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<p>We have processed the EMAG2v3 observed full field magnetic anomaly (Meyer et al., 2017) using the magnetic potential transformation to make a pseudo-gravity anomaly map for the South Atlantic between 15° S and 40° S. A pseudo-gravity transformation attempts to remove the dipolar complexity of a magnetic anomaly and produce the equivalent gravity anomaly assuming a constant ratio of magnetization to density contrast. We assume that magnetization is induced. Our South Atlantic study area encompasses the major bathymetric features of the Rio Grande Rise (RGR) and Walvis Ridge (WR), as well as the Brazilian and African rifted margins.</p> <p>On the Brazilian continental margin, there are high positive pseudo-gravity anomalies on the São Paulo Plateau (SPP) in the Santos Basin, as well as on the Florianópolis Ridge (FR). The distal Campos Basin also shows high positive pseudo-gravity anomaly. The southern Pelotas Brazilian rifted margin shows negative pseudo-gravity anomaly becoming positive oceanward on the Torres High. In the oceanic domain the Rio Grande Rise (RGR) shows three units of high positive pseudogravity anomalies. Although the RGR presents high amplitude pseudo-gravity anomalies, they are not homogeneous. The Eastern RGR has the most intense and linear N-S anomaly, while its Central unit has a circular pseudo-gravity anomaly and is more constrained in area. The Western RGR has a lower amplitude pseudo-gravity anomaly. The C34 magnetic anomaly region, separating the Eastern and Central RGR, shows a negative pseudo-gravity anomaly. Negative pseudo-gravity anomalies indicate that the assumption of entirely induced magnetization used in the pseudo gravity transformation is invalid and that significant long wavelength remnant magnetization exists. This may indicate heterogeneity of the magnetized layer as well as the effects of magnetic field reversals.</p> <p>On the African plate, very strong positive pseudo-gravity anomalies occur on the inner WR and the SW African continental margin. The positive pseudo-gravity anomalies of the WR and the beginning of the outer SW trending WR “tail” create a very strong continuous positive pseudo-gravity anomaly. Together with the South African rifted margin, it forms a strong positive anomaly with a “7” shape. Westwards of the C34 magnetic anomaly there are no significant large amplitude pseudo-gravity anomalies.</p> <p>The map of the pseudo-gravity has been restored using the GPlates reconstruction software. At 110 Ma, the SPP is near the inner WR and both show high amplitude positive pseudo-gravity anomalies. At 110 Ma, the FR is close to the most distal portion of the inner WR, both showing positive pseudo-gravity anomalies. At 85 Ma, the Central RGR, the western extremity of the inner WR and the start of the WR “tail” show conjugate positive pseudo-gravity anomalies. After the C34 anomaly, seen as an intense negative pseudo-gravity anomaly, the Eastern RGR and its conjugate WR “tail” both show positive pseudo-gravity anomalies and separate at ~ 65 Ma. The pseudo-gravity anomaly map indicates that the RGR and WR comprise distinct units which are correlated across the ocean and which correspond to the multiple oceanic ridge jumps reported in Graça et al. (2019).</p>

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ABSTRACT.

<p>The São Paulo Plateau (SPP) and the Florianópolis Ridge (FR), located on the Santos segment of the SE Brazilian margin in the South Atlantic, are large positive bathymetric features with a combined lateral dimension of approximately 500 km. An important question is whether they are underlain by thinned continental crust or by anomalously thick magmatic crust. Each hypothesis has implications for the breakup of the South Atlantic and the evolution of the overlying saline Santos basin.</p><p>Integrated quantitative analysis consisting of gravity inversion, RDA (residual depth anomaly) analysis and flexural subsidence analysis has been applied to a deep long-offset seismic reflection line running NW-SE across the SPP and FR. Gravity inversion predicts crustal basement thicknesses in the range of 12 to 15 km for the SPP and FR, deceasing to 7-8 km thickness at the extreme SE end of the profile. The SPP and FR are separated by a region of thinner crust approximately 80 km wide. Thinning factors from subsidence analysis for SPP and FR are typically between 0.6 and 0.7.</p><p>RDA values close to zero and a thinning factor of 1 were obtained for the region with 7-8 km thick crust at the SE end of the profile which are all consistent with normal oceanic crust rather than previously interpreted exhumed mantle. This oceanic crust is highly tectonised and corresponds to the location of the Florianópolis Fracture Zone.</p><p>Flexural backstripping and reverse thermal subsidence modelling were performed to calculate palaeo-bathymetry at breakup and give 2.5 km below sea level at the SE end of the profile consistent with this region being oceanic crust. Flexural subsidence analysis applied to base salt shows that the observed base salt subsidence requires a component of syn-tectonic subsidence as well as post-rift thermal subsidence, and that the salt was deposited while the crust was still thinning.</p><p>Joint inversion of time seismic reflection and gravity data to determine the lateral variation in basement density by comparing seismic and gravity Moho in the time domain gives a basement density under the SPP and FR of between 2600 and 2700 kg/m<sup>3</sup>. The same method gives a basement density of 900kg/m<sup>3</sup> for the oceanic crust at the SE end of the profile. The FR basement in the NW shows a basement density similar to that of the SPP while in its SE the basement density is much higher approaching 2950 kg/m3.  We interpret the relatively low basement densities of the SPP with respect to that of oceanic crust as indicating a continental rather than magmatic composition. A similar analysis to determine basement density applied to the Evain et al. (2015) seismic refraction profile in the same location also gives a SPP basement density that supports a continental composition.</p>