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Terrane Processes at the Pacific Margin of Gondwana (TAPMOG), 5-6th September 2003 New Hall, Cambridge
R.C. King, G.J. Potts and S. Flint Oroclinal bending of the Cape Fold Belt and Palaeogeographic Evolution of the SW Karoo Basin, South Africa.
The Cape Fold Belt (CFB) displays a pronounced bend from the east-west Gondwana trend of the Late Palaeozoic Swartberg range to the north-south trending Cerderberge range. This c. 80-degree bend approximates to an orocline using an updated classification scheme to that of Carey (1955). The CFB is comprises Ordovician to Permian age rocks of the Cape Supergroup (Table Mountain, Bokkeveld, Witteberg Groups) and Karoo Supergroup (Dwyka and Ecca Groups). Current work on the CFB is investigating the interaction of the oroclinal bend and associated Hexriverberge syntaxis with the evolution and formation of the Tanqua & Laingsburg sub-basins of the Karoo (Figure 1). Fieldwork has produced present day cross-sections through the Cederberge and Swartberg ranges. The cross-sections have provided a guide to the 3D geometry within the CFB and have demonstrated differences between the two ranges. Sequential restoration of present-day cross-sections combined with published profiles are being used to understand the palaeography of the region.
The Cederberge displays simple geometry with shortening estimates of well below 10% demonstrating low strain deformation. Fold amplitudes decrease rapidly to the east, to the Tanqua sub-basin margin, producing a frontal monocline geometry (Vann et al, 1986) where deformation is accommodated by folding not thrusting. The Swartberg range is more intensely folded, with higher amplitude folds and increased shortening estimates of >15%. Folds plunge steeply to the northeast beneath the Laingsburg sub-basin margin. Greater amounts of shortening are seen to the south and west.
The sediments of the in the Laingsburg sub-basin provide evidence of a northward propagating fold-thrust front during the Permian. The position of the CFB oroclinal bend and the shortening history of the two branches provide evidence of the tectonic development and palaeography of the South African margin of Gondwana. Carey, S.W. 1955. The Orocline Concept in Geotectonics. Pap. & Proc. R. Soc. Tasmania, 89, 255-288. Vann, I.R., Graham, R.H. and Hayward, A.B. 1986. The Structure of Mountain Fronts. J. Struct. Geol., 8, 215-227.
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