Tectonic evolution of the Rio Grande Rise - Walvis Ridge hotspot twins inferred from magnetic anomaly and seismic reflection data

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Although volcanic eruptions of lavas on land, like those on the island of Hawaii, are awe-inspiring, can cause significant destruction of property, and are spectacular to watch, they are insignificant in terms of the amount of erupted material when compared to eruptions of lava on the seafloor. The volume of lava from seafloor volcanic eruptions is so great that the entire floor of the oceans, over 70% of Earth's surface, is covered by lavas erupted from mid-ocean ridges or "hotspot" volcanoes, both of which tap molten material from the mantle and bring it to the surface. The class of volcanoes called "hotspots" arise from localized, persistent upwelling of mantle material and can erupt voluminous amounts of lava creating large igneous provinces. As Earth's tectonic plates move, over time, these hotspots create linear volcanic chains that can be used to infer the distance, direction, and rates of plate motion. The most iconic of these is the Hawaiian-Emperor chain in the Pacific Ocean, which exhibits an age progression from ~81 million years to the present along with a simple string of seamounts. The Walvis Ridge in the South Atlantic Ocean is another. Unlike the Hawaiian chain, however, the Walvis Ridge is more complex and spans a time that goes back ~130 million years. This research focuses on understanding the formation and evolution of the Walvis Ridge and its relation to its sister province, the Rio Grande Rise, which occurs on the eastern side of the Mid-Atlantic Ridge. These two igneous provinces were once a single entity, but during the late Cretaceous, ~85 million years ago, there was a rifting event that split them apart. This event resulted from reorganization of the Mid-Atlantic Ridge and movement of the originating hotspot off the ridge axis to the west where its magmatism continued and additional seafloor volcanoes were added to the Walvis Ridge. Recent evidence suggests that, during this breakup a microplate formed at the Mid-Atlantic Ridge. To examine this hypothesis and to better understand how the South Atlantic evolved, a geophysical expedition will go to the Walvis Ridge area and collect magnetic anomaly, bathymetry, and seismic data. The new magnetic data will be used to infer past locations of the Mid-Atlantic Ridge and the age of the seafloor, a crucial data set for making plate boundary reconstructions. Bathymetry data will show the morphology and structure of seafloor features and enable testing of the microplate hypothesis. Broader impacts of the work include international collaboration with German and Brazilian scientists. It also provides an important seismic site survey that will be used to help site drilling locations and boreholes for an upcoming NSF-funded International Ocean Discovery Program expedition, helping ensure its success. In addition, the project has a large student training component in which eight to ten students will go to sea, earn academic credit, and be trained in geophysical and sea-going technology and techniques. Outreach to the public will be done via blogs and a cruise project website as well as real-time broadcasts from the ship to University of Houston undergraduate classes. News releases about the cruise and its findings will also be generated to stimulate public interest in the project and science. With its ~130 million-year duration and prominent quasi-linear shape, the Walvis Ridge, which rises from the seafloor on the African plate in the southeast Atlantic Ocean, is one of the Earth's most important hotspot volcanic tracks because it serves as a keystone for the Indo-Atlantic hotspot reference frame. This feature is considered the product of a mantle plume arising from the edge of the African, large, low-shear velocity province and has a twin, the Rio Grande Rise, a basaltic plateau across the ocean on the South American plate. These two large igneous provinces likely formed together at the Mid-Atlantic Ridge during the Late Cretaceous. But, when the hotspot drifted off the ridge ~60-70 million years ago, magmatism continued but only on the Walvis Ridge. Due to its quasi-linear geometry, the Walvis Ridge is usually modeled as the trace of a single, age-progressive hotspot. However, radiometric age dating of volcanoes along its trace do not all fit with the hotspot model, with younger submarine volcanoes being dispersed among some of the older ones. Geochemical evidence of Walvis Ridge seamounts shows that ~60-70 million years ago, the Walvis track split into three sub-tracks. Recent studies of tectonic fabrics from the area suggest that a microplate may have formed between the Walvis Ridge and Rio Grande Rise igneous provinces during plate boundary reorganization, potentially disrupting the hotspot age progression and resulting in complex volcanic edifice morphologies and age relations. Because current geophysical data of the seafloor in the South Atlantic are sparse and the tectonic movement of the plates on both sides of the Mid-Atlantic Ridge is key to our models of plate motion, a 48-day geophysical that will collect magnetic, seismic, and bathymetric data will be carried out to resolve the relations between the Walvis Ridge and Rio Grande Rise. Goals of the project are to understand how these two large igneous provinces formed and evolved. Magnetic anomaly data will map isochrons C34-C30 in the area where the tectonic reorganization occurred and where magnetic anomalies are presently poorly mapped and defined. Identification of these anomalies and their geometry is key to defining past ridge positions and testing the microplate hypothesis. Seismic and bathymetric data will examine the structure and morphology of Valdivia Bank area on the Walvis Ridge as this bank is the twin of the main Rio Grande Rise plateau. Seismic data collected on the cruise will also serve as critical site characterization data for an upcoming International Ocean Discovery Program (IODP) expedition that is set to drill six locations on Walvis Ridge.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.