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Tectono-stratigraphic evolution and variability of the offshore Central South Atlantic basins --- Implications for margin evolution models

Author(s): Simon Higgins Statoil A.S.A., Norway
Grégoire Messager Statoil A.S.A.
Ian Sharpe Statoil A.S.A.
Mark Scott Statoil A.S.A.
Ulrike Freitag Statoil A.S.A.
Heike Gröger Statoil A.S.A.

Significant differences exist between current models addressing the mechanism of lithospheric thinning on rifted continental margins. Our understanding of these systems has traditionally been data-limited due to a number of factors, including the deep-water setting and geophysical imaging challenges. Recent acquisition of high-quality 3D seismic over super-regional swathes of distal rifted margins along with frontier deep-water drilling campaigns, provide new observations that can help constrain established models. In particular, the increased density and resolution of seismic and grav-mag data has allowed continuous interpretation over large areas and has highlighted the dramatic down-dip and along strike variability in rift-to-drift structural style and evolution. Consequently, recent observations blur much of the distinction between established conceptual end-member models of passive margin formation.

In this contribution we present a conjugate margin lithospheric-scale case study from the Central South Atlantic, and address the structural evolution from the proximal rift shoulder to the oceanic crust. Striking first-order similarities in the spatial and temporal progression of extensional deformation allows the definition of mega-regional structural domains, which can be mapped along both margins. Down-dip, domains show a similar progression to those documented in published schemes (e.g. Peron-Pinvidic et al. 2013); from proximal areas with limited crustal thinning, to lithospheric necking zones, to the distal margin where structural style and crustal geometries change dramatically. The Namibe rifted margin, Angola, is used to demonstrate domain development and definition, as well-exposed Cretaceous outcrops provide detailed tectono-stratigraphic observations (Sharp et al., 2016, this conference) that can be linked to the offshore.

The down-dip progression through distinctive domains observed in the Namibe Basin is also present in the neighbouring Benguela, Kwanza and Congo Basins to the north, as well as in the conjugate Santos, Campos and Espirito Santo Basins offshore Brazil. There are, however, important differences. At the basin scale we see variations in factors such as margin width, symmetry and orientation, subsidence history and accommodation space. Whilst within individual domains, characteristics such as fault style, depositional facies, distal margin geometry, timing of deformation and degree of volcanic addition can vary rapidly along strike. Some changes are gradual, such as sediment thickness and “sag” distribution, whilst others are abrupt, such as differences in pre-salt structural style across transfer zones. In the Distal Margin such dramatic variability along strike mean elements of both classic “magma-poor” and “magma-dominated” margins can be observed within tens of kilometers in the same basin.

Although the presented tectonic scheme is primarily based on South Atlantic basins, initial observations from the North Atlantic, and comparision to examples from published literature clearly indicate that rapid strike variability in structural style is a theme common to many rifted margins. Our understanding of rift-to-drift margin formation is evolving quickly as more data becomes available, and these new data and observations will require a revision of existing “end-member” models. The significance of model revision is manifold; from the fundamental mechanism of lithospheric thinning, to understanding palaeo-geographic evolution, to improved facies prediction, to better heat flow predictions through time.

Tectono-stratigraphic evolution and variability of the offshore Central South Atlantic basins --- Implications for margin evolution models
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