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[1] A regional-scale magnetotelluric (MT) experiment across the southern African Kaapvaal craton and surrounding terranes, called the Southern African Magnetotelluric Experiment (SAMTEX), has revealed complex structure in the lithospheric mantle. Large variations in maximum resistivity at depths to 200-250 km relate directly to age and tectonic provenance of surface structures. Within the central portions of the Kaapvaal craton are regions of resistive lithosphere about 230 km thick, in agreement with estimates from xenolith thermobarometry and seismic surface wave tomography, but thinner than inferred from seismic body wave tomography. The MT data are unable to discriminate between a completely dry or slightly "damp" (a few hundred parts per million of water) structure within the transitional region at the base of the lithosphere. However, the structure of the uppermost ∼150 km of lithosphere is consistent with enhanced, but still low, conductivities reported for hydrous olivine and orthopyroxene at levels of water reported for Kaapvaal xenoliths. The electrical lithosphere around the Kimberley and Premier diamond mines is thinner than the maximum craton thickness found between Kimberley and Johannesburg/Pretoria. The mantle beneath the Bushveld Complex is highly conducting at depths around 60 km. Possible explanations for these high conductivities include graphite or sulphide and/or iron metals associated with the Bushveld magmatic event. We suggest that one of these conductive phases (most likely melt-related sulphides) could electrically connect iron-rich garnets in a garnet-rich eclogitic composition associated with a relict subduction slab.

30Southern Africa, particularly the Kaapvaal Craton, is one of the world's best natural 31 laboratories for studying the lithospheric mantle given the wealth of xenolith and seismic data 32 that exist for it. The Southern African Magnetotelluric Experiment (SAMTEX) was launched 33 to complement these databases and provide further constraints on physical parameters and 34 conditions by obtaining information about electrical conductivity variations laterally and with 35 depth. Initially it was planned to acquire magnetotelluric data on profiles spatially coincident 36 with the Kaapvaal Seismic Experiment, however with the addition of seven more partners to 37 the original four through the course of the experiment, SAMTEX was enlarged from two to 38 four phases of acquisition, and extended to cover much of Botswana and Namibia. Comparisons between the resistivity image maps and seismic velocities from models 49 constructed through surface wave and body wave tomography show spatial correlations 50 between high velocity regions that are resistive, and low velocity regions that are conductive. 51In particular, the electrical resistivity of the sub-continental lithospheric mantle of the 52 Kaapvaal Craton is determined by its bulk parameters, so is controlled by a bulk matrix 53 property, namely temperature, and to a lesser degree by iron content and composition, and is 54 not controlled by contributions from interconnected conducting minor phases, such as 55 graphite, sulphides, iron oxides, hydrous minerals, etc. This makes quantitative correlations 56 between velocity and resistivity valid, and a robust regression between the two gives an 57 approximate relationship of Vs [m/s] = 0.045*log(resistivity [ohm.m]). 58 59

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