Potential-field and two-dimensional seismic analysis of a volcanic rifted margin: implications for crustal architecture and petroleum maturation off the west coast of South Africa

The South African west coast Orange Basin is a volcanic rifted margin formed during the Early Cretaceous break-up of west Gondwana. Volcanic margins are characterized by syn and post break-up magmatism represented by seaward-dipping reflector series (SDRS) as seen on seismic data. Poor seismic signal-to-noise ratio problems at depth compounded with the high impedance contrast of the volcanic successions make it difficult to interpret the geology preserved below them. This research integrates seismic and potential-field data with forward modeling to elucidate the architecture and nature of the rift-related geology. It is demonstrated that the Orange Basin is divided into two rift segments with diachronous seafloor spreading and that continental crust and associated rift basin extend far out to sea beyond the Orange Basin outer high. Furthermore, maturation modeling illustrates that magmatic processes may only have had a short lived and local effect on source rock maturation. The results also demonstrate that free-air gravity and total magnetic field intensity maps are good reconnaissance tools for identifying volcanic successions in rift basins. The documentation of the rift architecture of the Orange Basin provides a starting point for more sophisticated structural mapping of the margin and offers a basis for future work in rift mechanics. Moreover, the results presented herein provide evidence that linear magnetic anomalies on volcanic rifted margins may be confused with true sea-floor spreading chrons with implications of incorrect age estimates of continental drift.