Raft tectonics in the Kwanza Basin, Angola

• Published in: Marine and petroleum geology Vol. 9; no. 4; pp. 389 - 404
• Main Authors: Duval, Bernard, Cramez, Carlos, Jackson, M.P.A.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Oxford Elsevier Ltd 01.08.1992; Elsevier Science
• Subjects: Applied sciences; Crude oil, natural gas and petroleum products; Earth sciences; Earth, ocean, space; Energy; Exact sciences and technology; extension tectonics; Fuels; Geology and geochemistry. Geological and geochemical prospecting. Petroliferous series; Kwanza Basin, Angola; Prospecting and exploration; Prospecting and production of crude oil, natural gas, oil shales and tar sands; raft tectonics; salt tectonics; Tectonics. Structural geology. Plate tectonics; South Atlantic; Angola; Central Africa; Atlantic Ocean; Africa; raft tectonics; extension tectonics; salt tectonics; Kwanza Basin, Angola; Mesozoic; decollement; vertical seismic profiles; Upper Jurassic; salt; ocean opening; Cretaceous; chemically precipitated rocks; evaporites; passive margins; sedimentary rocks; rifting; Salt; Petroleum geology; Seismic sounding; Mechanism of action; Tectonics; Field study
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/0264-8172(92)90050-O

Raft tectonics (tectonique en radeaux) allows the exterme thin-skinned extension of overburden over a décollement 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 × 10 −16 to 3 × 10 −16 s −1.