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In the South Atlantic, a reorganization of the Mid-Atlantic Ridge began before anomaly C34n (83.6 Ma) and ended before anomaly C30n (66.4 Ma), complicating tectonics of Rio Grande Rise and older Walvis Ridge (WR), which formed together at the Mid-Atlantic Ridge. This reorganization is poorly understood because magnetic anomalies C30n-C34n are poorly defined near WR. We interpreted these anomalies along western WR to improve knowledge of Rio Grande Rise-WR tectonic development. Anomaly trends indicate that Valdivia Bank has an E-W age progression, perpendicular to that predicted by hot spot models. Anomaly spacing and width is irregular and anomalous near WR, implying a series of ridge jumps and possibly a microplate between anomalies C34n and C32n. Eastward ridge jumps transferred microplate lithosphere to the South American plate. This study shows that Late Cretaceous tectonic evolution of the Rio Grande Rise-WR large igneous provinces was more complex than previously understood.

Plain Language Summary Walvis Ridge and Rio Grande Rise are two large volcanic mountain ranges formed on the African and the South American plates, respectively, as the South Atlantic opened during Cretaceous and Cenozoic times. Between anomalies C34n (83.6 Ma) and anomaly C30n (66.4 Ma), a reorganization of the Mid-Atlantic Ridge complicated the tectonic development of these two igneous provinces. This process is not well understood due to the sparse distribution of magnetic anomalies C30n-C34n near Walvis Ridge. Analyzing magnetic anomaly data, we improved the interpretation of magnetic anomalies near Walvis Ridge. The updated magnetic anomaly pattern indicates that Valdivia Bank has an E-W age progression, rather than the N-S trend as predicted by hot spot models. Uneven anomaly spacing and width imply complex tectonics, including ridge jumps and possibly the formation of a microplate. Eastward ridge jumps transferred lithosphere from the African plate to South American plate. This study shows that the Late Cretaceous tectonic evolution of the Rio Grande Rise-Walvis Ridge large igneous provinces was more complex than previously understood.