Denudation along the Atlantic passive margin: new insights from apatite fission-track analysis on the western coast of South Africa

Abstract

Apatite fission-track (AFT) data from two traverses across the Great Escarpment of the western coast of South Africa are used to reconstruct the tectonic evolution and denudation history of this sector of the Atlantic passive margin. Fission-track ages range between 180 and 86 Ma. Modelling of this data identifies two distinct cooling events. The first event, between 160 and 138 Ma, is recorded only by the rocks above the escarpment in the Karoo area, and is tentatively linked to post-Karoo magmatism (c. 180 Ma) thermal relaxation. The second, between 115 and 90 Ma, results instead from a tectonically induced denudation episode responsible for the removal of up to 2.5 km of crust across the coastal zone in front of the escarpment and less than 1 km on the elevated interior plateau. Based on these results, it is suggested that the Cretaceous is the time when most of the elevated topography of Southern Africa was generated, with only a minor Cenozoic contribution.

Quaternary evolution of glaciated gneiss terrains: pre-glacial weathering vs. glacial erosion

“…10A). The linear nature of the valleys indicates that they coincide with large fracture zones; some of these have been interpreted as faults by Kounov et al (2009). These fracture zones evidently represent weaker rock that has been eroded more deeply than adjacent, less densely fractured rock.…”

“…Kounov et al, 2009;Luft et al, 2005;Rust & Summerfield, 1990). Denudation rates were markedly lower after the Late-Cretaceous, and the present-day topography of the high plateau was probably broadly established by that time (De Wit 1999;Gilchrist et al, 1994;Kounov et al 2009). During the Cainozoic; kaolonitic saprolite developed under humid conditions, with calcrete or silcrete duricrusts development under drier conditions (Brandt et al 2005;De Wit 1999;McCarthy et al 1985).…”

“…The Namaqualand gneiss terrain now occurs on a c. 1000 m high plateau, above the 'Great Escarpment' that characterises the margin of south-western Africa (e.g. Brandt et al 2005;Gilchrist et al, 1994;Kounov et al 2009). The area has been uplifting, at variable rates, since the rifting of southern Africa and South America in the Early Cretaceous, with a major phase of uplift and denudation during the Mid-Cretaceous, as indicated by the off-shore sediment record and apatite fission-track analyses (e.g.…”

“…The area has been uplifting, at variable rates, since the rifting of southern Africa and South America in the Early Cretaceous, with a major phase of uplift and denudation during the Mid-Cretaceous, as indicated by the off-shore sediment record and apatite fission-track analyses (e.g. Kounov et al, 2009;Luft et al, 2005;Rust & Summerfield, 1990). Denudation rates were markedly lower after the Late-Cretaceous, and the present-day topography of the high plateau was probably broadly established by that time (De Wit 1999;Gilchrist et al, 1994;Kounov et al 2009).…”

Soil Erosion Rates in South Africa Compared With Cosmogenic 3he-Based Rates of Soil Production

“…It is sobering to consider that the region has not experienced longterm denudation rates in excess of 50 m/Myr since the Cretaceous (Brown et al, 2002;Tinker et al, 2008;Kounov et al, 2009), and it may thus be said that humans are now mimicking the "Kalahari epeirogeny" (de Wit, 2007) in sculpturing the landscape. On the other hand, the rates of irregularly occurring processes such as erosion are timescale-dependent (Gardner et al, 1987), and thus present soil erosion rates might be less geologically significant for South Africa than a direct comparison with long-term rates would suggest.…”