Major influences on the evolution of the 2.67-2.1 Ga Transvaal basin, Kaapvaal craton

The Transvaal is one of three structural basins of the Transvaal Supergroup preserved on the Kaapvaal craton. The evolution of the Neoarchaean-Palaeop roterozoic Transvaal basin is ascribed predominantly to magmatism, palaeocl imate and eustasy, with plate tectonics playing a sporadic role. The superg roup comprises basal 'protobasinal' rocks, followed by the Black Reef Forma tion, Chuniespoort Group and uppermost Pretoria Group. Immature siliciclast ic and bimodal volcanic rocks of the protobasinal unit reflect a wide zone of rifting related to the c. 2.7 Ga Ventersdorp (Supergroup) mantle plume. Individual protobasinal successions were laid down in separate fault-bounde d basins, controlled at least partially by greenstone belt orientations in the Kaapvaal basement. Post-magmatic, post-rifting thermal subsidence accommodated Black Reef fluv ial sheet sandstones and the subsequent thick carbonate-BIF epeiric platfor m succession of the Chuniespoort Group. Subordinate mechanical subsidence a ccompanied this long-lived thermal relaxation. Intense weathering due to Ne oarchaean atmosphere composition greatly reduced elastic sedimentation and the greenhouse palaeoclimate further encouraged carbonate sedimentation. Gl obally enhanced sea levels, due to enhanced mid-ocean ridge growth conseque nt upon either global magmatic events or supercontinent break-up, also play ed a pivotal role in Chuniespoort epeiric basin evolution. Pretoria Group sedimentation is ascribed to two cycles of rifting and subse quent thermal subsidence. The first cycle appears to reflect plate tectonic ally induced rifting, with an epeiric sea drowning the rift basin despite r educed sea levels due to the first major global glaciation. The second Pret oria cycle is most likely related to a major continental flood basalt event , with thermal subsidence allowing a second and probably larger epeiric sea to advance onto the northern Kaapvaal craton. Evaluation of the inferred r elatively minor importance of plate tectonics in the evolution of the essen tially intracratonic Transvaal basin depends to a large degree on the age a ssigned to the Limpopo orogeny during which the Zimbabwe and Kaapvaal crato ns collided; recent evidence for a c. 2.0 Ga collision supports the concept that catastrophic global magmatic events gradually gave way to plate tecto nics during Transvaal basin evolution. (C) 2001 Elsevier Science B.V. All r ights reserved.