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'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. / Ph. D. / Far away from the edges of modern tectonic plates, old mountain ranges (~300 million years old) may experience changes in rates of erosion long after the forces that built those mountains have gone away. Tectonic forces that occur far away from the edges of tectonic plates could cause these changes in erosion rate or they could be a result of changes in climate that create conditions in which erosion rates could increase. One change in climate that could have caused faster rates of erosion in old mountain ranges is climatic cooling causing the repeated advance and retreat of glaciers in mid to high latitudes over the past ~2.6 million years. Glaciers are usually seen as having the ability to erode faster than non-glacial processes (e.g., rivers). However, not all glaciers have the ability to erode really fast. In old mountain ranges glacial erosion might be limited because the mountains are not being uplifted very fast supplying new land to be subjected to erosion. Also, big ice sheets that covered many old mountain ranges can be very cold, freezing ice to the ground and therefore acting to protect the underlying landscape rather than eroding it. In this dissertation the question of whether glaciation caused an acceleration in erosion rates in old mountain ranges far away from the edges of tectonic plates is investigated, specifically in White Mountains of New Hampshire and the western Scottish Highlands. To this end, erosion rates in these mountain ranges were determined prior to glaciation and then compared to erosion rates during glaciation and following deglaciation. The results of the research presented in this dissertation indicate that; (1) pre-glacial erosion resulting from tectonic forces far from the edges of tectonic plates was more important at producing relief and eroding the landscape than glaciers; (2) ice covering old mountain ranges was at least partially cold-based, inhibiting erosion; and (3) following deglaciation sediments in streams are mostly sourced from remobilized glacially produced sediments rather than direct bedrock erosion.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/78372 |
| Date | 19 July 2017 |
| Creators | Fame, Michelle Leigh |
| Contributors | Geosciences, Spotila, James A., Eriksson, Kenneth A., Owen, Lewis A., Romans, Brian W., Law, Richard D. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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