TOPOGRAPHY AND GEOID ANOMALIES DUE TO DENSITY HETEROGENEITIES AT THE BASE OF THE THERMAL LITHOSPHERE - APPLICATION TO OCEANIC SWELLS AND SMALL-WAVELENGTH GEOID LINEATIONS

The surface topography, gravity and geoid associated with loads situated at the base of the thermal lithosphere are computed. The model lithosphere is composed of a viscous lower layer with depth-dependent viscosity overlain by an elastic lid. When the viscosity varies strongly with depth, the amplitude of the surface topography decreases rapidly as the wavenumber characterizing the mass anomaly increases. The response for this two-layer lithosphere differs substantially from the response computed for the loading from below of an elastic lid. If the 200 km wavelength geoid anomalies observed in the oceans are due to convective instabilities at the base of the lithosphere, a positive geoid anomaly with an amplitude reaching 30 to 50 cm is expected over a cold downwelling for ages greater than 10 Myr. No detectable topography associated with the lineations should be present for ages greater than 20 Myr. Our model predicts that some topographic anomalies might be visible for younger ages. If 1000 km broad swells are due to light material at a depth of about 100 km, the associated geoid-over-topography ratios are quite reduced compared to those predicted by the long-wavelength linear relationship usually employed for estimating the apparent compensation depth.