Geochronology of the Cape Fold Belt;and the timing of Gondwanide orogeny in South Africa

Jeroen Hansma

    Research output: ThesisDoctoral Thesis

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    This research presents eleven new plateau ages from the Cape Fold Belt in South Africa to constrain the timing of orogenesis. New 40Ar/39Ar ages from handpicked muscovite grains of the Cape Supergroup show a shorter duration of mountain building than previously considered (33Ma; from 279.5Ma to 246.5Ma). Two age populations, at 250-255Ma and 270-280Ma are present in the dataset. I also present three detrital plateau ages >400Ma in the northernmost Witteberg Group. To complement 40Ar/39Ar age data I provide detailed microstructural observations, scanning electron microscope data, electron microprobe analyses on mica, powder X-ray diffraction, and anisotropy of magnetic susceptibility of samples. Microstructural features and fabric analysis highlight the importance of strain partitioning and non-homogenous deformation throughout the belt and that layer parallel slip is an important mechanism accommodating Cape Supergroup shortening. Pressure solution, fracture, and limited quartz plasticity are the dominant microstructural deformation mechanisms by which the Bokkeveld Group and Witteberg Groups deform. Dynamic quartz recrystallization in Table Mountain Group and Basement rocks is the expression of an increase in ductility deeper within the orogen. Pressure solution mechanisms result in grain size reduction of phyllosilicate minerals. This fine-grained muscovite recrystallization (causing a reduction in closure temperature of grains) is the mechanism proposed to explain an increase in plateau ages from the hinterland of the orogen outwards, a trend that is interpreted as muscovite cooling ages. The possibility of both age populations representing cooling (250-255Ma) and deformation ages (270- 280Ma) being present together with detrital ages is suggested, but such a hypothesis remains to be rigorously tested. These ages provide a high quality constraint upon Cape orogenesis, and confirm that grain ages from Beaufort Group Sediments in the Karoo Foreland Basin can be explained by a Cape Fold Belt provenance.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2013


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