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

一些证据表明，太平洋西北部的现代地形，包括从华盛顿北部到阿拉斯加的山脉，是在过去的800万到1200万年间形成的。然而，来自山脉东侧的植物化石表明，大约4500万年前，海拔高度是现在的两倍。如果该地区在推断的造山活动开始之前就处于高位，那么许多隆起的替代信号可能是气候变化的结果，而不是构造驱动的山脉隆起。相反，与气候变化相关的侵蚀率增加可能导致北美西部边缘的卸载，从而可能使狭窄的峰值以均衡的方式反弹。这个假设很难评估，因为在推测最近的造山活动开始之前，该地区的初始地形条件是未知的。本研究的目的是通过对北喀斯喀特山脉东部边界奥卡诺根山脉古地形的调查，确定一个初始状态，以测量北喀斯喀特山脉中新世晚期地表和高峰海拔的变化。利用磷灰石和锆石(U-Th)/He低温热年代学、始新世地表重建和景观演化情景热模拟，确定了奥卡诺根山脉的发掘历史和古地形。地形起伏代表了范围内山峰的最低高程。通过热年代学计算的古地形将与散布在整个山脉的始新世火山沉积物的侵蚀残余物中保存的地形进行比较。然后，奥卡诺根山脉的精确海拔历史将用于评估侵蚀和随之而来的弯曲均衡峰值隆起对喀斯喀特山脉高峰产生的贡献。评估侵蚀和弯曲均衡对太平洋西北部高起伏和高地形的影响，将有助于解决关于过去2000万年北美西部造山动力的悬而未决的争论。这些结果将有助于评估北半球冰川作用的开始可能导致不列颠哥伦比亚省西海岸山脉和华盛顿州北喀斯喀特山脉西部的高峰和山脊高度增加的观点。

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.