Slope tectonics-comparisons and contrasts of structural styles of salt and shale tectonics of the northern Gulf of Mexico with shale tectonics of Offshore Nigeria in Gulf of Guinea

Regional structural styles on the continental slope are revealed by well-imaged deep multi-channel seismic profiles from northern Gulf of Mexico and Offshore Nigeria in Gulf of Guinea. Both provinces are situated on passive continental margins developed during the breakup of Gondwana. The northern Gulf of Mexico as well as Offshore Nigeria are underlain by complex systems of thin skinned upslope extensional fault systems, downslope contractional fold and thrust systems along with large scale salt or shale structures. The interactions between the deformation of mid-Jurassic salt and various stages and patterns of sedimentation on the continental slope are responsible for both extensional growth faults and Tertiary contractional fold and thrust systems in the northern Gulf of Mexico. The slope tectonics style of the northern Gulf of Mexico passive margin is characterized by perennial upslope extension since mid-Jurassic, associated with multi-stage autochthonous and allochthonous salt deformations along with brief periods of late Tertiary shortening in the down-dip fold belts. Offshore Nigeria is a salt-free progradational margin characterized by upslope extension, various shale structures and downslope toe-thrust systems. Gravity spreading and gliding in response to rapidly prograding Tertiary sedimentation over under-compacted shales resulted in the structural styles of the slope of Offshore Nigeria. Large upslope extensional tectonics are linked to coeval and balanced downslope contractional deformations in the form of shale ridges, massifs and diapirs, and deep basin toe thrusts. Late Miocene-Early Pliocene contractional deformation is widespread across the entire foldbelt, and with later shortening, folding migrated in a landward direction.