Late-Cenozoic topographic evolution of the Cascade Range, Washington State, USA

The topography of the Cascade Range in Washington State reflects its long geologic, tectonic, erosion, and climatic history. This dissertation utilizes exhumation rate, paleoclimate, and geologic data, in addition to topographic analyses to evaluate hypotheses regarding the physiographic evolution of the range. Spatial relationships between topography, exhumation rate, climate, and erosion potential are analyzed to explore the influence of glacial erosion on the height and morphology of the Cascades. Finally, I constrain the extent to which isostatic compensation from valley incision affects peak altitude trends across the range.; A "triangulation" approach using many lines of evidence indicates that the Cascade Range has a polygenetic topographic history; in northern Washington the range likely had significant relief during the late Miocene, while in southern Washington it had modest relief. Both regions were subsequently uplifted and modified by erosion. In the central Cascades, non-volcanic summits rise ∼600 m above the average Quaternary glacial equilibrium line altitude (ELA) despite a ten-fold variation in exhumation rate. This evidence, along with hypsometric analysis and erosion modeling, suggests that glaciers limit the altitude of Cascade peaks by eroding most intensely and for the longest duration at the average Quaternary ELA, creating cirques at that altitude and steepening the slopes above cirques to the degree that they fail. This limiting of summit altitudes across the transition zone between the northern and southern Cascades created a landscape that was misinterpreted as an uplifted erosion surface and erased topographic differences that once existed due to the two regions' different physiographic histories. Independent constraints suggest that the flexural rigidity of the crust is >1023 N m and thus the effective elastic thickness is >24 km. This constraint limits the maximum effect differential erosion can have on Cascade Range relief to 700 and 300 m in the northern and southern areas of the range, respectively. Therefore, the remaining differences in altitude between the northern and southern Cascades in Washington must be due to differences in the geology or tectonics.