Rifting and Breakup in the South Atlantic – A Story of Ups, Downs, Magma, Water and Salt as Seen from a Mega-Regional Conjugate Seismic Dataset

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 interpreted to reconstruct the margins, treating them holistically as a single system with a shared but highly variable geological history. The re-evaluation presented here suggests that end-member models are not adequate to explain the development and instead, the basins and margins should be considered to exist on a spectrum of magma supply with variations in time and space. The dominant influences on volumes of intrusive and extrusive magmatism during rifting and breakup are underlying asthenospheric potential temperature and extension rates. We show how the kinematics of rifting and variations in the amount of magmatism and dynamic support during and after rifting affect the vertical motions at the surface through time and space placing clear and critical controls on the tectonostratigraphy and on the distribution and burial of critical source, reservoir and evaporite units in the South Atlantic. South of the Walvis Ridge – Rio Grande Rise (WRRGR), the margins on both African and South American sides are classically magma-rich and broadly symmetrical, resulting in much of the syn-rift period being dominated by sub-aerial conditions and voluminous extrusive magmatism and the formation of extensive magmatic seaward dipping reflector packages. Initial marine incursions can be demonstrated to occur away from active volcanic rift centres producing restricted, shallow marine conditions in well-defined basins during the Barremian to Aptian. During the post-rift, when additional subsidence occurs, a dominantly clastic shelf/slope system develops and buries the main source interval. During this phase there is significant differences between the now separated basins on the African and South American margins. Immediately North of the WRRGR we observe a significant dextral offset to the eventual spreading ridge location with the broad Sao Paulo plateau conjugate to the narrow Namibe basin. We demonstrate the critical role of this system along with the WRRGR itself in forming a barrier during the Lower Cretaceous and the implications for to the formation of the Great South Atlantic Salt. Further north still, we observe evidence for diachronous rifting, both in the dip and strike directions as well as a marked change from a magma-poor setting to a more magma-rich setting just prior to eventual break-up. These variations in rift architecture have a marked effect on the deposition and post-depositional deformation of the Aptian salt units. We discuss the importance of establishing a broad regional understanding of the kinematics and magmatic history of rift events when attempting to reduce exploration risk in the rift basins.

AAPG Datapages/Search and Discovery Article #90366 © 2020 AAPG Middle East Region Geoscience Technology Workshop, Rift Basin Evolution and Exploration: The Global State of the Art and Applicability to the Middle East and Neighboring Regions, Bahrain, February 3-5, 2020