THE EARTHS GRAVITY-FIELD AND PLATE-TECTONICS

A recent decomposition of the Earth's gravity field has allowed mass anomaly sources spatially related to plate tectonic features to be distinguished from those mass anomaly sources that are not. The earth's largest mass anomalies are not related. The mass anomalies contributing to the degree 2-3 field are inferred to be the result of topography at the core-mantle boundary, produce the world's largest geoid anomalies, and are unrelated to the plate tectonics occurring at our planet's surface. The degree 4-10 field shows major narrow bands of positive geoid anomalies with marked coincidence everywhere with plate convergence zones. The degree 4-10 mass anomaly appears to lie in the lower mantle beneath plate convergence zones, below the teleseismically determined downgoing subducted slabs, and may be an important driving force for plate tectonics, contributing to the stabilization of the location of subduction zones through time. Subtracting the degree 10 field from observed data yields residual anomalies. Both the residual geoid and gravity anomalies result entirely from mass anomalies that lie in the outer part of the Earth. The residual geoid anomalies have much lower magnitude than those of the degree 2-3 and 4-10 fields. Axes of spreading in the Indian Ocean, in the North Atlantic, and partially in the South Atlantic, lie along centers of positive residual geoid and gravity anomalies. The East Pacific Rise is without expression presumably because of its fast spreading rate with broad topography. Fracture zones are commonly well-defined because of their linearity, although their geoid and gravity amplitudes are small. Amongst spreading centers (without superimposed hot spots), fracture zones, and subduction zones, the subduction zones have the largest residual gravity anomalies. Although their residual geoid anomalies are generally less than 20 m, their residual gravity anomalies of 200 to 350 mGal show them to be the sites of the largest magnitude gravity anomalies of our planet. Residual geoid anomalies associated with the elevated topography of hot spot features reach 22 m.