Crustal thickness mapping of the central South Atlantic and the geodynamic development of the Rio Grande Rise and Walvis Ridge

Abstract

Abstract The origin of the Rio Grande Rise and Walvis Ridge within the central South Atlantic and its implications for the separation of South America and Africa during the Cretaceous are controversial. The recent report of the discovery from submersible sampling of continental material of Proterozoic age on the Rio Grande Rise suggests that the existing explanation involving ocean ridge – mantle plume interaction or simply excess ‘on-ridge’ magmatism for the formation of the Rio Grande Rise and Walvis Ridge needs to be re-examined. We use gravity anomaly inversion to map crustal thickness for the central South Atlantic area encompassing the Rio Grande Rise, Walvis Ridge and adjacent South American and African rifted continental margins. We show that the Rio Grande Rise consists of three distinct bodies of anomalously thick crust (Western, Central and Eastern) separated by normal thickness oceanic crust. The Central Rio Grande Rise forms a large elliptical body with maximum crustal thickness of 25 km. The Walvis Ridge also has a maximum crustal thickness of 25 km but has a narrower and more linear geometry. We use plate reconstructions to restore maps of crustal thickness and magnetic anomaly to Early Cretaceous times to examine the development of the Rio Grande Rise and Walvis Ridge. These restorations together with ages of magmatic addition suggest that the Central Rio Grande Rise and Walvis Ridge formed a single body between 90 and 80 Ma located on the ocean ridge plate boundary similar to Iceland today. On the basis of crustal thickness mapping, the plate restorations and the magmatic ages, we propose that the Rio Grande Rise was fragmented into its 3 parts and separated from Walvis Ridge by at least 4 ocean ridge jumps during the opening of the South Atlantic Ocean between approximately 90 and 50 Ma. Plate reconstructions of crustal thickness showing rotated structural lineaments imply that the separation of Eastern Rio Grande Rise and Walvis Ridge was highly complex involving simultaneous crustal extension and magmatic addition. We propose that the continental material reported on the Rio Grande Rise, if not drop-stones, was isolated from the main continental land-mass and transported into the ocean by these ridge jumps during the Cretaceous formation of the South Atlantic.