The Impact of High Precision Age Controls in Basin Modeling for Tectonic Studies: Karoo Basin, South Africa

McKay, Matthew P.; Dean, Justin; Weislogel, Amy

The Karoo Basin and adjacent Cape Fold Belt of South Africa record tectonism along the western Gondwanan margin from the Carboniferous through the Jurassic dissection of the Pangean 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 of 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 interbedded throughout the Permian Ecca Group within the Karoo Supergroup. 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 are coeval 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 (error unspecified). The compatibility between 40Ar/39Ar thermochronology and basin subsidence suggests that the Cape Fold Belt-Karoo Basin may represent a Late Paleozoic-Mesozoic intracratonic foldbelt-foreland basin system. If the age of deformational fabric development in the Cape Fold Belt is robust and not partially reset, then episodic subsidence events within the Karoo Basin could have been coupled with topographic crustal loading driven by Permian-Triassic shortening in the Cape Fold Belt.

The variation in tectonic subsidence within the basin modeling results is completely controlled by the age estimates for lithostratigraphic units. Lithostratigraphic age constraints are not always sufficient for generating a robust geohistory model in a basin with highly diachronous filling. Significant risk for major error exists when assessing burial histories using poorly-constrained chronostratigraphy.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013