The fundamental role of the Borborema and Benin–Nigeria provinces of NE Brazil and NW Africa during the development of the South Atlantic Cretaceous Rift system

Highlights

• The structural inheritance of the Neoproterozoic mobile belts controlled the process of lithospheric rupture of West Gondwana.

• This process began and ended around the Borborema and Benin–Nigeria Provinces.

• The onset of rifting, and the onset of sea floor spreading are diachronic complex, and spatiotemporal.

• At the space domain, six Structural Segments (or groups of marginal basins) were recognized, genetically linked with oceanic transform faults.

• At the time domain, five chronological rift stages were recognized as the result of sorting in time thirty-three relevant tectonic related geological events.

Abstract

The Borborema and Benin–Nigeria Provinces are composed of a complex network of Neoproterozoic mobile belts, and large-scale shear zones. The evolution of the South Atlantic Cretaceous Rift System was controlled by the structural inheritance of the Borborema and Benin–Nigeria Provinces. The process of lithospheric rupture began and ended around the Borborema and Benin–Nigeria Provinces, which behaved as a “lithospheric relay ramp” and act as the main obstacle to the opening of the South Atlantic. The onset of rifting, and the onset of sea floor spreading are diachronic complex, and spatiotemporal. At the spatial domain, the basins can be described by groups of marginal basins with genetic links to eleven oceanic transform faults, distributed along three rift branches and twelve rift zones, grouped at six structural segments and aborted rift arms. In the time domain, five chronological rift stages are here identified as the result of sorting 33 “tectonic-related events”, that affect the tectono-stratigraphic record of each one of these basins. Oceanic crust initiation was diachronic with emplacement initiating at three times from south to north: (i) at 130 Ma, seafloor spreading begin south of Florianópolis fracture zone; (ii) after salt deposition (113 Ma), between Florianópolis and Bode Verde fracture zone; and (iii) at the end of Albian (~110 Ma), between Bode Verde and Chain fracture zone, and at the entire Equatorial Branch. Not until the end of Albian (~110 Ma) a full, continuous, and stable mid-ocean ridge was established. The time and space distribution of LIP's and the St. Helena and Tristan da Cunha hotspots trails on the Brazilian side allowed us to corroborate these timings within our model. Upper crustal structures did not play a decisive role in the rift compartmentation at the Equatorial and Orthogonal branches, where lithospheric processes controlled the final plate boundaries.

Introduction

This is the second of a set of three papers dealing with the relevance of the Borborema and Benin–Nigeria provinces of NE Brazil and NW Africa during the opening of the South Atlantic. The first one (Matos et al., 2021) focused on the Pernambuco and Paraíba basins in Northeast Brazil, where it was summarized, reviewed, and presented new developments on the understanding of this segment of the South Atlantic Margin. The Pernambuco and Paraíba basins were developed at the heart of the Borborema and Benin–Nigeria provinces (BBNP) which inheritance played a key role during late rifting stages, with the development of an anomalous wide transition between the first true oceanic crust and the extended continental crust, explored by this research. The third one (Matos, 2021) focused on a tectonic analysis of the Cretaceous magmatism in South America, addressing Large Igneous Provinces (LIP's) and the hotspot trails of Tristan da Cunha and St. Helena plume heads on continental and oceanic crusts.

Other objective of this paper is to present a revised interpretation of South Atlantic opening using a summarized model to honor accumulated geologic observations along the South Atlantic Margin and to clarify the evolution of the South Atlantic Cretaceous Rift System (SACRS) in time and space. Fig. 01 illustrates a Late Jurassic/Early Cretaceous plate reconstruction emphasizing the main geological features that influenced the lithospheric rupture process. The black-dashed polygon refers to our area of interest, encompassing the South Atlantic Cretaceous rift basins. A List of Gondwana's main tectonic features and the acronyms used at this paper is presented in Table 1.

Although the basic geometric fit between Africa and South America has been recognized since Wegener's pioneering work, a precise fit is still debated. One of the first quantitative fits was by Bullard et al. (1965). These authors used the 500 fathom isobaths as a proxy for the edges of the continents and found an acceptable fit without any internal deformation of the continents. As plate tectonic theory developed and the concept of ocean-continent boundaries as defining the edges of continents emerged, it became apparent that the fit between Africa and South America was not perfect (see summary in Heine et al., 2013). The main problem was found to be reconciling the fit in the equatorial Atlantic with that in the far southern Atlantic, between South Africa and Argentina. If the equatorial fit is tightened, there is a gap in the south and if the south is tightened, there is a gap in the north. One way to at least partly reconcile this is by considering intra-continental deformation, so that the shapes of Africa and/or South America are altered. Again, Heine et al. (2013) is referred to as an excellent summary of the attempts. Based on a different approach, Pérez-Días and Eagles (2014) modelled plate divergence based on the growth of seafloor spreading data. They claim to have achieved better fits between Africa and South America. However, uncertainties exist on age, as well as on the type of oceanic crust sampled. Transitional proto-oceanic crust can be potentially interpreted as pure oceanic crust, and vice-versa, introducing uncertainties in the models.

Uncertainties on South Atlantic reconstructions come from age control and how the lithospheric boundaries change through time. Ideally, plate reconstructions should match isochrons such as sea floor spreading magnetic lineations.

Earlier South Atlantic plate reconstruction using COB matching result in incorrect reconstruction. The COB in the south, with recognizable sea floor spreading lineations outboard of it (Rabinowitz and LaBrecque, 1979), is clearly older than the COB in the equatorial zone, where there are no lineations. This effect is partly alleviated by using restored COB's for the fit, i.e., restoring COB's to their pre-rift location relative to their adjacent continents (Heine et al., 2013). We follow the Heine et al. (2013) methodology and take deformation within Africa into account but use a tight fit in the far southern Atlantic that results in a gap north of the equatorial zone. This gap lies between Demerara Rise and Guinea Plateau (Fig. 1), interpreted by Reuber et al. (2016) as filled by a Jurassic volcanic margin. A little further forward it will be discussed in detail how this gap in the reconstruction at 145 Ma was later closed.

One of the challenges for South Atlantic plate reconstructions is that the continent-ocean boundary (COB) is not an isochron, and COB's does not define the edges of rigid oceanic lithosphere (Eagles et al., 2015). Norton at al. (2016) uses the edges of SDR's to map the limits of oceanic crust. Mapping the limits of a true oceanic crust would avoid all the pitfalls of mapping the boundary of a commonly magmatic loaded-transitional crust. However, it is debated whether SDR's are sometimes volcanic filled rift compartments, or an isostatic response to a magmatic loading, which implies it can develop on top of continental crust and/or at a transitional-proto-oceanic crust. Paton et al. (2017) propose that SDRs form as portion of the uppermost part of proto-oceanic crust. Regarding nomenclature on continental breakup, we follow the terms used by Alves et al. (2020), recognizing the complexities of Intraplate Continental Margins (ICM's). Zalan et al. (2011), based on seismic, gravimetric and magnetometry data at Santos, Campos and Espírito Santo basins, typified this segment of the margin as a magma-poor passive margin (or magma-poor ICM)), suggesting a continuous belt of exhumed mantle at the transition between hyper extended continental crust and a true oceanic crust. Klingelhoefer et al. (2014), based on combined wide-angle and reflection seismic data, suggested a proto-oceanic crust beneath the easternmost Santos Basin, disregarding mantle exhumation at this segment of the South Atlantic. Schnürle et al. (2019) suggested the common presence of exhumed lower continental crust and proto-oceanic crust at the Santos Basin, but acknowledging that the dynamic interaction of lower crust and upper mantle at the transition from rifting and true sea floor spreading remains questioned. We will refer to this transitional domain as the ocean–continent transitions (OCTs).

We present a multidisciplinary research that integrates published data, with diverse data-resolution and levels of uncertainties on the time and space domain. Plate reconstructions from the Plates Project (UTIG) are used as a template, to discuss key evolutionary stages of the South Atlantic.

The Neoproterozoic mobile belts of the Borborema and Benin–Nigeria provinces of NE Brazil and NW Africa (BBNP) are crucial areas in the amalgamation of West Gondwana (Arthaud et al., 2008). This research discusses how these different types of inheritance may have controlled subsequent rifting and drifting events at the SACRS. When describing structural inheritance, the development of rift systems and the rifted margins of the South Atlantic, we are following the terminology of Manatschal and Müntener (2009), Manatschal et al. (2015), and Peron-Pinvidic et al. (2013). Manatschal et al. (2015) and Tugend et al. (2015) show how different types of inheritance control propagating rift/oceanic opening in the Alpine Tethys, Pyrenean–Bay of Biscay and Iberia–Newfoundland rift systems. Manatschal et al. (2015) recognized how different types of inheritance may control rifting events. A wide-variety of lithospheric extensional models exist along South Atlantic margin, and that do not necessarily fit the end members of magma-poor or magma-rich ICMs. Through a basin-to-basin analyses of the South Atlantic margin, we recognize two rift branches, named Dextral Equatorial Branch (DEB - from Demerara-Guyana Plateau to Chain FZ), and the Sinistral Southern Branch (SSB- from Kribi FZ to Florianópolis FZ), separated by a third branch, the Orthogonal Branch (OB- between Chain and Kribi FZs).