The distribution of intraplate volcanism in the Pacific Ocean basin: a spectral approach

Abstract

Summary. Although the major linear island chains in the Pacific Ocean basin have been extensively studied, relatively little attention has been given to the origin of the numerous seamounts and oceanic islands that are concentrated in the western Pacific. Spectral analysis of shipboard gravity and bathymetry data provides a means for statistically estimating the distribution of this volcanism in space and time. Estimates of the power spectrum of the bathymetry FB and the average gravitational admittance z (cross-spectrum correlation of gravity and bathymetry) were obtained as functions of age of the oceanic lithosphere by dividing the Pacific plate into eight age regions A-H. The power spectrum of the bathymetry is indicative of the overall bathymetric roughness (primarily volcanic) in each region. This roughness shows three gradations: regions A-D ( 140Myr), with many large volcanic features, are very rough. The average gravitational admittance is indicative of the degree of isostatic compensation of bathymetric features in each region and can be used to estimate the proportionh of total bathymetric power that was emplaced on relatively old lithosphere and the proportion that formed on young lithosphere at or near a mid-ocean ridge crest. The highest proportion of bathymetry that formed on old lithosphere occurs in region H (fI- 0.90) and the lowest proportion occurs in regions G and E (fI - 0.40-0.60). The power spectrum of the bathymetry and the gravitational admittance allow an estimate of the total bathymetric power PI = ~IPB accounted for by intraplate volcanism ('intraplate power') and the power PR = (1 - fI)PB which is of on-ridge origin ('ridge power'). At long wavelengths in regions E-G, the amount of on-ridge volcanism exceeds the amount of intraplate volcanism (PR > PI), indicating the presence of several large plateaus and rises which formed at ridge crests. This suggests the