On gravity from SST, geoid from Seasat, and plate age and fracture zones in the Pacific

Abstract

Our earlier measurements of the high degree and order (n, m>12) gravity in the central Pacific using the method of satellite‐to‐satellite tracking (SST) have been extended with an additional 50 passes of data. The SST method utilizes line of sight Doppler tracking between the very high geosynchronous ATS 6 spacecraft and the much lower (840 km) orbiting GEOS 3 spacecraft. The observed changes in velocity with time are reduced in relation to the well‐determined low degree and order (n, m≤12) Goddard Earth Model (GEM) field model, and accelerations are found by differentiation of the range rates. Because this new map is essentially identical to the earlier results, we have produced a composite map by combining all 90 passes of SST data. The resolution of the map is at worst about 5° and is much better in most places. A comparison of this map with conventional GEM models shows very good agreement. A reduction of the Seasat altimeter data has also been carried out for an additional comparison. Although the Seasat geoid contains much more high‐frequency information, it agrees very well with both the SST and the GEM fields. The maps are dominated (especially in the east) by a pattern of roughly east‐west anomalies with a transverse wavelength of about 2000 km. A further comparison with regional bathymetric data shows a good correlation of gravity and geoid with plate age. Each anomaly band is framed by major fracture zones whose regular spacing (≃10°) seems to account for the fabric in these fields. There are other anomalies that are accounted for by hot spots, and altogether, the immediate source of at least part of these Pacific anomalies is in the lithosphere itself. It therefore seems that most of the anomalies in the east half of the Pacific could be partly caused simply by regional differences in plate age. The amplitude of these geoid or gravity anomalies caused by age differences should decrease with absolute plate age, and large anomalies (∼3 m) over old, smooth seafloor may indicate a further, deeper source within or perhaps below the lithosphere. However, because of the significant effect of plate structure on the geoid and gravity fields, especially over young (<80 Ma) seafloor, lithospheric effects must be eliminated before these anomalies can be systematically ascribed to deeper sources such as small‐scale convection.