Crustal structure of the conjugate Equatorial Atlantic Margins, derived by gravity anomaly inversion

Abstract

The crustal structure of the Equatorial Atlantic conjugate margins (South America and West Africa) has been investigated using 3D gravity anomaly inversion, which allows for (1) the elevated geothermal gradient of the lithosphere following rifting and break-up and (2) magmatic addition to the crust during rifting and break-up. It is therefore particularly suitable for the analysis of rifted margins and their associated ocean basins. Maps of crustal thickness and conjugate-margin stretching, derived from gravity anomaly inversion, are used to illustrate how the Equatorial Atlantic opened as a set of stepped rift-transform segments, rather than as a simple orthogonal rifted margin. The influence of the transform faults and associated oceanic fracture zones is particularly clear when the results of the gravity anomaly inversion are combined with a shaded-relief display of the free-air gravity anomaly. A set of crustal cross-sections has been extracted from the results of the gravity inversion along both equatorial margins. These illustrate the crustal structure of both rifted-margin segments and transform-margin segments. The maps and cross-sections are used to delineate crustal type on the margins as (1) inboard, entirely continental, (2) outboard, entirely oceanic and (3) the ocean-continent transition in between where mixed continental and magmatic crust is likely to be present. For a given parameterization of melt generation the amount of magmatic addition within the ocean-continent transition is predicted by the gravity inversion. One of the strengths of the gravity-inversion technique is that these predictions can be made in the absence of any other directly acquired data. On both margins anomalously thick crust is resolved close to a number of oceanic fracture zones. On the South American margin we believe that this thick crust is probably the result of post-break-up magmatism within what was originally normal-thickness oceanic crust. On the West African margin, however, three possible origins are discussed: (1) continental crust extended oceanwards along the fracture zones; (2) oceanic crust magmatically thickened at the fracture zones; and (3) oceanic crust thickened by transpression along the fracture zones. Gravity inversion alone cannot discriminate between these possibilities. The cross-sections also show that, while 'normal thickness' oceanic crust (c. 7 km) predominates regionally, local areas of thinner (c. 5 km) and thicker (c. 10 km) oceanic crust are also present along both margins. Finally, using maps of crustal thickness and thinning factor as input to plate reconstructions, the regional palaeogeography of the Equatorial Atlantic during and after break-up is displayed at 10 Ma increments.Supplementary material: Detailed illustrations of the crustal-thickness mapping, the crustal cross-sections and the plate reconstructions are available at: https://doi

Evaluating the segmented post-rift stratigraphic architecture of the Guyanas continental margin

“…As introduced earlier, the 'basement' in this study is defined as the crust plus pre-and syn-rift sediments, that exist below the Central Atlantic post-rift succession. Previous studies (Mercier de Lépinay et al, 2016;Reuber et al, 2016;Kusznir et al, 2018) have characterised this interval, which is beyond the scope of this paper. However, a brief overview is provided summarising the evidence for the two models of basement composition.…”

“…The onshore geological map is from the Geological Map of South America (CGMW, CPRM, DNPM, 2003). Structural features are mapped after Gouyet et al (1994), Yang andEscalona (2011), Reuber et al (2016), Sapin et al (2016), crustal thickness modelled from 3D gravity anomaly inversion by Kusznir et al (2018), predicted Jurassic graben offshore (Griffith, 2017), and volcanics from Gouyet et al (1994) and Mourlot (2018). Fig.…”

“…Surrounding the Demerara Rise thinned continental crust (Kusznir et al, 2018) and volcanic basement composed of seaward dipping reflectors (SDRs; Reuber et al, 2016), the oceanic crust displays a diachronous accretion due to the dual-phase rifting history. Within the Guyana-Suriname basin the oceanic crust is inferred to be Jurassic in age, whereas to the north and east of the Demerara Rise, in the Equatorial Atlantic domain, oceanic crust is at least 45 Myr younger, accreted during the Upper Cretaceous (Fig.…”

Superimposed Rifting at the Junction of the Central and Equatorial Atlantic: Formation of the Passive Margin of the Guiana Shield

“…This rifting resulted from continental scale right‐lateral shearing between South America and Africa and formed an en‐échelon rift system (e.g., Benkhelil et al., 1995; Guiraud et al., 1992; Mascle et al., 1988; Ye et al., 2017). It produced a complex 3D lithospheric and crustal stretching pattern (e.g., Kusznir et al., 2018; Ye et al., 2019). Benkhelil et al.…”

“…Previous geological studies of the Equatorial Atlantic rift have mostly investigated the development of individual basins along the African rifted margin (e.g., Antobreh et al, 2009;Basile et al, 1998;Olyphant et al, 2017;Scarselli et al, 2018) or its conjugate south American counterpart (e.g., Loncke et al, 2016;Sapin et al, 2016;Yang & Escalona, 2011). Kusznir et al (2018) documented, at continental scale, the variability of the crustal structure along both margins of the Equatorial Atlantic. Ye et al (2019) have shown that the necking width of the African margin segments depended primarily on their obliquity to extension direction.…”