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Crustal structures across the central Kaapvaal craton from deep-seismic reflection data

CIGCES, Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa emails: maarten/justine{at}cigces.uct.ac.za

Eight deep-seismic reflection profiles (16 seconds TWT; generated by the vibroseis method) are presented and interpreted. The profiles cover a total length of 500km and are projected into a common section that strikes northwest to southeast across the central part of the Kaapvaal Craton, including the Colesburg Magnetic Lineament (CML). The profiles clearly image the crust-mantle transition (Moho) between 11 and 13 seconds TWT (~ 36 to 41km).

The sections link some of the brittle Ventersdorp (~2.7 Ga) faults near surface to more subhorizontal ductile shear zones and related features that transect the upper crust of the Kaapvaal Craton. Sharp shallow-dipping reflectors that predate the Ventersdorp faults subdivide the entire crust into four elongate panels (or slivers; >200km by 10 to 20km). These panels extend from below Vryburg in the west, across the CML, to the crystalline basement below the Witwatersrand Basin in the east. The upper-most crustal reflectors dip towards the west and indicate extensive east-directed tectonic accretion and emplacement of a ~10km-thick allochthonous upper crustal panel comprising the Amalia-Kraaipan granite-greenstone terrane, across a Witwatersrand basement panel. The latter, in turn, is underlain by an elongate crustal panel that dips west from the surface near Welkom, reaching the lower crust below the centre of the section. The CML that subdivides the craton into an eastern block and a western block at surface is not a feature that can be easily linked to an underlying suture in the lower crust and mantle. Instead, this lineament has a shallow origin related either (1) to the 3.0 to 2.8 Ga Amalia-Kraaipan greenstone belt lithologies (that include extensive BIF, and/or (2) to possible thin back-thrust wedges of highly magnetic shales of the lower Witwatersrand sequences with (as previously suggested by Corner et al., 1990) shallow east-dipping magnetite-rich basement granites.

The entire crustal section resembles a tectonically stacked series of crustal slivers derived from the (north-) west that amalgamated in the Neoarchean (2.92 to 2.76 Ga) along east-propagating thrusts that cut up-section from middle to lower crust to reach the surface next to the upper Witwatersrand foreland basin. Distinct shallow dipping reflectors below the Moho suggest that the upper mantle was involved in this "shingling" tectonics (c.f. Cook, 1986). Any related suture zone must have been positioned to the west of the present craton boundary. The lowermost crustal panel is probably composed of old TTG- basement (>3.2 Ga) that was extended and partially melted during the Ventersdorp event (~2.7 Ga), leaving a near-flat smooth and annealed Moho.

A seismic profile across the southern margin of the Kaapvaal craton images the tectonic boundary with the Mesoproterozoic Natal Namaqua Mobile Belt (NNMB), and is consistent with existing geological and geochemical models in which the Kaapvaal craton is overthrust by the NNMB. The data suggest that cratonic crust and probably mantle lithosphere underlie a significant section of the NNMB beneath Natal and the Eastern Cape.

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