Tilting of continental interiors over a source of dynamic topography : Late Cretaceous erosion of the South African plateau
Résumé
New sedimentary flux data confirm the observation that a large pulse
of erosion affected the South African Plateau in the late Cretaceous.
This episode of rapid erosion (less than 30 Myr) is likely to be related
to a major uplift phase that is apparently difficult to reconcile with
a possible mantle origin, namely the presence of low density body in
the underlying mantle causing flow and dynamical uplift of the continent.
Given its size, the growth and rise of this so-called "African superswell"
is likely to have taken one to several hundred million years.
Here we demonstrate by using a simple model for fluvial erosion that
tilting of the continent as it rides over a wide source of dynamic topography
can not only cause uplift of the plateau but also lead to substantial
erosion of large surface areas in a relatively short amount of time, because
the tilting produces a continental-scale drainage re-organization.
We show that embedding a lithological contrast, such as the one that
might have existed between a thick layer of Karoo volcano-clastic sediments
overlying basement, greatly amplifies the rate of erosion (and sedimentation)
during the tilting episode.We demonstrate that our scenario
is consistent with paleogeographic reconstructions of the position of the
African continent with respect to the African superswell, the temporal
and spatial evolution of kimberlite eruptions across southern Africa and
the past and present-day highly asymmetrical drainage geometry.