Lithologies preserve alpine-like topography in southern Africa
Abstract
The Cape Mountains of southern Africa present an exception to the often noted coupling of topography and tectonics. This relict postorogenic terrain is topographically comparable to active orogens in terms of relief and steepness yet returns some of the lowest denudation rates known, globally. Consequently, the Cape Mountains provide an outstanding demonstration that rugged topography alone is not sufficient to incite high denudation rates, and active tectonics need not be responsible for the maintenance of rugged topography. Within the Cape Mountains, relief commonly exceeds 1 km whilst hillslopes are often in excess of 30°. Despite the rugged topography, upland catchments return a regional median 10Be-based denudation rate of 5.2 ± 1.8 m/m.y. The preservation of rugged topography and suppression of denudation rates within the Cape Mountains are primarily attributed to the presence of physically and chemically robust quartzites and metasandstones that constitute the backbone of these mountains. Consequently, the Cape Mountains demonstrate the strong influence that lithology may exert on denudation rates; a phenomenon often obscured in regions of intense active tectonism or highly variable lithology. Bedrock denudation rates determined for catchment interfluves return a median value of 3.32 ± 1.3 m/m.y. The similarity between catchment-averaged and bedrock denudation rates is suggestive of geomorphic steady-state. These low denudation rates, in conjunction with the suggestion of geomorphic stability, are in agreement with the low denudation rates (<20 m/m.y.) estimated for southern Africa during the latter part of the Cenozoic by means of cosmogenic nuclide, thermochronology, and offshore sedimentation analyses. Accumulatively these data suggest the preservation of ancient southern African landscapes through much of the Cenozoic.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMEP13B0833S
- Keywords:
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- 1130 GEOCHRONOLOGY / Geomorphological geochronology