Martin J. Rigby

The Kaapvaal Craton, South Africa: no evidence for a supercontinental affinity prior to 2.0 Ga?

We briefly examine the possible antiquity of the supercontinental cycle while noting the likely unreliability of palaeomagnetic data >ca.1.8 Ga, assuming a gradual change from a magmatically dominated Hadean Earth to a plate tectonically dominated Neoarchaean system. A brief review of one of Earths oldest cratons, Kaapvaal, where accent is placed on the lithostratigraphic and geodynamic-chronological history of its cover rocks from ca. 3.1 to 2.05 Ga, forms the factual basis for this article. The ca. 3.1–2.8 Ga Witwatersrand–Pongola (Supergroups) complex retroarc flexural foreland basin developed while growth and stabilization of the craton were still underway. Accretion of relatively small composite granite-gneiss-greenstone terranes (island arc complexes) from both north and west does not support the formation of a Neoarchaean supercontinent, but may well have been related to a mantle plume which enhanced primary gold sources in the accreted terranes and possibly controlled the timing and rate of craton growth through plate convergent processes. Subsequent deformation of the Witwatersrand Basin fill with concomitant loss of ≤1.5 km of stratigraphy must have been due to far-field tectonic effects, but no known mobile belt or even greenstone belts can be related to this contractional event. At ca. 2714–2709 Ma, a large mantle plume impinged beneath the thinned crust underlying the Witwatersrand Basin forming thick, locally komatiitic flood basalts at the base of the Ventersdorp Supergroup, with subsequent thermal doming leading to graben basins within which medial bimodal volcanics and immature sediments accumulated. Finally (possibly at ca. 2.66–2.68 Ga), thermal subsidence enabled the deposition of uppermost Ventersdorp sheet-like lavas and sediments, with minor komatiites still present. Ongoing plume-related influences are thus inferred, and an analogous cause is ascribed to a ca. 2.66–2.68 Ga dike swarm to the north of the Ventersdorp, where associated rifting allowed formation of discrete ‘protobasinal’ depositories of the Transvaal (ca. 2.6–2.05 Ga Supergroup, preserved in three basins). Thin fluvial sheet sandstones (Black Reef Formation, undated) above these lowermost rift fills show an association with localized compressive deformation along the palaeo-Rand anticline, north of Johannesburg, but again with no evidence of any major terrane amalgamations with the Kaapvaal. From ca. 2642 to 2432 Ma, the craton was drowned with a long-lived epeiric marine carbonate-banded iron formation platform covering much of it and preserved in all three Transvaal Basins (TB). During this general period, at ca. 2691–2610 Ma, the Kaapvaal Craton collided with a small exotic terrane [the Central Zone (CZ), Limpopo Belt] in the north. Although farfield tectonic effects are likely implicit in TB geodynamics, again there is no case to be made for supercontinent formation. Following an 80–200 million years (?) hiatus, with localized deformation and removal of large thicknesses of chemically precipitated sediments along the palaeo-Rand anticline, the uppermost Pretoria Group of the Transvaal Supergroup was deposited. This reflects two episodes of rifting associated with volcanism, and subsequent thermal subsidence within a sag basin setting; an association of the second such event with flood basalts supports a plume affinity. At ca. 2050 Ma the Bushveld Complex intruded the northern Kaapvaal Craton and reflects a major plume, following which Kaapvaal–CZ collided with the Zimbabwe Craton, when for the first time, strong evidence exists for a small supercontinent assembly, at ca. 2.0 Ga. We postulate that the long-lived evidence in favour of active mantle (cf. plume) influences with subordinate and localized tectonic shortening, implicit within the review of ca. 3.1–2.05 Ga geological history of the Kaapvaal Craton, might reflect the influence of earlier Precambrian mantle-dominated thermal systems, at least for this craton.