Hybrid Break-Up: Spanning End-Member Models Towards a Full Tectonostratigraphic Understanding of the Central South Atlantic

Abstract

Rifted margins are often considered as end-members; magma-poor, and magma-rich. Investigations into magma-poor break-up processes (crustal hyper-extension, partial serpentinisation and exhumation of sub-continental mantle) have focused on the North Atlantic (e.g. Iberia – Newfoundland Rift System) and Tethyan rifted margins preserved in the Alps. Similarly, the processes of magma-rich breakup (continental crust “replaced” by mafic crust through volumous magmatic addition) and its characteristic SDR sequences have been studied on the North and South Atlantic margins, and onshore Greenland and Mozambique. However, it is becoming clear that in viewing margins as end-members, key developmental stages for many rifted margins are unaccounted for, as is the case for the central South Atlantic. Instead, we should consider the rifted margins as forming a spectrum and think in terms of a new class of margin, the hybrid-rifted margin. ION’s 72,000 km central South Atlantic conjugate BasinSPAN reflection seismic dataset has been utilised to produce a mega-regional tectonostratigraphic model. The model delineates crustal domains along the margins relating to continental stretching and break-up, highlighting diachronous rifting and partitioning of extensional strain across the margin resulting. The data also show how the central South Atlantic cannot be described in end-member terms, but instead should be considered as a hybrid margin. Our interpretation reveals a magma-poor rift beneath the sag-basin transitioning to magma-rich oceanwards. Mapping of the pre-salt sag sequences shows oceanward migration of extension forcing progressively younger strata to become effective syn-rift while their chronostratigraphically equivalents thermally subside (effective post-rift) continentward. We observe an outer high (magmatic construction) at the limit of oceanic crust forming a barrier to authocthanous salt suggesting the switch from magma-poor to magma-rich rifting exacted a fundamental control on the boundaries to the S. Atlantic Salt Basin. We conclude that a model invoking hybrid rifting is most appropriate to describe the Central South Atlantic and show that how it has led to significant implications for subsidence and heatflow through time. Understanding the interplay between tectonics and sedimentation across the margins through time is therefore key to reducing exploration risk in the central South Atlantic.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018