Post-Orogenic Exhumation and Glacial Erosion on the Flanks of the North Atlantic
Fame, Michelle Leigh
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Many passive margins experience pulsed exhumation events late in their history as a result of plate boundary distal geodynamic mechanisms or climatic events. The onset of late Cenozoic glaciation, often associated with enhanced rates of erosion, is one such possible cause of passive margin rejuvenation. However, along passive margins the effectiveness of Plio-Pleistocene glaciers at eroding the landscape may be limited by low tectonic rock uplift rates or as a result of erosionally inefficient cold based continental ice-sheets. In this dissertation the evolution of post-orogenic topography and the effect of glaciations on denuding landscapes along the North Atlantic Passive Margin, in the White Mountains of New Hampshire and the western Scottish Highlands, was investigated. Background exhumation rates averaged over 106-7 yr timescale were determined using apatite (U-Th)/He thermochronology. To resolve whether or not a change in exhumation rate occurred coincident with glaciation these background exhumation rates were compared to magnitudes of erosion averaged over the glacially relevant 103-4 yr timescale using the in situ terrestrial cosmogenic nuclide 10Be. In chapter two, 106-7 yr timescale exhumation and burial histories across the western Scottish Highlands were determined. The results show that post-orogenic burial and exhumation is mostly a result of plate margin distal tectonic and magmatic factors that are variable across short distances (i.e., <100 km). In chapter three, patterns and magnitudes of erosion during glaciation and following deglaciation in the Scottish Highlands were investigated. The results indicate that polythermal glacial erosion denuded low elevation portions of the Scottish Highlands and preserved summits. This produced relief but did not significantly lower the maximum elevation of the landscape. Following deglaciation Scotland�[BULLET]s sediment budget remains dominated by glaciogenic sediment. In the fourth chapter, magnitudes of background exhumation in the Presidential and Carter Ranges of the White Mountains, New Hampshire were compared to magnitudes of glacial erosion. The results indicate that most relief was produced prior to glaciation and that background exhumation rates in the Cenozoic are low (<0.01 mm yr-1). In the late-Cenozoic, cold- based glaciation prevented an acceleration of erosion in the White Mountains. The post- glacial sediment budget is made up of dominantly glaciogenic sediment. Overall, the main findings of this dissertation are; (1) post-orogenic burial, exhumation, and relief production are mainly the result of spatially heterogeneous plate margin distal vertical crustal motions; across passive margins covered by large continental ice sheets; (2) cold-based ice limits the magnitudes of late Cenozoic glacial erosion sediment budgets continue to be dominated by glaciogenic sediment, >10 ka after and (3) post-glacial deglaciation.
- Doctoral Dissertations