Comparison of seismic and electrical parameters of the Southern African lithosphere: Evidence for predominantly thermally-driven lateral variation

Seismic velocity is primarily a function of bulk properties of the media and electrical resistivity is usually primarily a function of the properties of a minor phase in the rock matrix (low order partial melt, presence of conducting irons, oxides, water, hydrogen diffusion, etc.), so one might not expect the two to correlate. However, for the Southern African lithosphere there does appear to be a definable systematic correlation, suggesting that both are driven by the same variations in lithostatic petro-physical parameters. Comparisons at various depths of slices from a new high-resolution (1.5 deg) seismic model, derived from surface wave inversion of events along continental paths, and of new electrical images from the SAMTEX experiment reveal correlations at both large and small scales. The existence of these correlations, which can be defined quantitatively by a quadratic regression between log(resistivity) and shear wave velocity, indicates that the two are functions of the same petro-physical parameters, namely temperature, pressure, physical state, magnesium number, and composition. Given that electrical conductivity is exponentially-dependent on temperature, temperature variation dominates over the other candidates. This infers that lateral variations in seismic velocity can also be primarily explained as due to temperature variation, and that compositional variation has a lesser effect.