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

[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 similar to130 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 similar to30 km seaward of the present position of the Drakensberg Escarpment indicate a total depth of denudation of 3.1 +/- 1.2 km since similar to91 Myr, with a phase of accelerated denudation of 2.1 +/- 0.9 km at a mean rate of 95 +/- 43 m/ Myr between similar to91 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 similar to78 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 Cl-36-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.