Abstract
New K-Ar age determinations on basalts, basaltic andesites, and ash flow tuffs from the central Western Cascades in Oregon range in age from 32 to 3 Ma. The ages decrease from west to east and with increasing elevation. Volcanism has been continuous throughout the evolution of the Western Cascades, with some periods of greater activity: 13-16 Ma, 22-26 Ma and 29-31 Ma. Relative volume estimates indicate that eruption rates decreased by a factor of 6 from 35 Ma to the present. The eruption rates during formation of the Western Cascades Volcanic Arc were influenced by changes in the direction and rate of convergence between the Farallon and North American plates. We have developed a model of the Tertiary convergence of these two plates, based on the mantle-fixed hotspot reference frame, which indicates a factor of 5 decrease in convergence rate (16.0 to 3.2 cm/a). Clockwise rotation of the western margin of the North American plate led to a decrease in the convergence angle since about 35 Ma. Apparently, slower, more oblique subduction resulted in a decrease in the volume of erupted magmas.