POLYGENETIC TOPOGRAPHIC EVOLUTION OF THE CASCADE RANGE, WASHINGTON STATE

We use GIS-based analyses of topography, geology, and exhumation to evaluate three existing models for the Neogene topographic development of the Washington Cascade Range. Models include: 1) post-Miocene uplift superimposed on a long-existing “Ancestral” Cascades, 2) post-Miocene uplift of an initially low-relief western Washington, and 3) a polygenetic model of high northern but low southern Cascades during the Miocene and additional syn- and post-Miocene uplift. Our analyses support the polygenetic model, originally proposed by J.Hoover Mackin. South of Snoqualmie Pass, vertical deformation and the spatial extent of the east-derived 15.5 Ma Columbia River Basalt (CRB) indicate that much of the modern elevation may be a result of post-Miocene rock and surface uplift. These observations suggest relatively subdued relief in the southern Cascades during the late Miocene. In the northern Cascades, the CRB is limited to the far eastern edge of the range, CRB deformation only reflects 30-60% of the total relief, and AHe exhumation rates are not consistent with a greater westward CRB extent, indicating high relief at the time of CRB emplacement. This two-part model for topographic development is consistent with geologic, paleontologic, and isotopic evidence for rainshadow formation during the middle Miocene. Finally, we suggest that glacial erosion following the Miocene erased much of the topographic evidence of the complex surface uplift history, leaving a single mountain range with a polygenetic topographic history.