Tectonic Fabric and Its Implications for the Formation of Rio Grande Rise and Walvis Ridge

Despite the efforts of geoscientists for decades, the mechanism for the formation of the Rio Grande Rise (RGR) and Walvis Ridge (WR) volcanic chains in the South Atlantic Ocean is not completely understood. Both chains are considered volcanic products of the Tristan-Gough hotspot, and formed as edifices on the South American (RGR) and African (WR) plates, respectively, as the South Atlantic opened. The morphology of these chains, thought to be a result of relative motion and interaction of the hotspot with the Mid-Atlantic Ridge (MAR), has been shown by recent radiometric age dating to be too complex to be explained by current, simplistic hotspot models. Furthermore, shipborne geophysical data in the South Atlantic are sparse, making accurate plate reconstructions difficult. New improvements in high-resolution satellite gravity (and related predicted-bathymetry) data allow the tracing of many tectonic features, including fracture zones and abyssal hills, which are directly related to the seafloor spreading process. In addition, new crustal spreading rates have been created from isochron-MAR measurements. With these new data, an improved study of the reconstruction and morphology of RGR and WR seamounts was performed. Evidence has been produced, which suggests that: 1) In general, relatively-high spreading rates and relatively-small hotspot-MAR distances existed at the time of accretion of the major RGR-WR seamount formations and 2) Eastward MAR “ridge jumps” toward the hotspot created asymmetric, spreading rate increases on some of the spreading corridors of the South American plate, which resulted in morphological differences between RGR and WR. Finally, there is evidence of a previously-unknown, defunct, microplate in the RGR-WR study area, which existed from about 88 Ma to 72 Ma and may have also affected RGR-WR morphology.