Tectonostratigraphical development of the Upper Karooforeland basin: Orogenic unloading versus thermally-induced Gondwana rifting

The origins of the Karoo Basin have never been adequately explained, but twomain models have been suggested: (1) a purely foreland basin (retro-arc) model; and (2) a polyphase successor basin, beginning with extension during Upper Table Mountain Group (Silurian) times to a foreland basin in the Early Karoo (Permian) and extension in the Upper Karoo (Upper Triassic-Jurassic). Subsidence and forward modelling on the known stratigraphy of the Karoo Basin do not fit a simple foreland basin model. This is particularly true for the Upper Karoo where the stratigraphy, stacking patterns and age of the succession suggest that the foreland basin model is not viable, and that continental extension played a major role in late basin development. Evidence for extension, and when this occurred, is provided by: (1) sequence stratigraphical identification of a major Middle-Late Carnian low stand sequence boundary unconformity, related to uplift of a newly emergent source area in the southeast; and (2) the presence of volcanic detritus in Upper Karoo sediments, chemically similar to the Karoo basalts, suggesting a possible chemogenetic link between them and, by implication, with early volcanism and incipient Gondwana rifting, now well-preserved in the rift sequences of East Antarctica. This early volcanism, which began not later than the Carnian (∼230 Ma), and provenance uplift, may be related to a thermal anomaly linked to mantle plume-induced updoming, crustal extension and faulting approximately 40 Ma prior to maximum uplift preceeding Karoo Basalt eruption at 183 Ma. Thermal uplift occurred some 200 km off the present southeast coast of South Africa, just south of the Karoo Hotspot, but away from the direct influence of any known hotspot or hotspot track. This uplift, aligned parallel to the southeast coast of South Africa, at a high angle to the Cape Fold Belt, was located close to the site of later rifting and separation along the Agulhas Falkland Fracture Zone.