The timing of the Cape Orogeny: New 40Ar/39Ar age constraints on deformation and cooling of the Cape Fold Belt, South Africa

Abstract

We conducted a 40Ar/39Ar thermochronological study of samples from three transects across the Cape Fold Belt of South Africa. Metamorphic conditions along these transects decrease from the greenschist facies in the southern hinterland to the anchizonal foreland to the north. Nine new 40Ar/39Ar plateau ages and two mini-plateau ages from handpicked muscovite are combined with one earlier published biotite mineral age and reveal two age modes. The older population ranges from 261±3 to 276±5Ma (n=4) and the younger from 248±2 to 254.6±2.1Ma (n=4). The remaining three ages (>400Ma) are detrital and reflect unreset ages. Electron microprobe analyses and X-ray diffraction studies indicate little mineralogical variation of micaceous material across the belt; samples dominantly host muscovite, clay minerals such as illite or smectite are absent. Microstructural characterization of rock fabrics across the belt reveals different mechanisms of strain partitioning and heterogenous deformation of different lithologies. Specifically, pressure solution, fracture, and limited crystal plasticity are the dominant deformation mechanisms in the shale-dominated Bokkeveld Group and Witteberg Groups in the foreland region, resulting in recrystallization of phyllosilicate minerals. Strain partitioning into narrow, locally deformed horizons of the Table Mountain Group is accompanied by dynamic grain boundary recrystallization in quartz. A weak trend of southward younging ages is interpreted as the preservation of deformation ages (ca. 275–260Ma) and younger cooling ages (ca. 255–245Ma), although it remains plausible that these populations represent two deformation events. These new age constraints significantly narrow the temporal window of Cape Fold Belt tectonic activity, and provide a firm basis for evaluating the depositional patterns in the foreland Karoo Basin.