Constraining plateau development in southern Africa by combining thermochronology, sediment flux, topography, and landscape evolution modeling
Résumé
The southern African Plateau is a dominant feature of African topography but there is considerable debate about when and how it formed. Mantle dynamics have been suggested to play an important role in the topographic evolution, but the time and specific mechanisms of topographic development are still contested. Three main intervals have been proposed for when most of the uplift occurred in southern Africa: 1) it was already elevated at the time of Gondwana breakup at ~150 Ma due to large igneous province activity based on models of rift flank uplift, 2) uplift occurred 100-80 Ma either due to deep mantle or lithospheric processes based on a major erosion phase detected in thermochronology and marine sediment flux, or 3) uplift occurred after ~30 Ma due to small scale convection in the upper mantle based on geomorphic planation surfaces and river profile analysis. Here, we test which of the three intervals of plateau development are plausible using erosion, sedimentation, and topographic data from southern Africa and a landscape evolution model. Recent work from several efforts has provided a clearer picture of the erosion history of the plateau surface and margins using low temperature thermochronology and the geometries of the depositional systems in the surrounding offshore basins. Landscape evolution model results are directly compared with apatite fission track and (U-Th)/He dates from across the plateau, sediment flux volumes in the surrounding marine basins, and present-day topographic metrics. We use an inversion method to constrain the range in erosional and uplift model parameters that can best reproduce the observed data. Results indicate two families of uplift histories are most compatible with the data. Both have limited initial topography and some topographic uplift and continental tilting starting in the east of the continent at ~95 Ma. In one acceptable scenario nearly all of the topography, ~1500 m, is created at this time with very little uplift in the Cenozoic. In the other acceptable scenario, only ~500 m of uplift occurs in the mid-Cretaceous with another ~850 m of uplift in the mid-Cenozoic. The data cannot easily distinguish between these two uplift patterns suggesting different proxies would be helpful to fully constrain the timing of plateau development and any climatic influences on the erosion history.