School of Earth Sciences - Theses

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    The low-temperature thermochronology of cratonic terranes
    Belton, David X. ( 2006)
    Cratonic terranes present many problems for geologists attempting to define those regions of the continental crust that are the core of today's continents. Inherent in the term is the great passage of time, and typically, the term defines consolidated Archaean or Proterozoic crust (Park and Jaroszewski, 1994). Cratons are further distinguished on the basis of tectonic activity. Marshak and his colleagues (1999) suggest that the lack of penetrative deformation or metamorphism is a useful definition but they further narrow this with the restriction of a Precambrian timeframe. Central to either view is the assumption of stability and perhaps senescence. And, since many aspects of geological research involve the detection of stratigraphic, structural or mineralogical change, stability implies a lack, or at least a minimum, of change. Thus the absence of these traditional markers of geological evolution or change, related to these processes, presents significant challenges in the study of cratons. This is particularly so in shield areas - those cratons with exposed basement rocks (Park and Jaroszewski, 1994). The extraordinary age of shield rocks and their apparent preservation at the surface, has encouraged research into the mechanics of landscape development and the individual evolution of landforms in the landscape, particularly by geomorphologists. From within this environment many thought provoking ideas have been proposed to explain the breadth of observations pertaining to almost every aspect of landscape process in these terranes. According to Summerfield (1991) the models of Davis, Penck, King and Budel have had the most impact in this field. The ideas presented by these workers remain a cornerstone in our understanding of the earth's surface, but in detail and in practice, the models have been shown to be antiquated. Nevertheless, landform evolution models such as those of L.C. King (1967) have held sway in southern Africa and elsewhere long after the underlying assumptions have been shown to lack validity. The purpose of this thesis is to investigate key aspects of landscape evolution in two regions, central Australia and southern Africa, where models have been espoused, arguing for extraordinary surface stability or alternatively a simple erosional history or pediplanation. Contemporary thermochronological techniques now permit us to investigate these regions in previously unavailable detail. The primary technique used in this work was apatite fission track analysis and an introduction to the fundamentals of the method is given in Chapter 1. The theoretical and practical aspects of the fission track method provided the basis for an innovative approach presented in Chapter 2. TASC is a scheme for analysing the raw fission track data so as to extract additional information about the rock's thermal history prior to undertaking traditional inverse modelling techniques. This method (recently described by the author in Ehlers et al., 2005) proved to be a powerful complement to the routine fission track analysis undertaken as part of the Australian and African case studies. Although first proposed for geological use in the 1960's, the fission track technique really only gained serious application with a number of technical and theoretical breakthroughs in the 1980's. Since then, growing understanding of the processes of annealing and how they might be modelled has allowed the technique continue developing. Chapter 3 is a discussion of this topic that expands on material previously published by the author and colleagues (Gleadow et al., 2002) and presents additional new work. Nevertheless, despite it's wide application in tectonic and basin studies amongst others, there remain many improvements to be made and problems to be solved. As part of this project, research into several areas presented the author with opportunities to contribute toward improvement in the apatite fission track technique, that have the potential to aid the study of cratonic terranes. The chlorine content of apatite has a profound influence on the sensitivity of the mineral for recording thermal events. Few current annealing models are capable of comprehensively addressing the variation of chlorine and other trace elements that appear to play a role in the annealing process. This issue is addressed in Chapter 4 where a universal annealing model is proposed to deal with the wide chemical variability observed in real apatites. For this theme, a fresh consideration of established empirical mathematical models was undertaken and all the current published annealing data was considered. Modern inverse modelling is based on a series of robust, but nonetheless empirical, equations that have withstood the test of time. However, with the aim of developing a more realistic and thus predictive model, Chapter 5 introduces an alternative, physicochemical to modelling the thermal annealing of fission tracks. This work attempts to draw firmer links between the processes of fission track formation, the mechanics of diffusion and the predicted response to variable temperature regimes. The first of the case studies is presented in Chapter 6 and is a comprehensive investigation of the long-term landscape evolution of the Davenport Ranges in the central Australian Craton. The study employs traditional petrographic methods as well as thermochronology and combines cosmogenic isotope analysis in an assessment of early landscape models. This chapter expands on work previously published by the author and co-workers (Belton et al., 2004) and has implications for our understanding of landscape evolution in the broader context of the Australian Craton. In order to maximise temperature sensitivity in slow cooled terranes, the relatively new thermochronological technique of (U-Th)/Helium analysis of apatite was tested on a suite of central Australian samples. The inconclusive results of this experiment prompted an investigation into the possible causes, and an important baseline study was conducted (Chapter 7). The study has implications for routine application of this new thermochronometer in cratonic and other terranes. More importantly the research identified a potential new thermochronometer with an even greater temperature sensitivity and near surface application for use in future landscape studies. Chapter 8 documents a larger, craton-wide study of the Mesozoic to recent landscape evolution of the Zimbabwe Craton. This work builds on material presented in earlier chapters and provides a broader view of the nature of crustal cooling, structural reactivation and landform development in the cratonic setting of southern Africa.