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

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 P̃B and the average gravitational admittance Z̃ (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 bathy- metric roughness (primarily volcanic) in each region. This roughness shows three gradations: regions A—D (< 80 Myr) have few volcanic features and are relatively smooth; regions E (80–100 Myr) and G (120–140 Myr) are of intermediate roughness; and regions F (100–120 Myr) and H (> 140 Myr), with many large volcanic features, are very rough. The average gravitational admittance is indicative of the degree of isostatic compensation of bathy- metric features in each region and can be used to estimate the proportion fI 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=fIPB 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 occurrence of a major outpouring of on-ridge volcanism during the Cretaceous at or near the Pacific/Farallon accreting plate boundary. At intermediate wavelengths in regions F and H, the amount of intraplate volcanism greatly exceeds the amount of on-ridge volcanism (PI ≫ PR), suggesting that the numerous seamounts in these regions are largely of intraplate origin. The age of this intraplate volcanism cannot be accurately estimated but available evidence from DSDP sites and geological samples suggest that it may at least in part be contemporaneous with the Cretaceous on-ridges event.