The tectono-sedimentary history of the Van Rhynsdorp Group

Abstract

Abstract provided by author:

The sedimentology, metamorphism and tectonism of the pre-Cape metasedimentary sequence, exposed between Bitterfontein, Nieuwoudtville and Klawer in the Vanrhynsdorp District, Cape Province, have been investigated. The sequence is named the Vanrhynsdorp Group and subdivided into sub-units which are correlated with the Nama Group of South West Africa/Namibia. Deposition of the Vanrhynsdorp Group is described in terms of longitudinal delta fan sedimentation down a late Precambrian Nama Gulf which was open-ended towards the south. Suturing progressed southwards along a proposed collision zone towards the west which shed exogeosynclinal, transverse turbidites paroxismically into the Gulf. As these synorogenic sediments were drawn towards the subduction zone, they themselves were affected by prevailing congressional forces which generated a northeastward verging, low-grade, marginal foreland thrust-fold belt on the subducting southern African plate. The tectonite developed in two phases. Firstly, either thin-skinned thrusting and ramping by simple shear or transverse shortening across a sinistral strike-slip shear zone in the basement, produced northeastward facing, transected F1 folds. It is postulated that a 7 to 4 kilometre thick nappe sequence belonging to the subduction complex, overrode the Vanrhynsdorp Group late during this 500 to 550 My F1 tectonic event, thus preventing any further thin-skinned thrust tectonics. As the tectonism progressed, F1 folds and thrusts were steepened and ultimately backfolded during a co-axial F2 event as a result of underthrusting along a basal decollement. Baric conditions during tectonism are comparable to intermediate pressure associations of the Otago/Sanbagawa type