HR: 11:50h
AN: T32C-07    [Abstracts]
TI: Polygenetic nature of the Cenozoic topographic evolution of the Washington Cascade Range
AU: * Mitchell, S G
EM: sgm1@u.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195
AU: Montgomery, D R
EM: dave@ess.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195
AB: We evaluate three existing models for the Cenozoic topographic evolution of the Washington Cascade range by analyzing the topography, geology, and exhumation patterns across the range. The models for topographic evolution include: 1) post-Miocene uplift of the entire range superimposed on pre-existing high-relief topography, 2) post-Miocene uplift of the entire range from an initially low-relief western Washington, and 3) post-Miocene uplift of a high-relief northern Cascades and low-relief southern Cascades. The third model, indicating a polygenetic topographic history, is consistent with our topographic data, as well as the previously contradictory geologic, paleaontologic, and isotopic lines of evidence regarding the Cascades physiographic history. South of Snoqualmie Pass, the east-derived, ~15 Ma 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 30-60% of the total relief. Thus, the northern Cascades were likely already a topographic barrier to the east-flowing flood basalts. After the deposition of the CRB, the entire range was subsequently uplifted and eroded. This polygenetic topographic history appears to be consistent with the geologic data and the paleontologic and isotopic evidence for rainshadow formation during the middle Miocene. Despite its longer history as a mountain range and the exposure of crystalline basement rocks, the maximum and mean altitudes of the north Cascades are not sharply distinct from those of the predominantly volcanic southern Cascades. 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 history.
DE: 1819 Geographic Information Systems (GIS)
DE: 1824 Geomorphology: general (1625)
DE: 8175 Tectonics and landscape evolution
DE: 8177 Tectonics and climatic interactions
SC: Tectonophysics [T]
MN: Fall Meeting 2005