Resolving the age of the first-order topography of southern Africa: new insight from joint (U-Th)/He and fission track dating of samples from deep boreholes

The topography of Africa is unusually high with respect to other continents and its origin remains strongly debated. Africa's topography is strongly bimodal as it is distributed between the high plateau areas in its central part (circa 1000 m), and significantly less elevated areas with higher relief around its borders. The geodynamical interpretation of this feature is not straightforward as the plateau is essentially surrounded by passive margins and oceanic ridges. However, abundant seismic studies have revealed a deep seismic anomaly beneath Africa and suggest that forces related to active upward flow within the mantle are dynamically sustaining its high elevation. If the large anomaly provides a mechanism explaining the south African plateau, a lot of questions remain on the timing of uplift. Geodynamic models allow Africa to go up or down but fail to put constrains on the age of the uplift. This is mainly because of a lack of tight constraints on the viscosity and density structure of the mantle, which lead to several models with uplift occurring either during Cretaceous or Miocene times. The question of the age of the plateau therefore remains unresolved. Thermochronology and techniques such as fission track and U-Th/He analyses provide tools to address this question by constraining the erosion history. In this study we take advantage of the availability of deep boreholes located all across south-Africa to sample truly vertical profiles through the plateau. The key advantage of this approach is that it enables constraints to be placed on the timing and amount of cooling resulting from relatively low amounts of erosion. A dozen boreholes from above and below the great escarpment have been sampled. We present fission track and U-Th/He results for three of them. The U-Th/He analyses are performed as single grain analyses with an average number of 15+ aliquots per sample for a total of 250+ single grain analyses in order to provide a high resolution chronology and to quantify the dispersion of single crystal ages. The shallowest borehole is 0.8 km and the deepest is 1.6, with most of them deeper than one kilometer. A spatial pattern is clearly evidenced with boreholes located above the escarpment showing older ages those located below. Thermal history inversion was performed using QTQt that enables a joint modelling of AHe, AFTA and fission track lengths. The data provide robust well-constrained evidence for a major period of cooling during the late Cretaceous (circa 90 Ma). From local measurement of the geothermal gradient and estimation of palaeo-thermal gradient we show that these data are best explained by an erosion event which could have led to removal of about 3.5 kilometers in the central part of the plateau. These estimates agree with the offshore sedimentary records on the eastern craton margin. We discuss the perspectives offered by joint analyses of thermochronological data and the utilization of surface erosion models based on CASCADE to provide new insight into south-African uplift and role of mantle dynamics.