AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Tectonic evolution of the Kwanza basin, offshore Angola

Abstract

The Kwanza basin is a 300km wide basin located in the Angolan margin and was formed as the result of the Late Cretaceous opening of the South Atlantic Ocean. This margin has been subject of multiple regional-scale studies on account of its possible evolution as a volcanic-poor hyperextended margin and its pervasive salt tectonism. Our study -based on seismic interpretation, well data, potential field modelling and sequential restoration,- presents a new vision of this margin. We contend that the Kwanza basin is possibly not a ‘pure’ example of volcanic-poor hyperextension and explore the possible mechanisms of vertical motion on the margin to account for present-day and past bathymetry. Crustal-scale domains typical of Atlantic margins can be defined in the Kwanza basin. In the shallow offshore, a proximal domain is characterized by a thick basement (up to 20km) with the Moho lying at around 25km depth. The Moho progressively rises up to 15km depth across a necking domain, where basement extensional faults are interpreted to cross-cut the entire crust. A distal domain defined further offshore is characterized by rotated basement blocks with a relatively flat Moho that lies at around 12-13km depth. A surprising element of this margin is the presence of a thick Early Cretaceous volcanic unit, that marks the transition between the distal domain and the oceanic crust. This volcanic unit is observed to extend parallel to the margin, along the continent-ocean transition and is interpreted to represent proto-oceanic volcanics. The proto-oceanic volcanics define the most distal margin of the Aptian evaporite basin. We have encountered no evidence of exhumed mantle in this margin. A structural evolutionary model has been constructed based on a regional cross-section. This model is based on sediment backstripping, compensating flexural isostasy, thermal subsidence, fault movement and halokinesis. The sequential restoration reproduces the bathymetric evolution of the basin from an original early rift geometry of the conjugate Angolan-Brasilian margin to the present day passive margin state. Continuous extension and syn-rift deposition is observed until the emplacement of the proto-oceanic volcanics and deposition of the Aptian evaporite bsain. The evaporite basin was originally continuous across the margin and it was only separated from the conjugate Campos Basin (Brasil) by the proto-oceanic volcanics. Based on our reconstruction, these evaporites where deposited in a shallow water setting, and their genesis was possibly linked to the proto-oceanic volcanism. Halokinesis was triggered immediately after oceanic breakup due to oceanward tilting of the margin, synchronous with the deposition of Albian carbonates. Continuous halokinesis from Albian to present time, created depocenters of Upper-Cretaceous and Tertiary sediments due to downbuilding and salt withdrawal. Progressive subsidence and tilting of the entire margin led to allochthonous salt being emplaced over oceanic-domain sediments during the Cretaceous.