Spanning the End-Member Break-Up Models: Towards A Full Tectonostratigraphic Model for the South Atlantic From Conjugate Margin Data

Abstract

Rifted margins are often considered independently and in terms of magma-poor and magma-rich end-members. Here, a mega-regional South Atlantic conjugate seismic dataset is used to reconstruct the margins, treating them as a single basin with a shared geological history.

Magma-poor breakup has often been proposed for the central S. Atlantic, while magma-rich breakup has been described in the Southern S Atlantic. Here, a re-evaluation of S. Atlantic data suggest that end-member models are not adequate to explain the development. Instead, they should be considered as hybrid-rifted margins existing on a spectrum of magma supply with variations in time and space.

Our observations provide insight into the mechanisms of continental break-up, including variable amounts of magmatism and dynamic uplift and their effect on margin tectonostratigraphic development. Well-correlated stratigraphic and crustal interpretations along the margins highlight diachronous rifting, variable amounts of magmatism and partitioning of extensional strain across the margin. Our crustal model consists of five crustal domains (continental, hyper-extended, magmatic, oceanic and oceanic plateau), and two important crustal boundaries; limit of continental crust (LoCC), and limit of oceanic crust (LoOC).

South of the Walvis Ridge – Rio Grande Rise (WRRGR), margins are classically magma-rich with type examples of volcanostratigraphic elements well-developed. However, north of the WRRGR elements of both end-member models are observed at different times and locations. We describe the central S. Atlantic as a hybrid-rift, with a hyper-extended magma-poor pre-salt rift becoming more magma-rich oceanwards. A demonstrably magmatic outer high at the LoOC forms a barrier to autochthonous salt suggesting the switch from magma-poor to magma-rich rifting exacted a control on the boundaries to the S. Atlantic Salt Basin.

We also show the role the WRRGR played in forming a barrier separating the segments of the S. Atlantic during the Lwr. Cretaceous and how renewed extension during latest Aptian allowed northward flow of seawater, through what we term the “Walvis Straits” contributing to the formation of the Great South Atlantic Salt.

We discuss the implications that a hybrid rifting model has for subsidence and heat flow through time and conclude that understanding the interplay between tectonics, magmatism and sedimentation across the margins is key to reducing exploration risk in the South Atlantic.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019