Abstract
Southern Africa is an elevated continental plateau located far from plate boundaries. The causes of plateau formation remain debated, but many have suggested that topographic uplift was due to mantle processes. Low temperature thermochronology and sedimentation rates in surrounding marine basins illuminate a distinctive pulsed erosion history, with a major phase of erosion in the Mid-Cretaceous often inferred to indicate plateau uplift at that time. However, the relationship between mantle processes, uplift, and erosion patterns is not simple. Previous work showed that the complex lithosphere responded differentially to a major thermochemical modification event and that lithospheric architecture exerted an important control on regional erosion patterns (Stanley et al., 2013; 2015). Additionally, the reactivation of faults locally controlled exhumation (Wildman et al., 2016; 2017). In addition to these structural controls, surface processes should have an important influence on erosion patterns and rates. Here we explore the role of river network geometry by focusing on the lower reaches of the Orange River, the large river system which drains the plateau interior. A strong signature of fluvial incision might be expected here, but the lithosphere of this complex region was strongly modified at approximately 90 Ma and the area has many pre-existing faults. We present new apatite (U-Th)/He (AHe) data from kimberlite and basement rocks in the downstream reaches of the Orange River system and integrate them with previously published AHe data from the larger region to investigate the relative roles of mantle processes, structures, and rivers on erosion patterns with the goal of better isolating the influence of river network geometry. The study area was affected by two Cretaceous erosion phases that were influenced by fault reactivation, but we show that the first phase ( approximately 120-110 Ma) predates regional mantle modification and suggest this is due to the shape of the Orange River drainage basin. We also identify spatially variable Cenozoic erosion which appears to be focused near present and paleo river channels. These results highlight the important role that river network geometry plays in modulating the mantle derived uplift signatures in the erosion history of the southern African Plateau.