Constraining the Influence of Tectonics Versus Climate on Orogensis in the North Cascades (Washington and British Columbia)

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Several lines of evidence suggest that the modern topography of the Pacific Northwest, encompassing mountain ranges from northern Washington to Alaska, formed within the last 8 to12 million years. However, plant fossils from the east side of the mountains suggest elevations were as much as two times their modern height approximately 45 million years ago. If the region was high before the inferred onset of mountain building, then many of the proxy signals for uplift could be the result of climate change rather than tectonically driven mountain uplift. Instead, increased erosion rates associated with changing climate could result in the unloading of the western edge of North America, potentially allowing narrow peaks to rebound isostatically. This hypothesis is difficult to evaluate because the initial topographic condition of the region before the inferred onset of recent mountain building is not known. The goal of this study is to define an initial state against which to gauge late Miocene changes in surface and peak elevation in the North Cascades through investigation of the paleotopography of the Okanogan Range, the eastern border of the North Cascades. The exhumation history and paleotopography of the Okanogan Range is determined using apatite and zircon (U-Th)/He low-temperature thermochronology, Eocene surface reconstruction, and thermal modeling of landscape evolution scenarios. Topographic relief serves as a proxy for the minimum elevation of peaks within the range. Paleotopography calculated via thermochronology will be compared to the relief preserved in erosional remnants of Eocene volcanic deposits scattered throughout the range. The refined elevation history of the Okanogan Range will then be used to evaluate the contribution of erosion and consequent flexural isostatic peak uplift to the production of high peaks in the Cascades.Evaluating the impact of erosion and flexural isostasy on high relief and high topography in the Pacific Northwest will help resolve an outstanding debate about the driving forces of mountain building in western North America over the past 20 million years. These results will allow evaluation of the idea that the onset of northern hemisphere glaciation might have resulted in increases in peak and ridge elevations in the western Coast Mountains, British Columbia, and western North Cascades, Washington state.