西非裂谷系Termit裂谷盆地地质结构及成因机制

Geological structure and dynamic mechanism of the Termit rift basin in West African rift system

基于地震和钻井资料,利用地震剖面解析与平衡剖面恢复等方法,对西非裂谷系Termit裂谷盆地的地质结构与运动学过程开展研究,探讨其成因机制。研究认为:①Termit盆地整体为狭长裂谷,以伸展构造为主,兼有伸展走滑及挤压性构造;平面上表现为“南宽北窄,向南撒开”结构,自北向南由地堑向半地堑逐渐过渡;北部发育边界断裂控制的地堑,南部以断超式半地堑为特征。纵向上,该盆地可以划分出5套地质构造层:以发育先存断裂为特征的前白垩系基底、下白垩统裂陷层、上白垩统坳陷层、古近系裂陷层和新近系-第四系后裂谷期坳陷层。②Termit盆地的形成受控于泛非构造基底;早白垩世受南大西洋张裂影响,基底的泛非构造薄弱带活化,Termit盆地发育裂陷Ⅰ期,形成盆地雏形;晚白垩世活动性减弱,以坳陷为主;古近纪主要受新特提斯洋俯冲汇聚的远程效应影响,Termit盆地发生强烈构造分异,发育裂陷Ⅱ期,形成近南北向系列断裂,逐渐形成现今隆坳格局。

Based on seismic and drilling data in the study area,the geological structure and kinematic process of the Termit rift basin were studied using seismic profile interpretation and balanced restoration to find out the dynamic mechanism of the basin.(1)The geological structure of the Termit Basin is represented as a narrow rift basin,with development of series of structural styles in extensional,extensional strike-slip and compressional stress setting.On plan,it is narrow in the north and wide in the south,and transitions from graben to half-graben from north to south;it features a graben controlled by the boundary faults in the north and a fault-overlapped half-graben in the south.(2)Before the Cretaceous,a series of hidden faults developed in the West African rift system,which laid the foundation for the development location and distribution direction of the Termit Basin;during the Cretaceous to Paleogene periods,the basin experienced two phases of rifting in Early Cretaceous and Paleogene,which controlled the initial structure and current structural shape of the basin respectively;during the Neogene to Quaternary,the basin was subjected to weak transformation.(3)In the Precambrian,the Pan-African movement gave rise to a narrow and long weak zone within the African plate,which provided the pre-existing structural conditions for the formation of the Termit Basin.In the Early Cretaceous,affected by the South Atlantic rifting,the Pan African weak zone was reactivated,resulting in the first stage of rifting and the basic structural framework of the Termit Basin.In the Paleogene,affected by the subduction and convergence of the Neo-Tethys Ocean,the African-Arabian plate extended in near E-W trending,and the Termit Basin experienced the second stage of rifting.The oblique extension in this period caused intense structural differentiation,and the current structural pattern of alternate uplifts and depressions took shape gradually.