Surface processes, and the spatial variation of denudation in particular, are key issues in understand long-term surface evolution of a passive margin.

This article shows how to convert thermal history information from a large apatite fission track data set from Namibia into

1. Palaeotemperature-,
2. Denudation-, and
3. Palaeotopography-

maps to achieve and visualise quantitative information of the denudation chronology of the passive continental margin of central Namibia. Watch three animated QuickTime movies to see Namibia's margin evolving.

Quantifying Long-Term Denudation

Quantifying long-term denudation required thermal modelling of the apatite fission track data by extracting the temperature history for every sample for any given time, since they provide information on timing and temperature respectively. The temperature information in turn has to be converted into an equivalent depth as a function of time. Extraction of thermal histories for large regional data sets was accomplished using forward modelling techniques, where the palaeo-temperature value for a particular time interval has been estimated. Data from across a region were contoured and placed over the topography, enabling the creation of a contoured image of the predicted palaeotemperature for any given time in the past.

In order to convert the thermal history information into estimates of denudation, some constraints on the palaeogeothermal gradient are required. Present-day heat-flow data, as well as palaeogeothermal gradients of 23 and 25°C/km, derived by ft-data (Raab et al, 2005), suggest that the variation in thermal gradients for Namibia is relatively restricted to values between 20 and 30°C/km. Based on this data, it was assumed for the models that the heat-flow was constant over time. To allow the model some spatial variation in heat-flow, the present-day heat-flow data from Pollack et al. (1993) were used. Further estimates of the palaeotopography can be made by loading the present-day topography with the estimated denudation. The paleotopography maps were then calculated using a simple elastic plate model of the lithosphere with an effective elastic thickness of 25 km, and allowing for isostasy.

Summary

The regional modelling suggests that samples within 100 km of the coast and in the area of the Northern and Central Damara Zone were at temperatures in excess of 110°C during the Late Cretaceous. Samples from the cratonic interiors and the highland respectively, were exposed to lower temperatures in the Early Cretaceous. Little cooling ocurred in the period between 130 and 90 Ma. The cooling history from the Late Cretaceous to the Early Tertiary (90 to 50 Ma) is dominated by more dramatic cooling for the coastal region and the Northern and Central Damara Zone. The majority of samples cooled in this period from tempratures in excess of 110°C to temperatures below 70°C. Samples in this region underwent only subdued cooling from the Middle Tertiary to present.

The modelling results do not only show the typical signature expected for passive margins, they also reveal the complexity of denudation in the Central Damara Zone, where deep seated shear zones dominate the regional basement structure. Animations provide an optimum way of viewing the overall temperature and denudation evolution over time. However, these figures can only show the general trend of the data, and are not intended to replace the careful interpretation on a local scale.

This research was conducted at The University of Melbourne in collaboration with Prof. R. W. Brown (Glasgow, UK), Prof. K. Gallagher (Rennes, France) and Prof. K. Weber (Göttingen, Germany). High resolution versions for teaching purposes are available on request.