HR: 14:40h
AN: H52D-05 INVITED     [PDF]
TI: Coupled precipitation and long-term erosion rates across the Washington Cascades
AU: * Reiners, P W
EM: peter.reiners@yale.edu
AF: Yale University, Geology & Geophysics PO Box 208109, New Haven, CT 06520 United States
AU: Ehlers, T A
EM: tehlers@umich.edu
AF: University of Michigan, Dept. of Geological Sciences, Ann Arbor, MI 48109 United States
AU: Mitchell, S G
EM: sgm1@u.washington.edu
AF: University of Washington, Dept. of Earth & Space Sci., Seattle, WA 98195 United States
AU: Montgomery, D R
EM: dave@geology.washington.edu
AF: University of Washington, Dept. of Earth & Space Sci., Seattle, WA 98195 United States
AB: The central Cascades mountains of Washington state form a 150-200 km wide, north-south oriented topographic high with a mean summit elevation of $\sim$1.5 km, and nonvolcanic peaks above 2.7 km. The largely post-late Miocene bedrock uplift that created this part of the range formed a barrier to Pacific-derived moisture, leading to a dramatic orographic rain shadow with mean annual precipitation rates $>$4 m/yr on the notoriously rainy west flanks and $<$0.2 m/yr on the sagebrush steppes to the east. To examine the timing and rate of rock exhumation across the range, as well as relationships between precipitation and erosional exhumation, we have measured over one hundred apatite (U-Th)/He ages in a transect across the range. Apatite He ages vary from 4 to 60 Ma in this part of the range, with youngest ages on the upper west flank, and the oldest ages near the summit, east flank, and far western side of the range. These ages can be inverted for model erosional exhumation rates, assuming constancy since closure of the He system, by accounting for the interdependence of cooling rate, closure temperature, and closure depth in a steady-state crustal thermal model, and correcting for mean local topography (10-km circle) on isotherm depths (method of Brandon et al., 1998). Using this model, erosion rates near the summit, east flank, and far west flank range from less than 0.05 mm/yr to about 0.10 mm/yr, while rates for the upper west flank are as high as 0.33 mm/yr. Patterns of fluivially based erosion potential indices (e.g., steepest descent slope and local channel discharge) show no correlation with the observed erosion rate variations across the range. Mean annual precipitation, however, is relatively well correlated with erosion rate across the Cascades, suggesting a strong coupling between the two processes. If the range is currently in topographic steady state, then not only erosion rates, but also rock uplift rates on the west flank are as much as an order of magnitude greater than elsewhere in the range, including at the summit. Alternatively, rock uplift rates may be constant across the range, in which case varying erosion rates reflect transient topographic development of the range.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1815 Erosion and sedimentation
DE: 8102 Continental contractional orogenic belts
DE: 8107 Continental neotectonics
SC: Hydrology [H]
MN: 2003 Fall Meeting