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

摘要

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 seis-micity 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 Be 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 ~(10)Be and ~(26)Al 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.