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Universidade Fernando Pessoa

Porto, Portugal

GEOLUANDA 2000

21-25 May 2000
Luanda, ANGOLA

Abstract:

We recognize three main contractional regimes in the Kwanza, Benguela, and Lower Congo Basins:

(1) Late Cretaceous gravity spreading,

(2) Mid-Tertiary uplift, and

(3) Neogene transpression.

Late Cretaceous Contraction During Classic Thin-Skinned Gravity Spreading.

Directly caused by Late Cretaceous gravity spreading and Mid-Tertiary uplift. Tilting of the young continental margin, the ultimate cause of this was thermal subsidence after continental breakup. A broad fold belt was created in what was formerly the lower slope and abyssal plain. The belt comprised small buckle folds spaced 1-2 km apart. Buckling deformed the thin, lower Pinda overburden. Folds died out or widened and coalesced upward. Some anticlines were unroofed by erosion and evolved into large passive diapiric walls. Locally, intense shortening formed narrow, imbricate thrust belts.

Mid-Tertiary Contraction During Widespread Coastal Uplift.

The direct cause of this was uplift of the Angolan coast, mostly in the Oligo-Miocene but continuing into the Plio-Pleistocene. Most of the African margin was epeirogenically uplifted by 1-3 km, which dwarfed the Oligocene eustatic fall of sea level. The ultimate cause of uplift is uncertain. Coastal uplift renewed gravity spreading and superposed contraction on formerly undeformed or extended areas. This created compressionally rejuvenated diapirs, thrusted diapirs, diapirically breached anticlines, and massive overthrusting of a condensed, elevated roof above a spreading and tectonically thickened allochthonous salt wedge.

Neogene Contraction During Local Transpression and Inversion.

The direct cause of this was probably differential movement of reactivated crustal blocks, whose slip directions depended on their orientation. The ultimate cause of reactivation is could be E-W compression of central Africa, imposed by ridge push from the oceans and by mantle-plume spreading in East Africa. Contraction occurred in settings where gravity deformation is implausible, such as on the shelf or uplifts onshore. The contraction was intense but highly localized. Deformation created thrust faults, buckle folds (Tobias), and pinched salt walls (Cegonha).
In all three contractional episodes, pre-existing diapirs acted as weak spots that responded strongly to lateral shortening while adjoining areas of thicker overburden were much less deformed. Hydrocarbon traps in contractional regimes include structural highs (buckle anticlines, rejuvenated diapir roofs, thrusted diapirs), salt seals (allochthonous and overhanging salt), and uplapping sand pinch-outs in the flanks of minibasins. 3-D mapping is required to test the closure of these structures, but these traps are less likely to be breached than are extensional traps.

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