DO ALL OCEANIC PLATEAUS ERUPT AT SPREADING RIDGES?

Oceanic plateaus are low-slope basaltic mountains that rise several kilometers above the abyssal plains and have broad areas >~10⁵ km². A widely-accepted hypothesis is that these large igneous provinces (LIPs) are the oceanic equivalent of continental flood basalts, erupted from the massive head of a nascent mantle plume. The result is thought to be an enormous volcano constructed of a vertical stack of long-runout lava flows. Accumulating evidence indicates that many oceanic plateaus record linear magnetic anomalies, a hallmark of seafloor spreading. Long-runout lava flows would destroy this magnetic character, so these plateaus must be formed via lava flows and intrusions constrained near the spreading ridge axis in a process akin to seafloor spreading, but forming thick crust. A notable example of a plateau with linear magnetic anomalies is Shatsky Rise, which formed during the Late Jurassic and Early Cretaceous at a triple junction with at least two ridges. Virtually the entirety of the plateau, including the large edifices Tamu and Ori massifs, is characterized by linear magnetic anomalies. Despite the fact that common practice is not to interpret magnetic anomalies over oceanic plateaus (lest the anomalies be contaminated by topography), a number of other oceanic plateaus clearly record linear magnetic anomalies plausibly recorded by seafloor spreading. This list includes Rio Grande Rise, Valdivia Bank (a part of Walvis Ridge), and the Azores plateau in the Atlantic Ocean and Joey and Roo Rises in the Indian Ocean. Many others, including the enormous Ontong Java Nui complex (Ontong Java, Manihiki, Hikurangi plateaus), likely formed at spreading ridges. Here I examine the list of oceanic plateau LIPs compiled by Coffin and Eldholm (Reviews of Geophysics, v. 32, 1994, p. 1-36) to see if any were clearly not formed at a spreading ridge. For many, this connection cannot be pinned down, but there is yet no convincing example of one formed off-ridge. The implication is that oceanic plateaus are a product of seafloor spreading.