Crustal structure and geophysical parameters of seamounts in the western Pacific as derived from topography and potential field anomalies

Abstract

Hydrographic and Oceanographic Department of Japan has conducted geophysical surveys by multi‐beam soundings, KSS - 30 type ship gravity meter, and sea‐surface proton magnetometer in the western Pacific as a part of continental shelf surveys sinc 1992. The surveyed area includes Jurassic Magnetic Quiet zone and a large number of seamounts were confirmed. Using the topographic depth soundings and potential field data (free‐air gravity and magnetic anomalies), Bouguer gravity anomalies on the assumed density values of 2300kgm - 3 and 2700kgm - 3 were compiled. The effective elastic thickness was estimated for the several seamounts. These values ranging from 5km to 30km, with the largeset occurrences in 10km are consistent with the thermal rejuvenation due to mantle plumes occurred in Middle to Late Cretaceous time. Geophysical parameters（volume, density, magnetization）of 85 seamounts were calculated. A volume histogram indicates bi‐modal pattern, one peek is around 1000～2000km 3, and the other 5000km 3. This bi‐modal pattern may be related with the different scale of magma plumes forming these seamounts. Density values of the seamounts were estimated from the correlation between calculated and observed free‐air gravity anomalies. These density values show a mono‐modal peek around a mean density of 2694±253kgm-3. Magnetizations of the seamounts are also estimated by the least square inversion method using a topographic model and a gravity basement model derived from Bouguer gravity of the assumed density of 2300kgm-3. The precision index parameters(GFR) on the topographic model give usually larger values than those on the gravity basement model. This may suggest the depressed structure observed in the gravity basement is not real but artificial resulting from filtering process. Histogram of magnetization intensities of the seamounts shows considerable wide range distribution, contrary to the density pattern. This may be ascribed to differences in magnetic field intensity at the origin or in metamorphoses of magnetic minerals composing seamounts. The Virtual geomagnetic poles (VGPs) of these seamounts generally coincide with the VGP curve estimated from DSDP and ODP results (Sager, 2006), however, Hanzawa and Katayama seamount (4,6)and seamount D4 (48) show considerable displacement from the VGP. The origin of this misfit may arise from tilting effects of the seamounts , non‐dipole components of magnetic field, or the older seamount’s formation ages in comparison with radiometric age of 80 Ma. Stage Euler poles derived from hotpot tracks(Koppers et al., 2001) failed to the reproduction of the APWP estimated by paleomagnetic method. Stage Euler poles reproducing the paleomagnetic APWP and South Pacific Isotopic and Thermal Mantle Anomaly (SOPITA) origin of West Pacific seamounts, are calculated and listed in this article for the consideration of the plate motions.