Cosmogenic 21Ne and 10Be reveal a more than 2 Ma alluvial fan flanking the Cape Mountains, South Africa
Cosmogenic 21Ne and 10Be analyses across a subhorizontal fossil alluvial fan flanking the Cape Mountains near Laingsburg (Western Cape, South Africa) indicate a formation age for this fan of at least 2 Ma. Maximum erosion rates obtained from samples of quartzite ridges protruding through the fan alluvium deposits are very low (<~0.4 m/Ma) and analytically indistinguishable from those derived from pebbles from the fan surface (<~1.5 m/Ma). Our results are consistent with the low denudation rates obtained from other cosmogenic nuclide analyses and predicted by apatite fission-track and offshore sedimentation analyses for southern Africa during the Cenozoic. They integrate already existing substantial evidence to show that relative tectonic stability has prevailed across southern Africa throughout this period, and that the present-day large-scale topography is largely inherited from the Cretaceous and has been only minimally resculptured by denudation during the Cenozoic.