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Format:
Print
Author:
Portenga, Eric William
Dept./Program:
Geology
Year:
2011
Degree:
MS
Abstract:
Bedrock outcrops are features of landscapes around the world. Outcrops are the backbone of mountain chains and their presence along ridgelines often marks the boundary between drainage basins. As mountain belts are uplifted, material removed from outcrops by erosional processes is transported through the drainage basin via fluvial processes. Though outcrops are common and these fluvial processes on-going, the rates at which these phenomena occur are poorly constrained on millennial timescales and longer.
In situ production of ¹⁰Be occurs when cosmic-rays trigyer spallation reactions in mineral structures at Earth's surface; thus, the concentration of ¹⁰Be can be used to infer residence times within the uppermost few meters ofthe crust. These concentrations can be used to model erosion rates in a variety of Earth surface materials that geologists sample, most commonly rock from the top surfaces of outcrops and fluvial sediment taken from active stream channels.
This thesis compiles publically available measurements of ¹⁰Be from outcrops (n = 418) and fluvial sediment (n = 1110), published in 82 studies, in a global meta-analysis. I do statistical analyses of environmental, physical, and topographical parameters to determine their relationship with erosion rates for various lithologic, climatic, and seismic settings. Erosion rates from drainage basins (average = 209 ± 33 m My⁻¹) are two orders of magnitude higher than the average outcrop erosion rate (12 ± 1.3 m My⁻¹) ; median erosion rates follow the same pattern (53 and 5.2 m My⁻¹ for drainage basins and outcrops, respectively). I conclude that 33% of global outcrop erosion rate variability is explained by six parameters (latitude, elevation, relief, mean annual precipitation and temperature, and seismicity) and that 56% of drainage basin erosion rate variability is explained by nine parameters: the same six listed above and basin slope, percent vegetation, and basin area.
This global context provides the background in which I am able to compare a subset of samples I collected from bedrock ridges in the central Appalachian Mountains (n = 72). Average erosion rates of 15 ± 1 and 9.7 ± 0.7 m Mil for bedrock outcrops in the Potomac and Susquehanna River Basins, respectively are similar to outcrop erosion rates previously determined for the region. Outcrop erosion rates are similar to those inferred for sub-basins of the Potomac River, but outcrops erosion rates are half as rapid as those inferred for sub-basins of the Susquehanna River. The average outcrop erosion rate for the field area (13 ± 1 m My⁻¹) is slower, but comparable to denudation rates effective over timescales, >10⁶ years, inferred from apatite fission track thermochronology and (U-Th)/He dating. By integrating my results with those of other studies, I am able to infer an overall lowering rate oftens of meters per million years for the central Appalachian Mountains.
Data presented here have significant implications for understanding erosion rates on multiple geographic and temporal scales: Mine is the first global analysis of bedrock outcrop and drainage basin erosion rates inferred from ¹⁰Be. With the data I have collected, the long-term (>10⁶ years) landscape evolution for the central Appalachian Mountains is better constrained by the integration oflong-lived denudation rates and more recent erosion rates.