南大西洋中段被动陆缘盆地下白垩统盐构造成因模式

Genetic model of Lower Cretaceous salt tectonics in passivecontinental margin basin of middle South Atlantic

随着巴西和西非海上巨型油气田的不断发现,盐相关勘探技术进步和数据资料快速积累,深入开展南大西洋被动陆缘盆地下白垩统盐岩成因环境及盐构造变形机理的研究,对于基础地质理论发展及海洋油气勘探开发具有重要的现实意义。南大西洋两岸被动陆缘盆地下白垩统阿普特阶盐岩构造具有明显的分带性特征,显示了从伸展构造到挤压构造连续过渡特点。巴西一侧,出现连续分布厚层盐岩,盐上和盐下地层协调变形。盐岩构造变形开始于洋壳出现的阿尔比期,漂移期新海底负地形空间的增长,触发了沉积载荷作用下的盐岩构造变形。随着被动陆缘沉积沉降,两侧陆地抬升(地幔作用),以及板块边界上构造应力的远程传递作用(南美),到新近纪持续发生重力作用下的构造变形。物理模拟实验以及在力学上实现正演再现的离散元数值模拟,表明南大西洋盐构造变形为重力构造作用成因,斜坡上的重力作用形成被动陆缘盆地的伸展—过渡—挤压的盐构造分带。局部高地抬升(被动陆缘断块抬升)扰动盐岩流动,盐岩颗粒具有较大的向上速度分量,形成盐株、盐底辟构造。而前缘构造阻挡(大洋地壳、SDRs形成、过渡型伸展地壳、出露的超镁铁质地幔等抬升造成)、盐上地层压力(载荷)则是坡下盐岩运动速率降低,出现聚集增厚现象的主要原因。南大西洋两岸的盐构造经历多期构造变形改造,主要涉及两期(阿尔比期和新生代)盐构造变形流动及其向大洋盆地的前展式扩张。沿着盐层滑动过程中重力滑动及重力扩展的混合作用模式,可以解释本区主体的盐构造样式。

With the ongoing discovery of giant offshore oil and gas fields in Brazil and West Africa and the progress of salt related exploration technology, there has been a rapid accumulation of data. This data is of great significance for the development of basic geological theory and the exploration and development of marine oil and gas, and in undertaking intensive research on the genetic environment and deformation mechanism of the Lower Cretaceous salt deposits in the passive continental margin basin of the South Atlantic Ocean. The lower Cretaceous Aptian salt tectonics in the passive continental margin basins on both sides of the South Atlantic Ocean has obvious zonation characteristics, showing the continuous transition from extension to contraction. On the Brazilian side, there are continuous distribution of thick salt deposits, and the strata above and below the salt deform harmoniously. The tectonic deformation of salt began in the Albian when the oceanic crust appeared. The growth of the negative terrain space of the new seafloor triggered the tectonic deformation of salt under the action of sedimentary load during the drift period. With the deposition and subsidence of the passive continental margin, the uplift of the two sides of the land(mantle action), and the long-distance transfer of tectonic stress on the plate boundary(South America), the tectonic deformation under gravity continued in the Neogene. The physical simulation experiment and the discrete element numerical simulations show that the salt tectonic deformation in the South Atlantic is caused by gravity tectonism, and the gravity on the slope forms the extension transition compression salt tectonic zoning of the passive continental margin basin. Local uplift(passive continental margin fault block uplift) disturbs salt flow as salt particles have a large upward velocity component, forming salt plant and salt diapir structure. The front tectonic barrier(caused by the uplift of oceanic crust, SDRs formation, transitional extensional crust, exposed ultramafic mantle, etc.) and the formation pressure(load) above the salt are the main reasons for the decrease of the movement rate of salt rock below the slope and the phenomenon of accumulation and thickening. The salt tectonics on both sides of the South Atlantic Ocean experienced multi-stage tectonic deformation, mainly involving two stages(Albian and Cenozoic) of salt tectonic deformation and flow and its forward spreading to the ocean basin. The mixed action mode of gravity sliding and gravity expansion during the process of sliding along the salt layer can explain the main salt structure style in this area.