Present denudation rates at selected sections of the South African escarpment and the elevated continental interior based on cosmogenic 3He and 21Ne

Abstract

Originally published as:Kounov, A., Niedermann, S., Viola, G., Andreoli, M., Erzinger, J. (2007) IntroductionSince the break-up of Gondwana, Southern Africa has been surrounded by elevated passive margins flanked by a classic escarpment (King, 1951;1967). The evolution of passive continental margins and their adjacent escarpments reflects the long-term interaction of tectonic and surface processes. It is now known that passive margin morphology is the result of positive and negative feedback between tectonic uplift and subsidence on the one hand and sedimentation and denudation on the other hand (Beaumont et al., 2000).Apatite fission track and cosmogenic nuclide studies along the escarpment-flanked continental margin of Southern Africa have recently revealed variable denudation rates for various morphological features of the passive margin, including its coastal plain, the seaward-facing escarpment and the elevated inland plateau (e.g. Fleming et al., 1999;Cockburn et al., 2000;Brown et al., 2000;Bierman and Caffee, 2001; Van der Wateren and Dunai, 2001;Brown et al., 2002;Raab et al., 2002;Bierman and Nichols, 2004;Tinker, 2005; Tinker et al., in review; Kounov et al., in review). These studies suggest that since the onset of the rifting phase, significant changes in denudation rates have occurred, in particular a substantial decrease when Cenozoic rates are compared to those for the Cretaceous. The high denudation rates during the Cretaceous are likely related to intense tectonic activity coupled with warm and humid climatic conditions, whereas the more arid climate that prevailed during the Quaternary and possibly throughout much of the Tertiary, and an apparent lack of significant uplift, were responsible for a remarkable subsequent decrease in the denudation African coastal escarpment and the interior plateau provide valuable estimates of the short-term (i.e., a few hundred thousand years) denudation rates. Present-day denudation rates at the summit of the escarpment and on the inland plateau are more than an order of magnitude lower than those in the Cretaceous. Values between 1.0 and 2.1 m/Ma obtained from resistant quartzite samples and between 1.5 and 3 m/Ma from dolerite sills suggest a strong lithology control on the erosion. The observed patterns of denudation across different landform sites on the high plateau and along its flanking escarpment suggest that the morphology of the sampling sites together with the local climate conditions have controlled the denudation processes.

(U-Th)/He thermochronometry constraints on unroofing of the eastern Kaapvaal craton and significance for uplift of the southern African Plateau

“…These studies identifi ed accelerated unroofi ng episodes in both Early and mid-to Late Cretaceous time, but only strictly limited Cenozoic denudation to ≤2100 m (assuming a cratonic geotherm) owing to the bottoming out in temperature sensitivity of the AFT method at ~60 °C. Cosmogenic studies resolve short-term (a few hundred thousand years) denudation rates that are an order of magnitude lower than those estimated for the Cretaceous (Kounov et al, 2007), but do not constrain Tertiary rates. Our data across the eastern escarpment are compatible with, and better refi ne, the broader unroofi ng brackets imposed by AFT and cosmogenic data elsewhere in southern Africa.…”

Tectonic subsidence history and thermal evolution of the Orange Basin

“…Post-break up margin uplift may be a consequence of a rising mantle superswell (Burke, 1996;Gurnis et al, 2000;Nyblade and Robinson, 1994), of residual effects of long-lived Mesozoic plume heads and tails (Nyblade and Sleep, 2003), of lithospheric folding and thermal softening in response to ridge-push forces (Ayele, 2002), of small-scale convection (Huismans and Beaumont, 2008), or even of a renewed pulse of lithospheric extension in response to a change in plate rotation poles (Guiraud and Bosworth, 1997;Nürnberg and Müller, 1991). For passive margins west and south of South Africa a temporal correlation is reported between phases of margin uplift, phases of increased denudation of the continent onshore (Kounov et al, 2007) and phases of Kimberlite emplacement (de Wit, 2007;Tinker et al, 2008), that points to processes affecting the lithospheric and sub-lithospheric mantle.…”

Sediment supply to the Orange sedimentary system over the last 150My: An evaluation from sedimentation/denudation balance

“…The drainage is organized in 2 sub-systems : (1) the present day Orange catchment draining a major part of the plateau and (2) series of small coastal catchments, located between the coastline to the westernmost part of the Orange divide, on the steep escarpment, and drained by short rivers perpendicular to the coastline. Recent mean denudation rates (last 10 3 -10 5 yr), based on cosmogenic isotope analysis and measured mostly in Namibia, in the south west corner of the plateau and along the Drakensberg scarp range from 0.5 to 5 m/Myr in vertical and about 10 m/Myr for the horizontal scarp retreat (Fleming et al, 1999;Cockburn et al, 1999Cockburn et al, , 2000Bierman and Caffee, 2001;Bierman and Nichols, 2004;Van der Wateren and Dunai, 2001;Kounov et al, 2007).…”