Electrical lithosphere beneath the Kaapvaal craton, southern Africa

Abstract

[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.

Deformation and hydration of the lithospheric mantle beneath the Kaapvaal craton, South Africa

“…Recent electrical conductivity measurements suggest that the lithosphere beneath the central Kaapvaal craton is currently defined by a high resistivity layer 200-250 km thick (Evans et al, 2011). High seismic velocities imaged in body and Rayleigh wave tomography also indicate a 250 km thick root beneath most of the craton (Chevrot and Zhao, 2007;James et al, 2001).…”

“…Cratons are domains of thick lithosphere with cold geotherms (Boyd et al, 1985;Chevrot and Zhao, 2007;Evans et al, 2011;Jaupart and Mareschal, 1999), which have remained stable for long geologic periods since their formation in the Archean (Pearson et al, 1995). However, while the mantle roots of some cratons, such as the Kaapvaal in South Africa, survived from reworking by plate tectonics and mantle convection since the Archean, others, like the north China and Wyoming craton, were partially destroyed in more recent times (e.g., Eggler and Furlong, 1991;Menzies et al, 1993).…”

The link between electrical conductivity anomalies and rheological boundaries

“…Suggested conductive agents include graphite in the Slave Craton (Jones et al, 2001) , hydrated minerals in the Kaapvaal Craton (Evans et al, 2011) and compositional changes of mantle minerals in the Dharwar Craton (Patro et al, 2009). New results from the Gawler Craton see a similar feature underlying the surface expressions of the Gawler Range Volcanics and the Hiltaba Suite (Thiel and Heinson, 2013).…”

Mapping the 3D extent of the Northern Lobe of the Bushveld layered mafic intrusion from geophysical data

“…Our 3D MT inversion emphasizes the resistivity structure of the upper 1-20 km 288 rather than the deeper structures as in previous models [Evans et al, 2011;Khoza et al, 289 M a n u s c r i p t 14 inverting the vertical magnetic field transfer function data. The refined MT inversion 291 model incorporates previously modeled data [Evans et al, 2011;Khoza et al, 2013] to 292 cover the entire Bushveld Complex along the KAP line and extend southwest of our 293 study area to cross exposed Transvaal units that could be buried in our study area.…”

“…1) based on gravity [Council for Geoscience, 1997a], magnetic 62 [Ledwaba et al, 2009] and MT [Evans et al, 2011;Khoza et al, 2013] data constrained M a n u s c r i p t a contrast of 320 kg/m 3 between high velocity/density crust (3020 kg/m 3 ) and mantle 200 inferred to relate to mafic underplating was calculated for the Kgaswane model [Cole et 201 al., 2014;Kgaswane et al, 2012]. Subtraction of these gravity effects from the observed 202…”