RAPID FORMATION OF THE SHATSKY RISE OCEANIC PLATEAU INFERRED FROM ITSMAGNETIC ANOMALY

SHATSKY Rise, in the northwest Pacific Ocean, is probably the oldest e xtant oceanic plateau, and as with most such features, its origin is u ncertain. Both oceanic plateaus and continental flood basalts are thou ght to be formed by rapid, voluminous eruptions that occur when the 'h ead' of a newly born mantle plume ascends to the base of the lithosphe re1-3. High eruption rates have been estimated for flood basalts (for example, 1.5 km3 yr-1 for the Deccan Traps2) from dating of lava flows , but the inaccessibility of oceanic plateaus makes it necessary to ex trapolate dating information from a small number of samples and sites4 . Here we estimate the eruption rate of Shatsky Rise by a method that is indirect, but has the virtue of 'sampling' the entire volume of the plateau above the surrounding sea floor. The main, southern part of t he plateau has a positive magnetic anomaly, corresponding to a reverse d geomagnetic polarity at the time of eruption. Using age constraints to identify the longest period of reversed polarity during which the p lateau could have formed, we estimate that 2 x 10(6) km3 of material e rupted at a minimum rate of 1.7 km3 yr-1. This is somewhat less than t he rate of 8-22 km3 yr-1 estimated for the Ontong-Java Plateau4, but s till represents a massive eruption, consistent with the plume-head hyp othesis.