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.

Seasat Altimetry and the South Atlantic Geoid: 1. Spectral analysis

“…The geophysical part of the signal is itself a composite including effects related to ocean bottom topography, sediment thickness [Marsh et al, 1984;Sandwell, 1984a], compensation and flexure phenomena (for instance, McKenzie and Bowin [1976] and Haxby and Turcotte [1978]; see review by Watts [1985]) and mantle convection (for instance, Parsons and Daly [1983] and Cazenave et al [1986]). Simple models of most of these effects relate ocean bathymetry (i.e., the watercrust or water-sediment interface) or the sediment-crust interface in a linear fashion to either the gravity field at the surface or to the geoid (see, for instance, Karner 1-1982]).…”

Lateral heterogeneity in the convecting mantle

“…The recent rapid growth of global geophysical data bases has been accompanied by a heightened interest in the implications and origins of the lateral heterogeneity in the earth's mantle which they have revealed. In particular, global variations of heat flow [Pollack and Chapman, 1977;Chapman and Pollack, 1980;Sclater et al, 1980], bathymetry [Parsons and Sclater, 1977], gravity [Gaposhkin, 1979;Lerch et al, 1979;Marsh et al, 1984], chemistry of igneous extrusions [DePaolo and Wasserburg, 1976;O'Nions et al, 1977;Allegre et al, 1983], seismic surface wave [Toksoz and Anderson, 1966;Knopoff, 1972;Woodhouse and Dziewonski, 1984] and body wave [Julian and Senqupta, 1973;Dziewonski et al, 1977;Dziewonski, 1984] velocities, and frequency shifts of fundamental spheroidal modes of elastic gravitational free oscillation [Masters et al, 1982], all provide loose constraints on possible models of lateral heterogeneity. As these data continue to be refined, it may ultimately be possible to use them to directly constrain models of mantle flow on the basis of the characteristic lateral heterogeneity implied by the different flow models.…”