The Geomorphic Response of the Passive Continental Margin of Northern Namibia to Gondwana Break-Up and Global Scale Tectonics

Abstract

Abstract by author:

Apatite fission track thermochronology data provide an independent and direct constraint on the spatial and temporal variations of denudation in onshore regions. The method allows the identification of major eroding source areas over time. With this information, a record of the cooling history can be established, potentially identifying offsets across major structures, as well as important periods of accelerated denudation and their distribution along the continental margin. Moreover, if vertical relief profiles are available, a palaeogeothermal gradient can be reconstructed. This thesis has successfully contributed to understanding of the low temperature thermotectonic evolution of the passive margin of northern Namibia using apatite fission track thermochronology. The study presents a detailed regional examination with a new set of 158 apatite fission track ages which reveal temperature and denudation evolution since continental break-up. Samples were selected to analyse the cooling pattern of the crust in one- (Chapter 4), two- (Chapter 5), and three-dimensional space (Chapter 6). In order to obtain site specific information about the cooling history in particular regions, three vertical relief profiles have been analysed. The thermal histories extracted from these profiles allowed the calculation of denudation rates, the net amount of denudation which occurred locally, and the determination of palaeogeothermal gradients (Chapter 4). The significance of this chapter is that palaeogeothermal gradients could successfully be revealed (22-26°C/km), and hence eliminates uncertainties in extrapolating present-day gradients into the past. This information is very important in that it provides a highly needed and so far unknown input parameter for the calculation of denudation. The net amount of denudation calculated for the Central Damara Zone is 4 to 5 kilometers, since the Late Cretaceous, for the Okenyenya and Brandberg complex respectively. Denudation affecting the Highland in the Windhoekgraben area could also be tightly constrained to be on the order of 3. 5 kilometers. A key finding in applying \\aft analysis along a 550 km long transect across the major structures in northern Namibia was the determination of timing and magnitude of a vertical offset across the Omaruru Lineament-Waterberg Thrust (Chapter 5). The net vertical displacement of the crust could be calculated to be on the order of at least 2 kilometers. A particularly valuable outcome of this reseach was applying thermal modelling techniques to confidently quantify the uniform effect of a phase of accelerated denudation in the Late Cretaceous over the transect. Advanced modelling procedures proved to be ideally suited to reveal the large scale trends, rates, and patterns of denudation in three-dimensions across northern Namibia (Chapter 6). The modelling results have not only shown the typical signature expected for passive margins, but they also revealed 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 (Appendix E). The information and ideas formulated and presented in this thesis represent a significant advance in our understanding of passive margin evolution of northern Namibia. They highlight the importance of considering the impact of pre-existing structures on the architecture and magnitude of denudation events, which have shaped the present landscape