Tectonic Evolution of Oceanic Plateaus and Hotspot-Ridge Interactions: Walvis Ridge – Rio Grande Rise, South Atlantic and Tamu Massif, Pacific Ocean



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Oceanic plateaus are large basaltic provinces on the seafloor formed due to massive magma outpourings from an upwelling mantle. Many of these plateaus were formed at or near spreading ridges, suggesting that hotspot-ridge interaction was involved in their formation. However, the extent of ridge influence in their evolution remains unknown due to poor data coverage owing to their large size and remote locations. Magnetic anomaly patterns and geomorphology of these plateaus can provide important clues into their tectonic evolution. In this study, the tectonic evolution of three oceanic plateaus – Walvis Ridge (WR) and Rio Grande Rise (RGR) hotspot twins in South Atlantic Ocean and Shatsky Rise in Pacific Ocean – was investigated using magnetic, bathymetry, and seismic data. In the South Atlantic, a major reorganization of the Mid‐Atlantic Ridge (MAR) began before anomaly C34n (83.6 Ma) and ended before anomaly C30n (66.4 Ma), complicating the tectonics of RGR and older WR that formed together at the MAR. This reorganization is poorly understood because magnetic anomalies, C34n‐C30n, are poorly defined near WR and RGR. Chapter 2 presents an initial review of magnetic anomaly picks near WR and attempts to trace down missing anomalies. Subsequently, large amounts of magnetic data from WR and RGR were collected onboard research vessels Thomas G. Thompson (2019) and Nathaniel B. Palmer (2018). In Chapter 3, magnetic anomaly maps were generated for WR and RGR using all existing magnetic data and reconstructed to their past configurations to study the tectonic evolution of the plate reorganization. The anomaly patterns indicate that WR and RGR formed around a microplate between C34n-C33n, producing various edifices at or near a reorganizing ridge. In the northwest Pacific, a significant part of southern Shatsky Rise was mapped using multibeam sonar onboard R/V Falkor during a 2015 cruise. In Chapter 4, a new high-resolution bathymetry map of southern Shatsky Rise was generated and geomorphological implications of its tectonic and sedimentary evolution were studied in detail. The map reveals that the Tamu Massif is segmented, consisting of four smaller rises that suggest it formed by a series of ridge centered eruptions along a moving triple junction.



Walvis Ridge, Rio Grande Rise, Oceanic Plateaus, Hotspot-Ridge Interactions, Magnetic Anomalies, Bathymetry, Triple Junction, Microplate, Secondary Volcanism


Portions of this document appear in: Thoram, S., Sager, W. W., & Jokat, W. (2019). Implications of updated magnetic anomalies for the Late Cretaceous tectonic evolution of Walvis Ridge. Geophysical Research Letters, 46(16), 9474–9482.