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Polygenetic topography of the Cascade Range, Washington State, USA

Sara Gran Mitchell^,*,** and David R. Montgomery^*

^* Department of Earth and Space Sciences, Box 351310, University of Washington, Seattle, Washington 98195, USA

Corresponding author: smitchel{at}holycross.edu

We evaluate models for the Cenozoic topographic evolution of the Cascade Range of Washington State by analyzing the topography, geology, and exhumation patterns across the range. The models for topographic evolution include: 1) post-Miocene surface uplift of the range from initially low-relief topography, 2) post-Miocene surface uplift of the range superimposed on pre-existing high-relief topography, and 3) post-Miocene surface uplift of high-relief Cascades in northern Washington and low-relief Cascades in southern Washington. The third model—a polygenetic topographic history—is most consistent with our analysis of topographic data and the previously contradictory geologic, paleontologic, and isotopic lines of evidence regarding the Cascades physiographic history. Between Snoqualmie Pass and the Columbia River, the east-derived, 15 My-old Columbia River Basalt (CRB) reaches nearly as far west as the modern drainage divide and is warped upward to the crest of the range. These observations require subdued relief on the east flank of the southern Cascades during the late Miocene. In the northern Cascades, the CRB does not extend into the range and CRB deformation only reflects 30 to 60 percent of the total relief. Thus, the northern Cascades were likely already a topographic barrier to the west-flowing flood basalts. Moreover, a polygenetic topographic history appears most consistent with the geologic data and the paleontologic and isotopic evidence for rainshadow formation during the middle Miocene. Despite differences in topographic history and rock type, the maximum and mean altitudes of the Cascade Range decrease smoothly from northern to southern Washington. We suggest that post-uplift erosion, particularly glacial erosion, has reduced the topographic distinctions between the northern and southern Cascades, resulting in a single mountain range with a polygenetic topographic history.