Electrical anisotropy of South African lithosphere compared with seismic anisotropy from shear-wave splitting analyses

Electrical anisotropy in southern Africa, inferred from the analysis of magnetotelluric (MT) data recorded as part of the Southern African MT Experiment (SAMTEX), is compared with seismic anisotropy inferred from an SKS shear-wave splitting study in the same region. Given the vastly varying penetration depths in the survey area, electrical anisotropy is derived in terms of approximate depth, rather than frequency. Electrical anisotropy directions for crustal depths (<35 km) show more distinct variability than those for upper mantle depths, and, not surprisingly, appear to be strongly related to large-scale geological structures. Our results for upper lithospheric mantle depths (>45 km) are not consistent with the fast-axis directions inferred from the SKS analyses. Upper mantle electrical results appear to be mostly a consequence of the geometry of large-scale geological structures and provide evidence that some crustal structures are distinct at depth, while others seem to be confined to the crust. Our results indicate that the causative region for the seismic anisotropy in the lithospheric mantle has either a correspondingly weak electrical anisotropic signature, or is more prominent at greater lithospheric depths than those we investigate here.