Rock uplift, erosion, and tectonic uplift of South Africa determined with cosmogenic Aluminum -26 and Beryllium-10

Erica D Erlanger, Purdue University

Abstract

The high topography of southern Africa has been attributed to a variety of uplift mechanisms. Some suggest southern Africa is supported by recent and ongoing mantle-driven dynamic topography, while others suggest it has been high since at least the Cretaceous. These competing hypotheses can be simplified into three models: 1) recent, rapid uplift of up to ∼200 m/My since the Pliocene; 2) slow, ongoing uplift of <10 m>/My; and 3) no Neogene or Quaternary uplift. It has remained difficult to evaluate these models because rates of uplift are poorly constrained and because it is difficult to separate mantle-driven uplift from isostatic rock uplift. To resolve the uplift rate and mechanism, I determined 1) rock uplift from river incision rates and a displaced marine terrace; and 2) modern erosion rates and paleo-erosion rates. River incision rates and paleo-erosion rates were calculated from a flight of strath terraces along the lower Sundays River Valley, located on the southeastern coast of South Africa. I dated the river terraces and the marine terrace with cosmogenic 26Al and 10Be measured in quartz sediment, using an isochron burial dating method and standard burial dating methods. The river terraces range in age from ∼0.3-4 Ma and vary in tread height from ∼6-88 m above the present river level, providing an excellent opportunity to evaluate river incision and rock uplift rates over million-year timescales. Dividing the terrace heights by the ages yields a long-term incision rate of 16.9 ± 1.2 m/My for the Sundays River. The marine terrace near Durban yields an age of 4.26 ± 0.68 Ma. Assuming a sea level highstand of 25 ± 5 m above present sea level at the time of terrace formation yields a rock uplift rate of 10 ± 3m/My. To estimate the component of uplift attributable to isostasy, I measured modern erosion rates using 10Be in quartz for catchments representing various physiographic regions: the coastal plain, the escarpment piedmont, the Great Escarpment, the Lesotho Highlands, the inner piedmont, and the interior plateau. Along the coastal plain, erosion rates vary from ∼9-22 m/My. The escarpment piedmont is eroding at ∼12-37 m/My, while the Great Escarpment is eroding at ∼86 m/My, and the summit of the escarpment is eroding at ∼24 m/My. Erosion rates in the Lesotho Highlands are ~40/My, and the interior of South Africa is eroding the slowest at ∼5 m/My. The average paleo-erosion rate for the Sundays River is ∼6 m/My. These erosion rates can be incorporated into a simple flexural model of the crust to calculate isostatic rebound due to erosional unloading. The model shows an isostatic rebound rate of 9 ± 4.5 m/My at the Sundays River for a uniform elastic thickness (Te) of either 60 or 80 km. At Durban, isostatic rebound is 8 ± 4 m/My (Te= 60 km) and 9 ± 4.5 m/My (Te= 80 km). Subtracting the isostatic component from rock uplift rates yields tectonic uplift rate estimates of up to 8 ± 5 m/My at the Sundays River, and 2 ± 5 m/My (Te= 60 km) and 1 ± 5 m/My (Te= 80 km) m/My near Durban. These results are inconsistent with recent and rapid uplift during the past several million years, although they do not preclude ongoing slow, mantle driven uplift, or no uplift and subsidence at Durban.

Degree

M.S.

Advisors

Granger, Purdue University.

Subject Area

Geophysics|Geomorphology

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