Denudation and uplift at passive margins: the record on the Atlantic Margin of southern Africa

The onshore region of a passive margin forms an integral part of the geolog ical evolution from continental break-up to later sedimentation in the offs hore basins. The dominant surface process in the onshore region is denudati on, which acts to remove any direct evidence of surface uplift. However, de nudation can be constrained on geological time-scales through low temperatu re thermochronological data, such as that obtained through apatite fission track analysis. Here, we present a suite of such data from the Atlantic Mar gin of southern Africa. The data have been modelled in terms of their tempe rature histories since the Jurassic. These temperature histories have been combined with heat-flow data to estimate the equivalent depth of denudation over these time-scales. Average denudation rates are of the order of a few tens of metres per million years, but show considerable variations both te mporally and spatially. These results demonstrate that passive margins expe rience complex patterns of denudation. Three landscape-evolution models are considered. Our results imply the downwarping model is inappropriate. The other two models, scarp retreat and pinned drainage divide, predict trends similar to those observed but the complexities inherent in the data and the evolution of passive margin topography do not allow us to resolve one from the other. In practice, both models probably operate to some extent as a m argin evolves. Estimates of palaeotopography have been made, assuming simpl e isostatic response models to denudational unloading. Flexural models with effective elastic thickness (EET) of 25 km predict elevations 2 km and mor e above the present-day values, while models with EET of 0 km predict eleva tions up to 750 m higher than the present day. These models ignore any post -break-up tectonic uplift and need independent constraints on surface eleva tion to assess their validity.