High quality seismic data recently acquired offshore Namibia have revealed the complex interplay between sedimentation and rifting that accompanied the breakup of Gondwanaland. Seismic sequence analysis has demonstrated a complex series of superimposed phases of rifting. Jurassic early-rift sediments were deposited in a Basin & Range setting with marine embayments. Mid- to Late Jurassic extension probably culminated in acidic to intermediate volcanicity. Subsequently, late-rift fill sedimentary accumulations were terminated locally by a major period of plateau-forming effusive activity. This was coincident with the start of Early Cretaceous spreading in the South Atlantic. Shelf, slope and basinal facies are clearly defined in the post-rift succession. Although virtually undrilled, the area is considered to have very considerable petroleum potential. Seismic stratigraphy will continue to play a vital role in defining the critical juxtapositions of source rock, reservoir and seal.
Nine Mesozoic‐Tertiary basins of different orientations lie along the Argentine continental margin, over a distance exceeding 2,000 km: these are the Salado, Colorado, Valdes, Rawson, San Jorge offshore, North Malvinas (San Julian), West Malvinas and Magallanes (Austral) Basins on the Continental Shelf, together with the Continental Slope. These basins formed following the latest Jurassic — Early Cretaceous extension that accompanied the onset of South Atlantic rifting. Strain was modified by earlier basement fabrics, with consequent transtension. The Cape Fold Belt (Permo‐Triassic) provided a NW‐SE grain to pre‐Mesozoic cover off NE Argentina. In the central‐southern sector, several phases of oblique NEwards Pacific subduction and terrane accretion during the Permo‐Triassic formed back‐arc basins and volcanic belts, producing a more variable fabric orientated close to NNW‐SSE. Atlantic basin fill, of Lower Cretaceous‐Tertiary clastics, was deposited as a result of rift‐shoulder erosion and Atlantic flooding, following eastward tilting. Basin fill thickness is typically 2–4 km, but locally exceeds 6km. The hydrocarbon potential of these basins hinges as much upon the preservation of source rocks within the pre‐rift succession as it does on that of those within the base‐rift succession, and subsequent Atlantic anoxic events.
Over 14 000 km of high-resolution multifold seismic data together with gravity and magnetic data provide the opportunity to examine the mechanism and history of rifting on the Namibian continental margin. The region is a completely developed divergent margin containing a pre-rift megasequence of interior cratonic sag origin; two rift basin megasequences dominated by siliclastic deposition; a transitional megasequence and thermal sag megasequences dominated by overlapping progradational wedges. In the Orange Basin acid to intermediate volcanic rocks erupted at the end of the pre-rift phase in Mid- to Late Jurassic times and this was followed by regional uplift. Basaltic volcanic activity was associated with the synrift phases and widespread volcanic rocks developed during the second synrift event, related to the Tristan da Cunha-Walvis Ridge mantle plume. Regional seismic mapping indicates that rifting migrated from south to north with time. The geophysical data enable the recognition of major structural elements which include the Eastern Graben Province, the Medial Hinge Line, the Central Half Graben and the Marginal Ridge. The geometry of the rift basin is asymmetric and can be explained by a simple shear mechanism. South of Walvis Ridge extension was accommodated by movement along a major normal fault which is listric at depth. The basin depocentre represented by the Central Half Graben lies landward of this major fault and is offset from the area of thinnest crust as interpreted from the gravity data.
The 600-km-long Limpopo Mobile Belt is discussed within the frame of a Proterozoic supercontinent model [Piper, 1976]. Evidence is presented that the Rhodesian and Kaapvaal cratons may have been separated by distances of more than 1000 km of oceanic cmast. From about 3350 Ma ago the Kaapvaal Craton appears to have been driven intermittently N, NW, and then WNW against the RhodesJan Craton forming the NE-SW trending collision zone, the Limpopo Mobile Belt, and all the major fold and fracture patterns found. This movemen• would be similar to the oblique movement of the Pacific plate into the Aleutian trench. When collision ceased around 2500 Ma ago, it is likely that the Great Dyke and other complexes intruded along release fractures formed at right angles to the compression. faults. The rocks within the LMB may be broadly subdivided into a garnet-free basement (Sand River Gneisses, R.S.A.) overlain by highly metamorphosed and complexly deformed garnet-bearing cover gneisses, the Beltbridge Group (Gumbu, Malala Drift, and Mount Dowe Groups, R.S.A.). These are intruded by the Ultramafic and Anorthosite Suites (Undifferentiated Messina Suite in R.S.A.) and the charnockitic enderbitic, Singelele, and Bulai gneisses (porphyritic granites) (Figure 1). Burke and Dewey [1972], Dewey and Burke [1973], and Burke et al.
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