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

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.

Sediment supply to the Orange sedimentary system over the last 150My: An evaluation from sedimentation/denudation balance

“…This assumption is contradicted by more recent published data showing a significant amount of Tertiary sediments preserved along the Namibian margin (Maslanyj et al, 1992;Light et al, 1992aLight et al, , b, 1993Stewart et al, 2000; Aizawa et al, 2000). Over 1 TWT second of Neogene sediments are actually locally preserved in the Walvis basin.…”

Evidence of a widespread paleo-pockmarked field in the Orange Basin: An indication of an early Eocene massive fluid escape event offshore South Africa

“…In general, Cenozoic depocenters have an elongated shape parallel to the steep slope and form a thin (50-250 m thick) cover on the continental shelf (Dingle et al, 1983;Light et al, 1992;McMillan, 2003). The Cenozoic-Cretaceous transition is marked by an easily recognizable late Maastrichtian-early Paleocene seismic reflector named reflector D (=Davie) by Emery et al (1975), horizon L by Gerrard and Smith (1982), and reflector SCB-D (Weigelt and Uenzelmann-Neben, 2004;Fig.…”

Seismic stratigraphy and subsidence analysis of the southern Brazilian margin (Campos, Santos and Pelotas basins)

“…Based on previous studies (Rabinowitz and LaBrecque, 1979;Light et al, 1992;Clemson et al, 1997) Meisling et al (2001) suggested that seafloor spreading started in the Cape Basin in the middle Hauterivian (anomaly M9; w130 Ma), in the Walvis Basin (conjugate to the Pelotas Basin) in the early Aptian (w120 Ma) and in the Namibia Basin (conjugate to the southern Santos Basin) in the late Aptian (w113 Ma). Salt deposition was widespread in the Santos and Campos basins and was apparently restricted to a short time span in the late Aptian (Davison, 2007;Karner and Gambôa, 2007;Mohriak et al, 2008;Torsvik et al, 2009).…”