CONTRIBUTION OF ISOSTATIC RESPONSE DUE TO VALLEY INCISION TO ALTITUDE TRENDS OF THE CASCADE RANGE, WASHINGTON STATE

Peaks in the Cascade Range in northern Washington are on average ~1000 m higher than in southern Washington. The degree to which this difference in altitude is a result of: A) differential valley excavation, or B) variations in hillslope length and average slope, was tested using a 3-dimensional model for isostatic rock uplift and calculations of hillslope length and slope, respectively. The magnitude of potential rock uplift determined by the model is highly dependent on the flexural rigidity (D) and the related effective elastic thickness (T_e) of the crust of this region. The rigidity of the crust was constrained using published estimates and by estimating the depth of the seismogenic zone in the area (D > 1023 Nm and T_e > 24 km). With these constraints, isostatic compensation due to differential erosion could have added a maximum of ~700 m and 400 m of height to peaks in the northern and southern Cascades, respectively; the actual effect was probably significantly less. Deeper valley incision, glacial or fluvial, in the northern Cascades can account at most for ~300 m of the 800 m difference in peak altitudes between north and south. Similarly, variation in valley spacing and slope has only a modest (< 350 m) effect on the difference in mean altitude between northern and southern regions. Thus, much of the difference in altitude between the northern and southern regions of the Cascades in Washington must be due to the topographic influence of tectonics, geology, and crustal thickness, rather than purely geomorphic effects like valley incision and geometry.