Gravity and bathymetry in the central Pacific Ocean

Abstract

Surface-ship and submarine pendulum gravity measurements have been compiled in a new free air gravity anomaly map of the central Pacific Ocean in the region of Hawaii. The main features of the map are large amplitude positive anomalies (up to +700 mGal) over the Hawaiian ridge, large amplitude negative anomalies (up to −136 mGal) flanking the ridge, and a broad belt (>250 km) of positive anomalies (+25 to +50 mGal) bordering the negative anomalies. The map has been used to construct 1°×1°, 5°×5°, and 10°×10° free air anomaly averages. The main feature of the 5°×5° average map is a long-wavelength (∼2200 km) positive anomaly (up to +15 mGal) over the southeastern end of the Hawaiian ridge. A long-wavelength positive anomaly is also seen on the 10°×10° average map, which agrees well with the Gem 6 satellite-derived solution to harmonic degree 16. Computations suggest that crustal structure of the Hawaiian ridge is unlikely to contribute significantly to these long-wavelength positive anomalies. The positive anomalies correlate closely with the Hawaiian swell upon which the Hawaiian ridge is superimposed. The regression lines representing 1°×1° and 5°×5° averages of gravity against topography slope at 21 mGal/km and 22 mGal/km, respectively. These slopes are smaller than those over other regions where the lithosphere is warped for large distances, suggesting that if the swell is warped, it must be compensated. The form which the compensation takes is uncertain, but it may be related to some pattern of flow beneath the lithosphere which maintains both the swell and the associated long-wavelength gravity anomalies.