Abstract
The South African landscape displays important lithological and topographical heterogeneities between the eastern, western margins and the plateau. Yet the underlying mechanisms and timings responsible for this peculiar layout remain unclear. While studies have proposed a post-Gondwana uplift driver, others have related these heterogeneities to a more recent evolution induced by deep mantle flow dynamics during the last 30 million years. This theory seems supported by the rapid increase of sediment flux in the Orange basin since the Oligocene. However, the triggers and responses of the South African landscape to dynamic topography are still debated. Here we use a series of numerical simulations forced with Earth data to evaluate the contribution of dynamic topography and precipitation on the Orange river source-to-sink system since the Oligocene. We show that, if the tested uplift histories influence deposits distribution and thicknesses in the Orange sedimentary basin, they poorly affect the large-scale drainage system organisation and only strongly impact the erosion across the catchment for two of the four tested dynamic topography cases. Conversely, it appears that paleo-rainfall regimes are the major forcing mechanism that drives the recent increase of sediment flux in the Orange basin. From our simulations, we find that climate strongly smoothed the dynamic topography signal in the South African landscape and that none of the currently proposed dynamic topography scenarios produce an uplift high enough to drive the pulse of erosion and associated sedimentation observed during the Palaeocene. These findings support the hypothesis of a pre-Oligocene uplift. Our results are crucial to improve our understanding of the recent evolution of the South African landscape.
Highlights
Conflicting timings and drivers have been proposed to explain the singular South African tilted topography as well as the subsequent increase of the sediment flux in the Orange basin over the last 30 Ma
It appears that paleo-rainfall regimes are the major forcing mechanism that drives the recent increase of sediment flux in the Orange basin
We find that climate strongly smoothed the dynamic topography signal in the South African landscape and that none of the currently proposed dynamic topography scenarios produce an uplift high enough to drive the pulse of erosion and associated sedimentation observed during the Palaeocene
Summary
Conflicting timings and drivers have been proposed to explain the singular South African tilted topography as well as the subsequent increase of the sediment flux in the Orange basin over the last 30 Ma. We focus on new studies that show a pulse of sedimentation in the Orange basin over the last 25 Ma (Baby et al, 2018, 2020) combined with drainage studies (Roberts & White, 2010, Paul et al, 2014), geomorphological features (Dauteuil et al, 2015), and thermochronology data (Green et al, 2017) which uphold a late Cenozoic uplift These studies link the last increase of sedimentary flux with either dynamic topography associated with LLSVP (Gurnis et al, 2000) or small-scale convection (Burke, 2008). Present-day topography of the South African plateau, 2- on the Orange River drainage pattern and 3- on the Orange basin sedimentation over the last 30 Ma
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