RAFT TECTONICS IN THE KWANZA BASIN, ANGOLA

Raft tectonics (tectonique en radeaux) allows the extreme thin-skinned extension of overburden over a decollement of thin salt or other evaporites. Rafts are allochthonous fault blocks no longer in mutual contact. In the Kwanza Basin, the type area for raft tectonics, Jurassic-Lower Cretaceous rift fill was succeeded by a cratonic Aptian lowstand progradational wedge. At about 119 Ma, the Massive Salt capped this wedge just before the South Atlantic Ocean began opening. Active spreading caused a eustatic sea level rise and the accumulation of transgressive systems tract carbonates. About 200 km of downdip space for extension on the tilted continental margin was created, mainly by the glide of allochthonous rafts onto fresh oceanic crust. At about 110 Ma, the overburden began to extend when only a few 100 m thick, forming many small, tilted, phase 1 rafts. These older rafts were yoked together by Upper Cretaceous sedimentation before rupturing into huge, non-rotated glide blocks during phase 2 rafting from 55 to 10 Ma. Tertiary sediments accumulated asymmetrically in strike-parallel depocentres created by deep, widening grabens between phase 2 rafts. These sediments rest directly on salt or subsalt strata with a tectonic jump of 60-90 Ma. Strain rates for both phases of rafting varied from 2 x 10(-16) to 3 x 10(-16) s-1.