Pseudo-Gravity Transformation of Magnetic Anomaly Data for the South Atlantic and Adjacent Continent: Implications for Crust and Lithosphere Composition

The transformation of magnetic anomalies to magnetic potential or pseudo-gravity simplifies the complexities of magnetic anomaly interpretation. We present new pseudo-gravity maps, derived from EMAG2v3 data, for the South Atlantic Ocean and adjacent continent to reveal insights into crust and lithosphere composition.

The pseudo-gravity transformation of the full-field magnetic anomaly consists of three steps: (i) reduction to the pole (RTP); (ii) integration to determine magnetic potential; and (iii) scaling to determine the pseudo-gravity, assuming a constant ratio of density contrast to magnetization. This transformation produces a gravity-like anomaly that would be observed if the magnetization were replaced by a density distribution of proportional magnitude. We use magnetic data from the public domain EMAG2 v3 as the primary input. The South Atlantic region was divided into 90 tiles of 5º to account for regional variations in magnetic inclination and declination. An important assumption for the transformation is that the magnetization is induced. The pseudo-gravity mapping shows large amplitude positive anomalies consistent with the assumption of induced magnetization; negative anomalies identify areas of remnant magnetization. In the oceanic domain, alternating positive and negative anomalies reflect magnetic reversals and are not an artifact.

On the South American plate, the Rio-Grande-Rise exhibits three distinct high positive pseudo-gravity anomalies, reflecting thick magmatic crust. The São-Paulo-Plateau in the Santos Basin, Florianópolis-Ridge and Torres-High also show high positive anomalies. The Campos Basin has lower-amplitude positive anomalies, suggesting localized magmatic crust. In the continents, high-amplitude positive anomalies correlate with magmatic intrusion. The high positive pseudo-gravity anomalies form an NW-SE arc from magmatic material in the Paraná Basin to the São-Paulo-Plateau, continuing through the Florianópolis-Ridge and Rio-Grande-Rise.

Strong positive anomalies along the Namíbia-Ridge, Walvis-Ridge, and southwestern African margin form a "7"-shaped uniform anomaly, corresponding to magmatic crust. In contrast, the oceanic crust offshore Orange Basin shows a north-south region of negative anomalies. No significant anomalies are seen in the Tristan-Gough Guyot Province (nor are they seen on the Vitória-Trindade or Tristan-Gough Guyot Province hot-spot tracks).

A comparison of the pseudo-gravity map with crustal thickness from gravity inversion shows that many large, high-amplitude positive pseudo-gravity anomalies in the oceanic domain correlate strongly with anomalously thick crust (>12.5 km thickness) on the Rio-Grande-Rise, São-Paulo-Plateau, Florianópolis-Ridge, Namíbia-Ridge and Walvis-Ridge. Plate reconstruction of pseudo-gravity anomalies and crustal thickness suggest that the Namíbia-Ridge and western Florianópolis-Ridge, both with thick magmatic crust, have a common origin at approximately 110 Ma but were later separated by the Florianópolis-Fracture-Zone. The spatial relationship of observed high positive pseudo-gravity anomalies on magmatically intruded crust in the South American and African continents, the hybrid or magmatic crust on their rifted margins and thickened magmatic crust within the South Atlantic shows the major role of mantle inheritance in their origin.