LONG-WAVELENGTH AND INTERMEDIATE-WAVELENGTH GEOID UNDULATIONS OVER THE AFRICAN PLATE

The study of the undulations of the geoid, acquired from radar satelli te altimeter data, has proved to be essential in understanding geotect onic processes in the world's oceans. In comparison, the continents ar e not studied in the same derail because continental geoid cannot be d erived from satellite altimetry. Geopotential models of the earth's gr avity field derived from satellite-tracking data combined with terrest rial information can be used to compute the geoid over both the contin ent and the ocean. Geoid undulations were thus derived over the Africa n plate. The long-wavelength components of the geoid undulations are i ndependent of surface topography and 'warp' the African plate's lithos phere. From an analysis of the geoid residuals calculated by substract ing the spherical harmonics of degree 2-10 from that of degree 2-30, w e show that these correlate with major geological features of the Afri can continent. In the oceans, the geoid residuals are congruent with g eoid anomalies derived from satellite altimetry. The association of ge oid residuals with geological entities on the continents and with lith ospheric cooling in the oceans indicates that intermediate-wavelength geoid undulations reflect, primarily, lateral contrasts in mass distri bution within continental crust and lithosphere. We demonstrate that t he causative geotectonic processes may be identified by imaging the ge oid impedance, computed as the dimensionless ratio of geoid residuals to topography. Results indicate that continental plateaux and Precambr ian shields are associated with intermediate to high impedance, wherea s sedimentary basins are, with notable exceptions, coincident with hig h geoid impedance. The causative processes are compatible with seismic tomography and include lithospheric thinning, formation of low-viscos ity zones within the lithosphere, as well as upper-mantle upwelling.