SALT STRUCTURES ON THE WEST-AFRICA PASSIVE CONTINENTAL MARGIN AND THEIR GENETIC MECHANISM

The styles of salt structures developed on the west-Africa margin and their evolutionary mechanism and controlling factors are studied upon 3-D seismic data. The thin-skin structures were formed due to the gliding of the overlying strata,characterized by a compressional deformation zone in the frontier area and an extensional deformation zone on the backside, with a transitional zone in between. Salt rafts, pre-rafts and monocline structures develop in the extensional area, whereas the transitional deformation zone is characterized by various extensional diapirs. Squeezed diapir and allochthonous sheets are well developed in the compressional deformation zone. Gravity gliding is the driving force of the salt tectonics on the passive margin throughout the forming time. Gravity spreading derived from the differentiation of sediment load significantly controls the salt tectonics in the stage while the margin is mature, but weak in the early time. Tectonic activity at the continental margin is the main factor which triggers salt tectonics which starts after the cessation of tectonic activities. Differentiation of sediment load influences the decollement of strata overlying the salt, and the topography below the salt layer has significant impacts on the salt flowing, the slipping velocity of the strata,the generation of related faults and the responses of sedimentation.