INTEGRATING BURIAL HISTORY AND ISOSTATIC MODELS TO EVALUATE TECTONIC SUBSIDENCE MODELS: KAROO BASIN, SOUTH AFRICA

The Karoo Basin and adjacent Cape Fold Belt of South Africa record tectonic activity along the western Gondwanan margin from the Carboniferous through the Jurassic dissection of the supercontinent. Previous studies have suggested that the Karoo Basin may be a retroarc foreland basin created as a result of lithospheric flexure in response to tectonic loading in the Cape Fold Belt. However, paleotectonic reconstructions indicate the distance between the Panthalassan margin and the Karoo Basin may have exceeded 1000 km, making this interpretation suspect, and leading to recently postulated models that link early tectonic subsidence of the Karoo Basin to epeirogenic, mantle-driven dynamic subsidence. We present new results from 1-D basin models that incorporate newly available stratigraphic age controls from volcanic ashes in the Permian Ecca Group. Basin subsidence analysis suggests that periods of rapid tectonic subsidence occurred during the intervals of 280-277, 270-257, and 248-243 Ma. These rapid subsidence events coincide with deformational events in the adjacent Cape Fold Belt as constrained by previously published ⁴⁰Ar/³⁹Ar ages of syn-metamorphic cleavage micas that record periodic, post-deformational cooling at 298, 278, 258, 247, and 230 Ma (± unspecified error). The compatibility between ⁴⁰Ar/³⁹Ar thermochronology and basin subsidence, suggests that the Cape Fold Belt-Karoo Basin may represent a Late Paleozoic-Mesozoic intracratonic thrust belt-foreland basin system. Despite some arguments concerning partial thermal resetting of the mica thermochronology ages, the age of deformational fabric development in the Cape Fold Belt may be robust and not reset, as has been suggested. If this is the case, then episodic subsidence events within the Karoo basin were likely coupled with topographic crustal loading driven by Permian-Triassic shortening in the Cape Fold Belt.