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Volume 24 Issue 9 (September 2014)

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Article, pp. 4-11 | Full Text | PDF (5.2MB)

A cosmogenic view of erosion, relief generation, and the age of faulting in southern Africa

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Paul R. Bierman1*, Ryan Coppersmith2, Kathryn Hanson3, Johann Neveling4, Eric W. Portenga5, Dylan H. Rood6

1 Geology Department and Rubenstein School of the Environment and Natural Resources, University of Vermont, Burlington, Vermont 05405, USA
2 Coppersmith Consulting Inc., 2121 N. California Blvd. Suite 290, Walnut Creek, California 94596, USA
3 AMEC Environment & Infrastructure, Inc., 180 Grand Avenue, Suite 1100, Oakland, California 94612, USA
4 Council for Geoscience, 280 Pretoria Street, Silverton, Pretoria 0184, South Africa
5 School of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK, and Department of Environment and Geography, Macquarie University, North Ryde NSW 2109, Australia
6 Scottish Universities Environmental Research Centre (SUERC), University of Glasgow, East Kilbride G75 0QF, Scotland, UK, and Earth Research Institute, University of California, Santa Barbara, California 93106, USA, and Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94550, USA

ABSTRACT

Southernmost Africa, with extensive upland geomorphic surfaces, deep canyons, and numerous faults, has long interested geoscientists. A paucity of dates and low rates of background seismicity make it challenging to quantify the pace of landscape change and determine the likelihood and timing of fault movement that could raise and lower parts of the landscape and create associated geohazards.

To infer regional rates of denudation, we measured 10Be in river sediment samples and found that south-central South Africa is eroding ~5 m m.y.−1, a slow erosion rate consistent with those measured in other non-tectonically active areas, including much of southern Africa. To estimate the rate at which extensive, fossil, upland, silcrete-mantled pediment surfaces erode, we measured 10Be and 26Al in exposed quartzite samples. Undeformed upland surfaces are little changed since the Pliocene; some have minimum exposure ages exceeding 2.5 m.y. (median, 1.3 m.y.) and maximum erosion rates of <0.2 m m.y.−1 (median, 0.34 m m.y.−1), consistent with no Quaternary movement on faults that displace the underlying quartzite but not the silcrete cover.

We directly dated a recent displacement event on the only recognized Quaternary-active fault in South Africa, a fault that displaces both silcrete and the underlying quartzite. The concentrations of 10Be in exposed fault scarp samples are consistent with a 1.5 m displacement occurring ca. 25 ka. Samples from this offset upland surface have lower minimum limiting exposure ages and higher maximum erosion rates than those from undeformed pediment surfaces, consistent with Pleistocene earthquakes and deformation reducing overall landscape stability proximal to the fault zone.

Rates of landscape change on the extensive, stable, silcretized, upland pediment surfaces are an order of magnitude lower than basin-average erosion rates. As isostatic response to regional denudation uplifts the entire landscape at several meters per million years, valleys deepen, isolating stable upland surfaces and creating the spectacular relief for which the region is known.

DOI: 10.1130/GSATG206A.1

Manuscript received 18 Dec. 2013; accepted 13 Mar. 2014.

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