As increasing emphasis is placed upon the role surface processes play in regulating tectonic behaviour, the need for accurate measurements of denudation rate has become paramount. The quantity and quality of denudation rate studies has grown with the advent of cosmogenic radionuclide techniques, capable of recording denudation rates over timescales of 100 to 1000000 years. This study seeks to utilise cosmogenic 10Be concentrations measured in alluvial sediments in order to further develop this method and to investigate rates of basin-wide denudation in the San Bernardino Mountains, an active orogen associated with the San Andreas Fault system. The theory which underpins measurements of basin-wide denudation rates with cosmogenic radionuclide analysis is evaluated in light of recent understanding of production mechanisms. Field testing of the assumptions required by the basinwide denudation rate model highlights the importance of sampling thoroughly mixed sediments. Denudation rates ranging over three orders of magnitude are measured by applying the cosmogenic radionuclide technique in thirty-seven basins throughout the San Bernardino Mountains. Results show a relationship between denudation rate and slope which provides quantification of the threshold slope angle in high relief granitic environments and suggests tectonic activity is the first order control of denudation rates in these mountains. Mean annual precipitation is shown to exert no significant influence over the rates measured in the San Bernardino Mountains. Questions concerning denudation rates recorded over differing timespans are addressed using the cosmogenic technique, (U-Th)/He thermochronometry, incision into dated horizons and modern day sediment flux data. This comparison reveals that a decrease in rates with distance from the San Andreas Fault has been consistent throughout the lifespan of the San Bernardino Mountains and provides further evidence that a tectonic mechanism is driving denudation in this region. The relevance of both spatial and temporal scale in geomorphic studies is considered in light of these results, highlighting the need for a greater appreciation of their role in the interpretation of basin-wide denudation rates.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22264号 / 理博第4578号 / 新制||理||1657(附属図書館) / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)准教授 松四 雄騎, 教授 千木良 雅弘, 教授 岩田 知孝 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

The Cairngorm Mountains contain an outstanding assemblage of glacial landforms from both the deglaciation of the last British Irish Ice Sheet and the Younger Dryas readvance. Glaciers are recognised as sensitive indicators of past and present climate change and, thus, these landforms provide information about past climate and glacier-climate interaction that can be used to contextualise the present climate change. Previous interpretations have left doubt over the extent and style of the Younger Dryas readvance. In addition, the pattern and timing of deglaciation in the southern Cairngorms and, particularly, how local and external ice masses interacted is unclear. New geomorphological mapping from aerial images and fieldwork has been compiled in a GIS for a 600km2 area of the Cairngorm Mountains. This has allowed a complex pattern of ice-dammed lakes and local and regionally sourced ice margins to be reconstructed during the retreat of the last British Irish Ice Sheet. The mapping has been combined with new cosmogenic surface exposure ages taken from areas of hummocky moraine previously subject to differing age interpretations. The effect of moraine denudation on apparent 10Be ages has been checked by inverse modelling of the 10Be concentration vs. boulder height. The results indicate more extensive Younger Dryas glaciation, with glacier reconstructions and equilibrium-line altitudes (ELAs) comparable with the surrounding areas. Reconstruction of both valley and plateau-fed glaciers are presented, with modelling of local topoclimatic factors, such as radiation, avalanche and snow drifting, combined with precipitation gradients, explaining most of the variation within the glacier ELAs. The geomorphological evidence and palaeoclimatic inferences are important, alongside a growing number of palaeoglaciological studies, in acting as evaluation areas for current numerical models of ice-sheet growth and decay.