Break-up Processes in the Presence of Plume Magmatism: New Insights into the Tectonostratigraphic Development of the South Atlantic

Abstract

The conjugate rifted margins of the austral South Atlantic are classically magma-rich and display extremely well developed examples of all the volcanostratigraphic elements commonly observed on magma-rich margins globally: stretched continental crust, inner and outer SDR (seaward dipping reflector) packages, an outer (volcanic) high, a zone of high-velocity lower crust, and relatively thick early oceanic crust. Rifted continental margins are often considered independently due to a paucity of conjugate high-resolution reflection seismic profiles. Here, newly acquired mega-regional, conjugate seismic datasets are, through palaeogeographic reconstructions, considered as they once were; a single basin with a shared geological history. Observations from these seismic data provide new and important insights into the principle mechanisms involved in highly magmatic continental break-up. Through a well-correlated stratigraphic and crustal structure interpretation, a new tectonostratigraphic model is presented. This model, describes the development of magma-rich margins influenced by plume magmatism and may have global applications to equivalent margins. The model consists of four distinct crustal domains; continental, magmatic, oceanic and oceanic plateau, and two important crustal boundaries; the limit of continental crust and limit of oceanic crust. These crustal domains are delineated with respect to, and reflect the effects of, variable melt volume during continental stretching and break-up. The tectonostratigraphic model also describes strongly diachronous post-rift and drift phase subsidence and highlights the role the Walvis Ridge – Rio Grande Rise system played in the separation of the central and austral segments of the South Atlantic Ocean. Analysis of subsidence patterns on both conjugates, compared to the drowning of the Walvis – Rio Grande Ridge systems, reveals intriguing correlations between distribution of major source rock intervals and evaporite deposition in the Lower Cretaceous through time and space. The observations and processes described here underpin the development of a regional petroleum systems model, allowing prediction of regional heatflow through time as well the likely location of source and reservoir lithologies along the entire austral South Atlantic Basin.

AAPG Datapages/Search and Discovery Article #90325 © 2018 AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin, Lisbon, Portugal, May 2-3, 2018