Lithospheric structure, evolution and diamond prospectivity of the Rehoboth Terrane
and western Kaapvaal Craton, southern Africa: constraints from broadband magnetotellurics.
Muller, M.R., A.G. Jones, R.L. Evans, H.S. Grütter, C. Hatton, X. Garcia,
M.P. Hamilton, M.P. Miensopust, P. Cole, T. Ngwisany, D. Hutchins, C.J. Fourie,
H.A. Jelsma, S.F. Evans, T. Aravanis, W. Pettit, S.J. Webb, J. Wasborg, and The SAMTEX Team
A 1400 km-long, 2-D magnetotelluric (MT) profile, consisting of 69 sites at 20 km
intervals, across the western part of 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 lithospheric structures of the Rehoboth Terrane and Ghanzi-Chobe/Damara Belt have not
been imaged previously by geophysical methods.
Temperature is the primary control on the resistivity of mantle minerals, and the MT
derived lithospheric thickness therefore provides a very reasonable proxy for the
“thermal” thickness of the lithosphere (i.e., the thickness defined by the intersection
of a conductive geotherm with the mantle adiabat), allowing approximate present-day geotherms
to be calculated.
The work indicates the following present-day average lithospheric thicknesses, to an accuracy
of about 20 km, for each of the terranes traversed (inferred 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).
A clear relationship between the electrical resistivity structure of the lithosphere and
the tectonic stabilisation-age of the terrane is in evidence.
Good agreement between the inferred present-day lithospheric geotherms and surface heat
flow measurements indicate the latter are strongly controlled by variations in
A significant difference in lithospheric thickness is observed between the Eastern and
Western Kimberley blocks, and is consistent with previous seismic tomography images of
the Kaapvaal Craton.
The present-day lithospheric thickness, and reduced depth extent into the diamond
stability field, is able to account for the absence of diamondiferous kimberlites in the
Gibeon and Gordonia kimberlite fields in the Rehoboth Terrane.
Previously published mantle xenolith 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 mW/m2 ),
very similar to that of Eastern Kimberley Block of
the Kaapvaal Craton, at some time prior to the Mesozoic eruption of the kimberlites.
The timing and nature of both the thermal equilibration of the Rehoboth Terrane, and
the subsequent lithospheric heating/thinning event required to account for its
present-day lithospheric structure, are not well constrained.
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
mantle depletion in the Rehoboth Terrane is found to be significantly reduced with respect
to the Eastern Kimberley Block: 117 km versus 138 – 167 and it appears most likely that
the depletion depth in both terranes is explained by refertilisation of the lower
Lithos, 112S, 93-105. (Note: In Special Issue of Volume 112 devoted to 9th IKC.)
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Alan G Jones / 12 February 2010 /