Base‐salt relief controls salt‐related deformation in the Outer Kwanza Basin, offshore Angola

Abstract

AbstractSalt‐influenced passive margins are widespread and commonly hydrocarbon‐rich. However, they can be structurally complex, with their kinematic development being poorly understood. Classic models of salt tectonics divide such margins into updip extensional, mid‐slope translational and downdip contractional kinematic domains. Furthermore the faults, folds, and salt walls associated with each kinematic domain are typically assumed to form perpendicular to the maximum principal stress, which in gravitationally driven systems means broadly perpendicular to base‐salt dip. We use high‐resolution 3D seismic reflection data from the Outer Kwanza Basin, offshore Angola to show that these models cannot explain the diversity of salt structures developing on passive margins, especially those defined by considerable relief on the base‐of‐salt surface. Overburden seismic‐stratigraphic patterns record the basinward translation and rotation, allowing us to reconstruct the origin and evolution of the salt structures. We show structures in the transitional domain of the Outer Kwanza Basin display three dominant trends, each characterised by different structural styles: (a) salt walls perpendicular to the overall base‐salt dip, (b) salt walls parallel to the base‐salt dip and (c) salt walls oblique to the base‐salt dip. We show that each set of walls has a unique history, with synchronous phases of extension and compression occurring in adjacent structures despite their close spatial relationship. Our analysis suggests that, in the Outer Kwanza Basin, the structural evolution of the salt and overburden is predominantly controlled by translation over relief on the base‐salt surface formed above fault scarps associated with a preceding phase of rifting. Changes in the downdip volumetric flux and velocity of the salt over topographic features can cause local extension or contraction of the salt and its overburden, associated with local acceleration or deceleration of the salt, respectively. This interaction with base‐salt relief creates locally variable stress fields that deform the salt and its overburden, overprinting the broader, margin‐scale salt tectonics typically associated with gravity gliding and spreading.