SUBAERIAL DENUDATION OF PASSIVE MARGINS - REGIONAL ELEVATION VERSUS LOCAL RELIEF MODELS

Variations in denudation rates, which are an important factor in the modification of the sub-aerial topography of passive margins, have frequently been modelled as being proportionate to regional elevation. To be valid, this assumption requires that mean slope gradient and the related property of local relief show a strong positive relationship to regional elevation since slope angle is a primary determinant of denudation rate. Analysis of the relationships between these variables based on the 1:250,000 scale topographic map coverage of southern Africa demonstrates that regional (modal) elevation explains only 0.5% of the variance in mean sine slope gradient and only 0.14% of the variance in local relief (measured over 15' x 15' areas). These results demonstrate that regional elevation cannot be used to predict denudation rates for passive margins, such as those of southern Africa, where a dissected coastal zone is separated from a high interior plateau by a major escarpment. On the basis of a previous regression analysis relating local relief to sediment and solute transport by rivers the data for southern Africa indicate denudation rates of > 80 m Ma-1 over much of the coastal zone and < 20 m Ma-1 across most of the elevated interior. Extrapolation of these results over geological time, however, necessitates either assumptions about the local relief of palaeotopography or independent evidence of long-term denudation rates.