Rapid erosion of the Southern African Plateau as it climbs over a mantle superswell

Abstract

We present new sedimentary flux data confirming that a large pulse of erosion affected the Southern African Plateau in the Late Cretaceous and is likely to be related to a major uplift episode of the plateau. This short phase of erosion (i.e., less than 30 Myr in duration) has commonly been difficult to reconcile with a mantle origin for the plateau anomalous uplift: given its size, the rise of the African superplume is likely to have lasted much longer. Here we demonstrate by using a simple model for fluvial erosion that tilting of the continent as it rides over a wide dynamic topography high cannot only cause rapid uplift of the plateau but also trigger continent-wide drainage reorganization, leading to substantial denudation in a relatively short amount of time. The amplitude and short duration of the sedimentary pulse are best reproduced by assuming a strong erodibility contrast between the Karoo sedimentary and volcanic rocks and the underlying basement. We also present a new compilation of paleoclimate indicators that shows a transition from arid to very humid conditions approximately at the onset of the documented erosional pulse, suggesting that climate may have also played a role in triggering the denudation. The diachronism of the sedimentary flux between the eastern and western margins of the plateau and the temporal and geographic coincidence between the uplift and kimberlite eruptions are, however, better explained by our tilt hypothesis driven by the migration of the continent over a fixed source of mantle upwelling.

Neo-Tethys geodynamics and mantle convection: from extension to compression in Africa and a conceptual model for obduction

“…These successive volcanic events sign the presence of a long-lasting mantle upwelling underneath South Africa during a long period, also responsible for the evolution of dynamic topography in eastern Africa (Burke 1996;Moucha and Forte 2011;Torsvik et al 2014). This conclusion is further corroborated by the occurrence of kimberlites from ϳ200 to ϳ50 Ma with a younging from east to west (Jelsma et al 2009(Jelsma et al , 2004Torsvik et al 2010) suggesting that South Africa has slowly overridden the plume, before the latter migrated northward (Braun et al 2014). The plume influence is also suggested by erosion and uplift of the Southern African Plateau in the Late Cretaceous (Fig.…”

Mantle dynamics, isostasy, and the support of high terrain

“…Inferences from geothermometry and geobarometry of xenoliths brought up from depth by kimberlites suggest that the upper mantle temperatures before roughly 100 Ma were roughly 100°C cooler than those after that time [Bell et al, 2003;Janney et al, 2010]. For a coefficient of thermal expansion of 3 × 10 À5°CÀ1 , and a depth range of 200 km over which temperatures rose by 100°C, the change in height would be 600 m. The recent evidence that erosion of southern Africa was more rapid in mid to late Cretaceous time than since that time supports the inference that an atypically high landscape formed in Cretaceous time [Braun et al, 2014;Flowers and Schoene, 2010;Stanley et al, 2013] by chemical, mineralogical, and/or thermal alteration of the lithosphere.…”

Dynamic topography and lithospheric stresses since 400 Ma

“…Braun et al [2014] have already explained this tilt (in a mantle reference frame) by the drift of the continent over a fixed mantle upwelling, and constrained it with observations of pulses of erosion. Braun et al [2014] have already explained this tilt (in a mantle reference frame) by the drift of the continent over a fixed mantle upwelling, and constrained it with observations of pulses of erosion.…”