled by the plate reconfiguration during the Middle to Late Eocene and subsidiary dextral strike-slip movement of the Tan—Lu fault zone,the Southwest Bohai Sea sub-basins experienced the transition from N—S directed stretching to NW—SE directed stretching,segmented by a transient shortening event.Meanwhile,the NW-trending preexisting fault suffered trans tensional deformation,while the NEE-trending faults deformed logically in the newly changed stress field,causing the reworked structural configuration and the depocenter migration.The interaction between the mantle upwelling and the strike-slip faulting resulted in the differential tectonic evolution of the Southwest Bohai Sea sub-basins and the Jiyang Depression.During the depositional period of the 3rd member of Shahejie Formation,the Bohai Sea was located in the rifting center of the Bohai Bay Basin,which is characterized by the intense rifting relative to strike-slip deformation,causing the inherited NW-trending dominated structural pattern.While the Jiyang Depression is far away from the rifting center,which begins to be influenced by the strike-slip deformation,causing the structural configuration to transform into NEE-directed.The relative intensity of stretching induced from strike-slip faulting leads to the heterogeneous structural configuration in space,and the occurrence of the diachronous tectonic transition within the Bohai Bay Basin.