Archaean greenstone belt tectonism and basin development: some insights from the Barberton and Pietersburg greenstone belts, Kaapvaal Craton, South Africa

Abstract

The sediments in two of South Africa's major Archaean greenstone belts, the Barberton and Pietersburg greenstone belts, span an age range of some 800 million years. Both greenstone belts represent remnants of extensive fold and thrust belts with complex, but different polyphase tectonic histories. The oldest sediments were deposited between circa 3470 and 3490 M.a. on oceanic like crust preserved in the Barberton belt, possibly at the same time as sedimentation on similar oceanic crust preserved in the Pietersburg belt. Thereafter, the geologic evolution of these two belts diverged considerably. In the Barberton belt, there is clear evidence that the oceanic crust and sediments were obducted onto an intra-arc basin environment within 50 million years of its formation. The sequence was later further imbricated by northwest directed thrust stacking between 3300-3200 M.a. Basin development during both periods of thrusting took place in close proximity to active “calc-alkaline” arc systems. Deformation of the sediments within these basins took place while the same sediments were being deposited. Sedimentation took place predominantly in subaqueous environments, ranging from submarine mid-fans below the photic zone to tidal flats and deltaic plains. The sediments represent a polyhistory successor-type basin: early basins developed along a complex subduction related plate boundary; these basins later evolved into foreland depositories along and within collisional environments of an accretionary orogen. Late in the history of the Barberton greenstone belt (circa 3100 M.a.), the rocks were in places thermally reactivated and probably subjected to extensional processes; these processes overlapped in time with the main episodes of economic gold mineralization, and are of “early Witwatersrand-basin” age. The oceanic-like crust (including associated sediments) preserved in the Pietersburg belt was not significantly deformed until at least 500 million years after its formation: tectonic imbrication of this crust occurred between about 2900 and 2700 M.a., during north-directed thrusting. Sedimentary basins formed in response to this thrusting and associated bulk-flattening, and the sediments in these basins were deformed during sedimentation. Sedimentation took place predominantly in subaerial environments, in alluvial fans of an active foreland basin. It is likely therefore that the oceanic-like crust had previously been tectonically emplaced onto a continental environment as an allochthonous sheet, but the timing of this is not clear. The foreland basin of the Pietersburg belt which developed on top of this simatic basement is almost certainly part of the greater upper Witwatersrand basinal systems; it was most likely formed in an intermontane tectonic setting of the Cordillera-type such as that along the Mesozoic-Cenozoic margins of western North America. Late in the history of the Pietersburg belt ( circa 2.7 G.a.), the rocks were tectonically deformed in a major strike-slip fault system with both transpressional and transtensional kinematics. Sediments which were deposited and deformed in local contemporaneous basins, form an integral part of the greenstone belt's lithotectonic sequence, and are probably of Ventersdorp age. Unravelling the kinematic details of the basin evolution in both greenstone belts is now a major challenge for further understanding of the tectonic evolution of these belts, and almost the entire 800 million years of Archaean history of the Kaapvaal craton as recorded in these belts. A better understanding of the intimate relationship of tectonism to basin formation from greenstone belt studies, is of fundamental importance to understanding the famous Witwatersrand gold deposits and the source of this gold. This major task can only be achieved through better integrated and focused field and laboratory studies, in which precise chronostratigraphy must play a central role.