Uplift rates of southern Africa from incision of the Sundays River, South Africa

Southern Africa is bounded by a passive margin escarpment, and is characterized by widespread and discrete uplifted surfaces that are recognizable by their elevation, low relief, and deep weathering profiles. The timing and rate of uplift of the surfaces remain debated, with estimates of uplift rates varying from near zero to about 500 m/My. Uplift has been attributed both to mantle-driven dynamic topography and to erosional isostasy. One possible cause for uplift is the African superplume, a low-velocity mantle structure believed to be rising beneath southern and eastern Africa. Alternatively, the uplifted surfaces could be passive markers of isostatic rock uplift, due to localized erosion along the Great Escarpment. To resolve this question requires measurements of both uplift rates and the distribution of erosion rates across the landscape. We will present data on river incision rates, erosion rates, and paleo-erosion rates in the Sundays River Valley, located on the southern coast of South Africa. This valley hosts the best preserved flight of fluvial terraces in southern Africa. The ages of these terraces range from modern to several millions of years old, providing an excellent opportunity to evaluate uplift rates over million-year timescales. We are dating the river terraces with cosmogenic 26Al and 10Be in quartz sediment, using an isochron burial dating method. This method allows us to date sediments buried at depths of only a few meters, and exposed in roadcuts, riverbanks, and construction excavations. In addition, we have observed Stone Age archaeological artifacts incorporated in some of the terraces. The technological style of the artifacts strongly constrains the maximum age of the terrace sediments. Preliminary archaeological data indicate that the uplift rate in the Sundays river valley is from 30-100 m/My.