Erosional unloading, hillslope geometry, and the height of the Cascade Range, Washington State, USA

摘要

Peaks in the Cascade Range in northern Washington State are on average similar to 800 On higher than in southern Washington. The influences of differential valley excavation and variations in hillslope length and average slope on these altitudinal trends were tested using a 3-dimensional model for isostatic rock uplift and calculations of hillslope length and slope respectively. The magnitude of isostatic peak uplift calculated 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. Crustal rigidity was constrained using published estimates and by estimating the depth of the seismogenic zone in the area (D > 1 x 10(23) Nm and T-e > 24 km). With these constraints, isostatic compensation due to differential erosion added < 700 m and 300 m, or < 25% overall, of height to peaks in the northern and southern Cascades, respectively. Deeper valley incision in the northern Cascades accounts for < 300 On of the 800 m difference in peak altitudes between north and south. Similarly, variation in valley spacing and slope account for < 350 m of the difference in mean altitude between northern and southern regions. Hence, at least several hundred m difference in altitude between the northern and southern regions of the Cascades in Washington must be clue to tectonic, geologic, or geophysical factors rather than surficial and geomorphic effects like isostatic response to valley incision and hillslope geometry.