DISTRIBUTION OF LARGE PACIFIC SEAMOUNTS FROM GEOSAT ERS-1 - IMPLICATIONS FOR THE HISTORY OF INTRAPLATE VOLCANISM/

We characterize the seamount distribution on the Pacific Plate using t he gridded vertical gravity gradient (VGG, or geoid curvature) derived from Geosat and ERS-1 satellite altimetry. The VGG amplifies short-wa velength information and suppresses longer wavelength components, maki ng it suitable for seamount detection purposes. Furthermore, the VGG o ver seamounts has a much more pronounced zero crossing than that of th e free-air anomaly (FAA); the distance to the zero crossing can be use d as a proxy for seamount radius. After removing a regional field obta ined by robust median filtering we identify seamount amplitudes and lo cations from local maxima in the VGG grid. The radius of a seamount is more difficult to estimate since seamounts tend to cluster and overpr int each other's signals. Individual seamounts are modeled as Gaussian , axisymmetric objects loading an elastic lithosphere; the VGG over su ch features can be approximated by a simple analytical expression whic h we use to determine the zero-crossing distances for overlapping seam ounts. By using the VGG the maximum amplitude and distance to the zero crossing become largely independent of the elastic plate thickness an d infill density. We do forward modeling of Gaussian seamounts and the ir gravimetric response and create a look up table that relates seamou nt FAA (in milligals), VGG (in Eotvos), and zero-crossing distance (in kilometers) to actual height and radius (in kilometers). The frequenc y-size distribution of these predicted seamount heights follows a powe r law for heights between 2 and 8 km. The seamount density (number of seamounts per area) is greatest in the central Pacific. We confirm ear lier results suggesting that the majority of large seamounts are locat ed in the western region of the Pacific Plate, on older crust. As crus tal age increases, so does seamount density, peaking on 100-130 m.y. c rust, supporting suggestions of high magmatism in the Cretaceous. We d emonstrate that there may be an empirical relationship between the sea mount VGG amplitude and the age of the lithosphere at the time of seam ount formation and invert this relationship to predict seamount ages f rom VGG amplitudes. These pseudo ages have large uncertainties but, ne vertheless, may be used to investigate temporal fluctuations in Pacifi c intraplate volcanism. Our results indicate that seamount intraplate volcanism attained a maximum level in the mid-Cretaceous to Late Creta ceous, about 70-120 Ma, apparently contemporaneous with the formation of large oceanic plateaus in the Pacific.