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Thrust faults and fold vergence in the Palaeozoic middle and upper Witteberg Group, Cape Supergroup (Cape Fold Belt), Steytlerville: an interpretation of their relationship

Department of Geology, University of Port Elizabeth, P.O Box 1600, Port Elizabeth, 6000, Republic of South Africa., e-mail: glapwb{at}upe.ac.za

Four sections through middle and upper Witteberg Group strata north of Steytlerville were investigated in detail to ascertain the phenomenon of opposite vergence of folds, as well as the relationship between folding and thrusting. Strata of the middle Witteberg Group are thinly-bedded and predominantly arenaceous, whereas those of the upper Witteberg Group show variation in bed thickness, lithological composition and proportion of arenaceous to argillaceous material. East-southeast striking thrust faults are commonly developed throughout the study area, and have a complex history of development. They display characteristic ramp and flat geometry, with south-southwest-dipping forethrusts predominating over north-northeast-dipping backthrusts. Thrust/fold relationships show that thrust faults formed prior to, during and after folding. Fold styles vary from gentle to tight, and fold axes of folds plunge shallowly east-southeast and west-northwest. The orientation of east-southeast-striking axial planes of folds and their predominantly northward vergence indicate that folds formed during a north-northeastward-directed compression event, during the Cape orogeny. The few examples of southward vergence of fold structures are explained in some cases by formation of folds during backthrusting, and in other cases folds formed prior to fore-and backthrusting. The implications of these findings are that strata of the middle and upper Witteberg Group have inherent structural complexities that make it difficult to define the lithostratigraphy.