The proposed boundaries of geological terranes in northeastern Botswana are mainly based on regional magnetic and gravity data, because there are not many outcrops available due to the thick Phanerozoic cover rocks. The extent of the Zimbabwe craton into Botswana as well as the location of the boundaries to its neighbouring mobile belts (Limpopo Belt, Magondi Mobile Belt and Ghanzi-Chobe Belt) are not very well known. Magnetotelluric (MT) profiles of the Southern African MagnetoTelluric EXperiments (SAMTEX) are present in this area and provide information about lithospheric strike directions and the resistivity distributions as well as possible locations of terrane boundaries, which verify some of the proposed terrane boundaries and suggest modification for others (e.g., Ghanzi-Chobe Belt to Magondi Mobile Belt terrane boundary and the western boundary of the Zimbabwe craton).

The Southern African Magnetotelluric Experiment (SAMTEX) is the largest ever land-based magnetotelluric (MT) project. The main objective of the project is to define the geo-electric structure across the region in order to gain a better understanding of Archean and Proterozoic tectonic processes. Only the MT profiles crossing the Rehoboth Terrane, the Neoproterozoic Ghanzi-Chobe/Damara belts (collectively termed the DMB) and the southern Angola craton are the focus of this study. One of the ways in which geo-electrical structural information is obtained is by detailed analysis of the measured impedance tensor. The Groom and Bailey decomposition technique was applied to the MT data and indicates significant depth and along-profile variations in geo-electric strike and dimensionality on all transects crossing these three tectonic units (i.e. Rehoboth Terrane, Angola craton and the DMB). The geo-electric strikes are generally parallel to the north-east trending tectonic fabric as inferred from the magnetic data, but the significant strike variations with depth are expressions of heterogeneity in the lithospheric structure. The Rehoboth terrane, south of the DMB, exhibits a strongly one dimensional (1D) to moderate two dimensional (2D) structure, with preferred strike directions in the range 200-450 20for the crust-mantle period (i.e. depth) range, indicating little crust-mantle decoupling. The DMB appears to be moderately 2D at lower crustal and upper mantle depths (10-100 s) with no consistent/preferred strike direction and significant phase differences between the conductive and resistive directions. North of the DMB and into the Angola craton there are significant variations in geo-electric strike direction and dimensionality at most sites for lower-crustal and upper mantle lithosphere. Our results further indicate that the profiles have to be divided into smaller areas having similar strike directions to allow for 2D modelling and inversion.

A 1400 km-long, 2-D magnetotelluric (MT) profile across the Archaean Kaapvaal Craton, the Proterozoic Rehoboth Terrane and the Late Proterozoic/Early Phanerozoic Ghanzi-Chobe/Damara Belt reveals significant lateral heterogeneity in the electrical resistivity structure of the southern African lithosphere. The profile indicates the following present-day average lithospheric thicknesses, to a precision of about ± 20 km, for each of the terranes traversed (inferred conductive geotherms in brackets): Eastern Kimberley Block of the Kaapvaal Craton 220 km (41 mWm-2), Western Kimberley Block 190 km (44 mWm-2), Rehoboth Terrane 180 km (45 mWm-2) and Ghanzi-Chobe/Damara Belt 160 km (48 mWm-2). Previously published mantle xenolith pressure-temperature (P-T) arrays from the Gibeon, Gordonia and Kimberley fields, however, suggest that the Rehoboth Terrane had equilibrated to a cooler conductive palaeo-geotherm (40 – 42 mWm-2 ) very similar to that of Eastern Kimberley Block of the Kaapvaal Craton, at some (unconstrained) time prior to the Mesozoic eruption of the kimberlites. A model consisting of the penetration of heat transporting magmas into the lithosphere, with associated chemical refertilisation, at an early stage of Mesozoic thermalism appears to be the most plausible model at present to account for both the present-day lithospheric structure of the Rehoboth Terrane and an earlier, cooler palaeo-geotherm. Some problems, however, remain unresolved in terms of the isostatic response of the model. Based on a compilation of xenocryst Cr/Ca-in-pyrope barometry observations, the extent of depleted mantle in the Rehoboth Terrane is found to be significantly reduced with respect to the Eastern Kimberley Block: 117 km versus 138 – 167 km. It appears most likely that the chemical depletion depth in both terranes, at least in the vicinity of kimberlite eruption, is accounted for by refertilisation of the lower lithospheric mantle.

Southern Africa, particularly the Kaapvaal Craton, is one of the world's best natural laboratories for studying the lithospheric mantle given the wealth of xenolith and seismic data that exist for it. The Southern African Magnetotelluric Experiment (SAMTEX) was launched to complement these databases and provide further constraints on physical parameters and conditions by obtaining information about electrical conductivity variations laterally and with depth. Initially it was planned to acquire magnetotelluric data on profiles spatially coincident with the Kaapvaal Seismic Experiment, however with the addition of seven more partners to the original four through the course of the experiment, SAMTEX was enlarged from two to four phases of acquisition, and extended to cover much of Botswana and Namibia. The complete SAMTEX dataset now comprises MT data from over 730 distinct locations in an area of over one million square kilometres, making SAMTEX the largest regional-scale MT experiment conducted to date.Preliminary images of electrical resistivity and electrical resistivity anisotropy at 100km and 200km, constructed through approximate one-dimensional methods, map resistive regions spatially correlated with the Kaapvaal, Zimbabwe and Angola Cratons, and more conductive regions spatially associated with the neighbouring mobile belts and the Rehoboth Terrane. Known diamondiferous kimberlites occur primarily on the boundaries between the resistive or isotropic regions and conductive or anisotropic regions.Comparisons between the resistivity image maps and seismic velocities from models constructed through surface wave and body wave tomography show spatial correlations between high velocity regions that are resistive, and low velocity regions that are conductive. In particular, the electrical resistivity of the sub-continental lithospheric mantle of the Kaapvaal Craton is determined by its bulk parameters, so is controlled by a bulk matrix property, namely temperature, and to a lesser degree by iron content and composition, and is not controlled by contributions from interconnected conducting minor phases, such as graphite, sulphides, iron oxides, hydrous minerals, etc. This makes quantitative correlations between velocity and resistivity valid, and a robust regression between the two gives an approximate relationship of Vs [m/s]=0.045*log(resistivity [ohmm])+4.5.

The Kaapvaal Craton is one of the world’s best natural laboratories for studying the lithospheric mantle given the wealth of xenolith and seismic data that exist for it. The Southern African Magnetotelluric Experiment (SAMTEX) was launched to complement these databases and provide further constraints on physical parameters and conditions by obtaining information about electrical conductivity structures within the lithosphere. Initially, magnetotelluric data acquisition was planned on profiles spatially coincident with the Kaapvaal Seismic Experiment. However with seven more partners joining the original four through the course of the experiment, SAMTEX was enlarged from two to four phases of acquisition, and extended northwards to cover much of Botswana and Namibia. The complete SAMTEX dataset now comprises MT data from over 730 distinct locations in an area of over one million square kilometres, making SAMTEX the largest regional-scale MT experiment conducted to date. Preliminary images of electrical resistivity and electrical resistivity anisotropy at 100 km and 200 km, constructed through approximate one-dimensional methods, map resistive regions spatially correlated with the Kaapvaal, Zimbabwe and Angola Cratons, and more conductive regions spatially associated with the neighbouring mobile belts and the Rehoboth Terrane. Known diamondiferous kimberlites occur primarily on the boundaries between the resistive or isotropic regions and conductive or anisotropic regions.