Implications of Updated Magnetic Anomalies for the Tectonic Evolution of Walvis Ridge

Walvis Ridge (WR) is a NE trending submarine ridge on the African plate in the South Atlantic extending from Namibia to the islands of Tristan Cunha and Gough. WR is a hotspot trail thought to be formed by the interaction of a mantle plume with the Mid-Atlantic Ridge. WR morphology is complex, starting as narrow ridge near the continental margin, converting to a plateau (Valdivia Bank) at right angles, forming en-enchelon ridges and splitting into three seamount chains (Guyot Province), two of which end in active volcanic islands of Tristan and Gough. Many hypotheses based on simple hotspot models have been put forth to explain the tectonic evolution of WR - multiple hotspots, moving hotspots, ridge-hotspot interactions - none of which satisfactorily explain the complex geomorphology. In this study, we investigate the tectonic evolution of WR during 84-68 Ma by interpreting magnetic anomalies C34-C30 near the ridge. We model newly obtained magnetic data around WR and re-interpret old magnetic data available from NOAA and JAMSTEC databases. Our focus is C34-C30 because it is an anomalous time in the formation of WR and coincides with formation of the Rio Grande Rise on the South American plate. During this period, a major plate reorganization occurred in the South Atlantic, WR evolved from an ocean plateau (Valdivia Bank) to en-echelon ridges and then to a series of seamount chains. Coincidently, there was a reduction of spreading rate from 70mm/yr (end of 34n) to about 40mm/yr (end of 30n) and several new fracture zones formed between WR and Rio Grande Rise. This study for the first time has successfully tracked chrons C33n to C34n along the length of WR, where previously no magnetic anomalies were identified. The newly identified magnetic anomalies will provide better constraints and help understand the tectonic evolution of WR in greater detail. The results from this study will help revise the kinematic plate model for South Atlantic and help decipher the tectonic history of WR and Rio Grande Rise. Our analysis of the magnetic anomalies suggests that WR should not be treated as a simple hotspot trail and that complex tectonic processes, including microplate formation, were involved in dictating the overall geomorphology.