Interpretation of Global Gravity Anomalies

Abstract

THE absence of any significant correlation between large scale topographic features of the Earth's surface and the undulations of the geoid indicates near perfection in the isostatic balance of continent-sized features, whereas local geological features do cause distinct gravity anomalies. The difference is explicable in terms of a lithosphere of order 100 km thick which can support gravitationally unbalanced features up to 100 km or so in lateral extent, but is underlain by a yielding asthenosphere, so that larger scale features are isostatically balanced. It is therefore natural to seek a deep-seated source for the broad-scale features of the geoid. Depths greater than about 900 km become implausible, however, because the implied stresses increase very sharply with the fineness of the detail of the gravitational field which density variations are required to match, that is, with increasing order of the geoid harmonics. We can, of course, suppose that mass anomalies are distributed at all depths in the mantle, but if it can be shown that the geoid is explicable in terms of an appropriate mass distribution at a single depth, such that the stresses are approximately equal for all harmonic orders in the spherical harmonic representation of the surface, then this is strong evidence that we have found the depth of the most significant anomalies. We have reported such an analysis¹ in which the plausible range of depths allowed by the uncertainty in the data was 250 km to 900 km. The analysis has since been repeated using the improved geoid coefficients of Gaposchkin and Lambeck², which give a preferred depth of 460 km for the mass anomalies.