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| TSG Annual Meeting, 8th - 10th January
2003 University of Liverpool |
| R.C. King, G.J. Potts and S. Flint
Oroclinal bending of the Cape Fold Belt (CFB) and its effect on the formation and evolution of the Karoo basin, South Africa. |
| Current work on the CFB is investigating the
interaction of the CFB oroclinal bend and associated Hexriverberge syntaxis
with the evolution and formation of the Tanqua & Laingsburg sub-basins. A
structural model is being developed to evaluate how the Hexriverberge syntaxis
may have partitioned the two sub-basins. This will, in turn, lead to an
understanding of the tectonic control of the CFB and/or syntaxis on sediment
supply routes into the sub-basins. Initial fieldwork produced present day cross-sections through the N-S trending Cederberge and W-E trending Swartberge ranges in orientations normal and parallel to the major trends of the fold axes. The cross-sections have provided a guide to the 3D geometry within the CFB and have generated some ideas of the formation of the CFB, with a frontal monocline in the Cederberge area and a simple fold belt in the Swartberge range. Sequential restoration of these and other sections will be used to model the evolution of basin floor paleogeography. The distinctive arcuate shape of the CFB approximates to an oroclinal bend and it has been compared with oroclines documented worldwide, on the basis of key structural characteristics. Since the original definition of an orocline (Carey, 1955) the work of Ries and Shackleton, (1976) and Marshak, (1988) on arcuate orogenic zones has evolved and expanded this definition to include many orogenic zones that do not fit Carey's original idea of an orocline. This leaves the present-day geologist with a confusing account of an orocline, giving rise to a need for a clearer definition. A guide has been formulated from a comparative study; essential characteristics of an orocline include: Angle of deflection >60º, arc-parallel fold hinges, open fold shapes, arc-parallel thrust faults and oblique extensional faults. Other important features have been designated as commonly present and occasionally present. This scheme allows the rapid and consistent identification of oroclines. The position of the Tanqua and Laingsburg sub-basins within the orocline may be critical to their development. Work continues to evaluate the effect of the orocline and/or syntaxis on basin evolution, using aerial photography and fractal analysis of key structures and field mapping. References Carey, S.W. (1955). The orocline concept in geotectonics: Part 1. Papers and Proceedings of the Royal Society of Tasmania, 89, pp 225-288. Marshak, S. (1988). Kinematics of orocline and arc formation in thin-skinned orogens. Tectonics, 7, pp 73-86. Ries, A.C., and Shackleton, R.M. (1976). Patterns of strain variation in arcuate fold belts. Philosophical Transactions of the Royal Society of London, 283, (A), pp 281-288. |
| TSG Annual Meeting, January
2004 University of Durham (TBC) |
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