Mesozoic exhumation of the southern Cape, South Africa, quantified using apatite fission track thermochronology

Southern Africa's topography is distinctive. An inland plateau of low relief and high average elevation is separated from a coastal plane of high relief and low average elevation by a steeply dipping escarpment. The origin and evolution of this topography is poorly understood because, unlike high plateaus elsewhere, its development cannot be easily linked to present day compressional plate boundary processes. Understanding the development of this regional landscape since the break-up of Gondwana is a first order step towards resolving regional epeirogenesis. We present data that quantifies the timing and extent of exhumation across the southern Cape escarpment and coastal plane, using apatite fission track analysis (AFTA) of 25 outcrop samples and 31 samples from three deep boreholes (KW1/67, SA1/66, CR1/68). Outcrop fission track (AFT) ages are Cretaceous and are significantly younger than the stratigraphic ages of their host rocks, indicating that the samples have experienced elevated paleotemperatures. Mean track lengths vary from 11.86 to 14.23 μm. The lack of Cenozoic apatite ages suggests that major cooling was over by the end Cretaceous. The results for three boreholes, situated seaward (south) of the escarpment, indicate an episode of increased denudation in the mid-late Cretaceous (100–80 Ma). An earlier episode of increased denudation (140–120 Ma) is identified from a borehole north of the escarpment. Thermal modelling indicates a history involving 2.5–3.5 km of denudation in the mid-late Cretaceous (100–80 Ma) at a rate of 175 to 125 m/Ma. The AFT data suggest that less than 1 km of overburden has been eroded regionally since the late Cretaceous (< 80 Ma) at a rate of 10 to 15 m/Ma, but do not discount the possibility of minor (in relative amplitude) episodes of uplift and river incision through the Cenozoic. The reasons for rapid denudation in these early and mid-Cretaceous episodes are less clear, but may be related to epeirogenic uplift associated with an increase in mantle buoyancy as reflected in two punctuated episodes of alkaline intrusions (e.g. kimberlites) across southern Africa and contemporaneous formation of two large mafic igneous provinces (~ 130 and 90 Ma) flanking its continental margins. Because Cenozoic denudation rates are relatively minimal, epeirogenic uplift of southern Africa and its distinct topography cannot be primarily related to Cenozoic mantle processes, consistent with the lack of any significant igneous activity across this region during that time.