Seismic Stratigraphic Constraints on Neoarchean - Paleoproterozoic Evolution of the Western Margin of the Kaapvaal Craton, South Africa

Abstract

The Kaapvaal Craton is partially covered by the supracrustal sequences of remnants of the Witwatersrand, Ventersdorp and Transvaal-correlated Griqualand West Basins that span about 800 million years (~3.0 to 2.2 Ga). This study describes and interprets eight newly available seismic reflection profiles of the Ventersdorp and younger cover sequences, acquired by the vibroseis method to 6 seconds TWT (~ 17km), and totalling ~720km in length. New stratigraphic and structural features are identified across three main regions: the Kaapvaal Craton’s western margin, the craton’s northern margin, (southwards from the Thabazimbi-Murchison Lineament across the western extremity of the Bushveld Igneous Complex) and the craton’s interior. Across the craton’s western margin, the volcano-sedimentary succession is shown to thicken towards its edge from 0 km to >17km over 150km. The Ventersdorp Supergroup underlies the Transvaal Sequence right to the western extremity of the craton. Close to the margin, we recognise a normal fault, here named the Moshaweng fault, with >6km normal displacement and an unknown distance of strike-slip displacement. Three prominent unconformities are identified, as well as ~300% tectonic thickening of the Hartley Formation in response to east-verging, Paleoproterozoic (~1.8 Ga) thrusting across the craton. Folding and thrusting of Griqualand West and Olifantshoek Supergroups across the craton are also apparent. A tectonic model for the evolution of the craton’s western margin involves the formation of an Atlantic-type margin by extension, volcanism and rifting over ~70 Ma, followed by 160 Myrs of thermal subsidence. Subsequently, Paleoproterozoic (1.7 to 1.9 Ga) compression involved folding and thrusting eastward across the margin. Along the northern margin of the craton, the seismic profiles traverse a syncline which trends east to west over several hundred kilometers and involves mainly Transvaal-aged rocks and sporadic Bushveld outcrop. Evident from these profiles are discrete Ventersdorp half grabens, up to 3km in depth, with bounding listric normal faults that trend east to west, subparallel to the Thabazimbi-Murchison Lineament (TML), and downthrow predominantly to the north. These faults are truncated by the well-imaged Black Reef unconformity. Across the craton interior, horst and graben structures, involving the Witwatersrand and lower Ventersdorp Formations, dominate the seismic profiles. Bounding listric normal faults (one example with minimum throw of 9.5km), trend east to west and northeast to southwest and downthrow to the north, south, northwest and southeast. The Black Reef unconformity is again distinctive, as is a Paleozoic unconformity (the Beaufort unconformity). The orientation of boundary faults of Ventersdorp-aged half-grabens along the craton’s margins and interior, south of the TML, suggests a complex process of extension tectonism for ~70 Myrs that overlaps in time with northward thrusting of volcano-sedimentary sequences to the north of the TML. On a regional scale, the data presented are consistent with a model for the Kaapvaal Craton that resembles, both in space and timing, the evolution of the Argentine Atlantic margin. This margin developed during an initial 40 to 70 Ma period of regional extension across a ~600 km wide volcano-tectonic rift zone behind an active (Andean) subduction zone, followed by regional lithospheric cooling and subsidence. Our results also confirm that the Kaapvaal Craton was part of a larger Meso-Archean landmass and thus supports growing realisation that models of Archean continental growth rates based on the present day aerial extent of Archean cratons, represent only minimum values.