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Interplay Between Extension, Salt and Pre-Existing Structure, Offshore Angola


Cretaceous and Tertiary structures in offshore-Angola are often characterized as the result of gravity gliding along a basal Aptian salt detachment. In several areas offshore this deformation can be sub-divided into three general structural domains: 1) up-dip extension and allochthonous raft blocks, 2) down-dip extension and rollovers and 3) an outer fold-and-thrust belt. However, large scale departures from this pattern in the form of anomalous Tertiary grabens with significant displacement and missing section can be documented throughout the region. Recent hydrocarbon discoveries near these structures have motivated the need for detailed understandings of the structural development of these grabens. Using multiple restoration techniques, we describe the interplay of extension and salt movement through time for a Tertiary graben that extends ∼75 km along-strike in offshore-Angola and provide evidence for the control of structural development by underlying, pre-existing structures. 2-D structural restorations of representative sections from 3-D seismic reflection surveys reveal the relative influence of Coulomb collapse and salt evacuation over the development of this graben at multiple time steps with >5 km of horizontal extension and >50% salt loss since mid-Miocene. In addition, our restorations provide a story of paleo-salt thickness with an estimated 2 km of salt at ∼15 Ma and, in places, no resolved salt at present. Furthermore, these restorations show this graben initiated above a prominent basement dip inflection likely due to stress concentrations at the Atlantic Hinge, a prominent tectonic feature along the West African margin. Pre-salt normal faults that offset the base of salt may have also controlled the degree of horizontal extension within this structure. To improve the structural analysis throughout the history of this large structure we performed 3-D geomechanical restorations over structural domains 1 and 2 (as defined above) where they are separated by this prominent Tertiary graben. These restorations provide the ability to investigate the kinematics, strain and salt movement throughout the development of the entire structure along-strike. Additionally, these volumetric restorations allow us to assess how this graben interacted with surrounding structures in three dimensions at several time steps which is difficult to evaluate using only 2-D restoration methods.