The electrical anisotropy of the continental lithosphere: Methods of analysis and results for southern Africa

The formation and deformation processes of continental lithosphere are poorly understood, and this paucity of knowledge increases exponentially further back in Earth’s history. In particular, how Mesoarchean-aged lithosphere, that roots all of the world’s Archean cratons, was formed is controversial, and none of the current models are without serious objection. One tool that can shed light on those processes is the electrical anisotropy and its variation laterally and vertically, in particular when combined with observations of seismic anisotropy. From these anisotropy observations can be inferred tectonic fabrics imprinted as part of the formation and deformation processes. Determination of electrical anisotropy has been demonstrated to yield key information, particularly of ductile shear zones (Ji et al., 1996; Eaton et al., 2004). Measurements of electrical anisotropy on widespread continental regions also are providing crucial data (Mareschal et al., 1995; Hamilton et al., 2006; Frederiksen et al., 2006; Padhila et al., 2006), although care must be exercised when undertaking the analyses and interpretation not to be influenced by crustal distortions (Lahti et al., 2005; Jones, 2006).
Magnetotelluric data from the SAMTEX (Southern African Magnetotelluric Experiment) project at over 500 sites in an area of over one million square kilometres in Botswana, Namibia and South Africa have been analysed using a variety of novel techniques, including transformation to approximate depth and cluster analysis. The anisotropy directions were band-averaged into three depth ranges: 1) the period band above the Moho (approx. 20-40 km), 2) the period band below the Moho (approx. 40 to 100 km), and 3) periods sensing below 250 km, i.e., into the asthenosphere. In the case of class (3) there were so few periods at each site that a multi-site average for groups of sites was performed.
The crustal results show high correlation with large-scale crustal features, such as tectonic boundaries and regional-scale lineaments. The upper lithospheric mantle results also exhibit correlation with some, but not all, of these features. The asthenosphere results show excellent correlation with absolute plate motion.