The Namib Rift: a rift system of possible Karoo age, offshore Namibia

Abstract

Abstract This paper describes a hitherto poorly documented, deep-seated rift system offshore Namibia, here named the Namib Rift. The structural history and rift geometry of this system are determined from reflection seismic data. No wells penetrate the rift-fill succession but from its stratigraphic context, regional thickness relationships and by analogy with onshore Karoo stratigraphy, the age of the rift is interpreted as broadly equivalent to the Karoo Supergroup. The rift is divisible into two major arms, with rift segmentation evident in the north. An angular unconformity separates a rift and post-rift sequence. This is interpreted as a Mid-Triassic hiatus related to a regional compressional events. The basement comprises the Pan-African Fold Belt along the coast, with the Damaran arm extending inland. In the Cape Cross-Walvis Bay area, the Autseib Lineament separates the Kaoko and Damaran arms. This lineament, and the adjacent basement fabrics, are shown to have strongly influenced rift geometry in this area. South of the Autseib Lineament, Damaran basement fabrics largely determined fault trends within the Namib Rift. To the north, Kaoko basement fabrics strongly influenced basement extensional faulting. This area was also subjected to Mid-Triassic compression. South of the Autseib Lineament, large-scale uplift was distributed across large folds whereas to the north, uplift occurred close to pre-existing faults. North-northeast directed compression was common to both areas, but the Autseib Lineament partitioned the structural response. McKenzie-type pure shear extension is favoured as the rift mechanism, with Mid-Triassic uplift modifying thermal subsidence. The final compressive event of the Cape Orogeny, coeval with the regional Mid-Triassic hiatus, terminated rifting in offshore Namibia.