GEOPHYSICAL CONSTRAINTS ON THE DEEP STRUCTURE OF THE WASHINGTON CASCADES, AND TECTONIC IMPLICATIONS

Several geophysical observations constrain models of the deep structure of the Washington Cascade range. Sparse, but generally consistent, heat flow measurements in the Cascades show a positive anomaly greater than 60 mW/m2. Gravity measurements combined with topography, reveal that the range is close to isostatic equilibrium. Crustal earthquakes are common on the western margin of the Cascades, extending into the Puget lowland, but are largely absent from the core of the range – an observation consistent with higher than normal crustal temperatures in the Cascades. A magnitude 5.4 earthquake in 1996 on the west flank of the Cascades is interpreted as a high angle reverse fault with the Cascades side up, consistent with active range uplift. Juan de Fuca slab earthquakes reach maximum depths of only 90-100 km beneath the western central part of the range, providing limited evidence about the deeper structure of the slab beneath the Cascades. Earlier teleseismic tomography analyses indicate that the slab may descend steeply to several hundred kilometers beneath the Cascades. Seismic structure investigations of the crust reveal a 10 km Moho depression beneath the central Cascades. The Moho is not observed seismically further west in the Cascadia forearc region. Since arc-normal compression doesn’t play a significant role in Cascade uplift, deep magmatic processes appear to offer the best chance of understanding the uplift of the Cascades.

A steady-state underplating model can be found that is consistent with our general knowledge of heat flow, gravity, topography, erosion rates, seismic structure, and reasonable rates of magma influx.