New geophysical evidence for extensional tectonics on the divergent margin offshore Namibia

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Abstract 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.