Denudational history along a transect across the Drakensberg Escarpment of southern Africa derived from apatite fission track thermochronology

Abstract

[1] The denudational history of a $500 km long transect across the Drakensberg Escarpment on the high-elevation passive margin of SE Africa is quantified on the basis of thermal history modeling of apatite fission track data for 15 deep borehole samples, supplemented by an additional 10 outcrop samples. A minimum of 4.5 km of denudation since formation of the margin $130 Myr ago is estimated for the coastal zone, with a marked Early Cretaceous episode of accelerated denudation broadly coincident with continental breakup. Samples from the Swartberg borehole (SW 1/67) located $30 km seaward of the present position of the Drakensberg Escarpment indicate a total depth of denudation of 3.1 ± 1.2 km since $91 Myr, with a phase of accelerated denudation of 2.1 ± 0.9 km at a mean rate of 95 ± 43 m/Myr between $91 and 69 Myr. Samples from the Ladybrand borehole (LA 1/68) west of the Lesotho Highlands indicate 1.7 ± 0.5 km of denudation since $78 Myr, with a phase of accelerated denudation at 82 ± 43 m/Myr from $78 to 64 Myr. Average denudation rates declined to about 10 m/Myr during much of the Tertiary. Although the apatite fission track data do not provide any direct constraints on the denudational history of the Lesotho Highlands, interpolation between the two boreholes, constrained by geological evidence and extrapolated in situ-produced cosmogenic 36 Cl-derived denudation rate estimates, suggests a pattern of denudation compatible with numerical modeling studies of escarpment evolution involving rapid river incision seaward of a preexisting inland drainage divide. These patterns of denudation are incompatible with constant retreat of the Drakensberg Escarpment from an initial position near the present coast. We suggest that the Drakensberg Escarpment formed by rapid post-breakup river incision seaward of a preexisting drainage divide located just east of the present escarpment location and became pinned at this divide with subsequent retreat rates of only 100-200 m/Myr.

Long lasting epeirogenic uplift from mantle plumes and the origin of the Southern African Plateau

“…[11] The uplift history of the southern African Plateau has been reviewed in several studies and is the subject of considerable debate [e.g., Partridge and Maud, 1987;Partridge, 1998;Brown et al, 2000;Gurnis et al, 2000]. A brief summary is provided here.…”

“…We must compare with global averages, and there is sufficient uncertainty in the average structure of cratonic lithosphere and heat flow globally that 25 km of lithsopheric thinning and an increase in heat flow of 2.5 mW m À2 cannot be resolved. However, xenolith geotherms from kimberlites of different ages provide some evidence that the lithosphere was thinned during the Mesozoic [Bell et al, 2003]. …”

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

“…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.…”

Long‐term landscape evolution: linking tectonics and surface processes

“…This pattern of kilometre-scale post-breakup denudation on the coastal plain, between the escarpment and the coast, and a marked episode of accelerated denudation broadly coincident with continental breakup, has now been broadly confirmed for several margins (e.g. southern Africa, Brown et al, 2000aBrown et al, , 2000bEritrea, Balestrieri et al, 2005), as well as being very clearly evident in more recent data for the SE Australian margin (Persano et al, 2002(Persano et al, , 2005. All these various data unequivocally confirm the critical role of flexural isostasy in passive margin evolution, and make it extremely unlikely that the downwarp model is sustainable on the southern African and southeastern Australian PCMs.…”