Publication 131

The Electrical Lithosphere beneath the Kaapvaal Craton, Southern Africa

Evans, R.L., A.G. Jones, X. Garcia, M. Muller, M. Hamilton, S. Evans, S. Fourie, J. Spratt, S. Webb, H. Jelsma, and D. Hutchins

Abstract

A regional-scale magnetotelluric (MT) experiment across the southern African Kaapvaal craton and surrounding terranes, called SAMTEX (Southern African MagnetoTelluric Experiment), 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 thermo-barometry 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 a slightly “damp” (a few hundred of ppm water) structure within the transitional region at the base of the lithosphere. However, the structure of the uppermost ~150km of lithosphere is consistent with enhanced, but still low, conductivities reported for hydrous olivine and Opx 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.

Source

Journal of Geophysical Research - Solid Earth, 116, B04105, doi: 10.1029/2010JB007883, 16pp. [PDF]


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Alan G Jones / 15 October 2011 / alan-at-cp.dias.ie