Strength anisotropy in Southern Africa from gravity-topography coherence studies: Implications for continental deformation

The relationship between gravity anomalies and topography, diagnostic of the elastic strength of the lithosphere and expressed in the wavelength domain by means of the coherence function, has been found to be directionally dependent in various portions of the North American, Australian, and Indian (sub)continents. While various mechanisms have been proposed as to the origin of azimuthal variations in the degree and mechanism of isostatic compensation per se, a fruitful way of interpreting this information has been to construe the orientations of mechanical weakness as indicators of fossil strain and compare them to the fast axes of seismic anisotropy, which are themselves an independent measure of strain, and to which they should be orthogonal under certain simplifying assumptions. Seen in this light, the study of mechanical anisotropy via gravity-topography coherence functions is a useful way to define the very essence of the mechanical lithosphere, as that portion which relates consistently to its seismic anisotropy, delineating a stable regime that has been coherently deformed by the action of orogenic processes over time (in the sense of Silver) and separating it from the actively deforming asthenosphere (in the sense of Vinnik) below it. In this study we present evidence for the mechanically anisotropic nature of the Southern African continent from a multitaper spectral analysis of two-dimensional coherence functions relating high-resolution Bouguer gravity anomalies and topography. Of the thirteen 750×750 km2 areas surveyed within the continental confines, five fall within an area for which seismic anisotropy has been measured by means of shear-wave splitting analysis, and 80%, or four of those, corroborate Silver's hypothesis of vertically coherent deformation in that the directions of mechanical weakness are perpendicular to the fast axis of seismic wave propagation. We discuss the limitations of our evidence and present it with a view to further work.