SUBSIDENCE ANALYSIS OF THE PERMIAN TANQUA DEPOCENTRE, SOUTHWESTERN KAROO BASIN, SOUTH AFRICA

Basin subsidence analysis, employing the backstripping method, indicates that two fundamentally different basin-generating mechanisms controlled the Tanqua depocentre development in the southwestern Karoo Basin, South Africa. On one hand, the subsidence curves display initial dominantly decelerating subsidence suggesting an extensional regime and thermal control due to heating of the lithosphere possibly in a strike-slip setting. While on the other hand, a cooling phase followed the initial heating phase, and subsequent rapidly accelerating subsidence most likely a foreland basin compression which suggests flexure of the lithosphere. Based on this work, the subsidence phases of the Tanqua depocentre implies that the southwestern Karoo Basin was superimposed on a passive continental margin, i.e. on a lithosphere which was already mechanically extended and had thermally subsided. This implies that subsidence was initiated and mainly controlled by mechanical (i.e. detachment faults of basement blocks) rather than thermal events (i.e. sediment burial). In this context, the southwestern Karoo Basin evolved from a "passive" continental margin into an Andean-type continental foreland basin; hence characterizing a fully evolved post-rift setting along the southwestern Gondwana margin.