A Magnetic Anomaly Map of Valdivia Bank (South Atlantic): Implications for Oceanic Plateau Formation at a Spreading Ridge

Abstract:

Valdivia Bank (VB) is a Late Cretaceous oceanic plateau offshore of Namibia that is part of the Walvis Ridge hotspot track in the southeast Atlantic. It is considered to be a volcanic product of the Tristan-Gough hotspot formed at the Mid-Atlantic Ridge. The tectonic evolution of VB is not well understood due to sparse distribution of geophysical tracks in the region. To better understand the structure and evolution of VB, we acquired a total of ~15,000 km of new magnetic data over and around the plateau. We used this new dataset as a backbone and integrated previously collected magnetic data (30 tracks collected over 60 years) in this region to compile a coherent dataset of magnetic anomalies over VB. After removing the contribution from internal and external magnetic fields and performing crossover analysis to correct for navigational errors, we generated a high-resolution magnetic anomaly map. Most of VB is characterized by a positive magnetic anomaly, representing the Cretaceous Quiet Zone just prior to Chron 34 (83.6 Ma). A linear negative anomaly, oriented nearly N-S, crosses the west flank of VB and has been interpreted as Chron 33r (79.1 Ma) (Thoram et al., Geophys. Res. Lett., 2019). The Chron C33r anomaly narrows towards the south end of VB indicating that the anomaly is not completely preserved in this section. Another linear negative anomaly trends NE over the SE flank of VB where it is coincident with a buried rift valley. The cause for the bifurcated Chron C33r is not clear. The intervening positive anomaly could be a bit of Cretaceous Quiet Zone inserted by a westward ridge jump. Alternatively, the positive crust could have been remagnetized during a normal polarity period owing to Eocene volcanism that has affected VB. The magnetic structure of VB, with linear magnetic anomalies crosscutting the plateau, implies a lateral age progression formed via seafloor spreading, rather than the expected vertical pile of lava flows that would form from a large volcano.