A Link between Topography and Volcanism of the Oregon Cascade Range

It has been suggested by prior studies (geomorphology of an ancient river channel and δ18O values of fossil teeth) that the topography of the Oregon Cascades increased ~700-800 m since 5 Ma and ~700-900 m since 7 Ma. However, subduction-related volcanic activity of the Cascades arc began ~42 Ma, and it is doubtful that the Cascades topography had been low throughout its evolution before 5-7 Ma. In this study, oxygen isotope ratios of authigenic and pedogenic minerals from weathered ash and tuff layers in the Tertiary sedimentary rocks in north-central Oregon were measured. The δ18O values of both pedogenic carbonates and authigenic clay minerals show a decreasing trend of ~3-4‰ since about 7 Ma. These results corroborate the Late Miocene surface uplift of the range by ~600-900 m. However, the previously documented δ18O values of Late Oligocene pedogenic carbonates are up to 10‰ lower than the δ18O values of our Neogene carbonate samples. Also, the measured δ18O values of pre-Neogene authigenic clay minerals show a continuous decrease of up to 8‰ with increasing age. These results suggest that paleoelevation of the range in the Late Eocene to Early Oligocene was ~1.8-2.2 km higher than the Neogene and ~1.2 km higher than today. It seems that these δ18O trends are temporally correlated to the volcanic production rate of the Cascades. Volcanic production rate of the Oregon Cascades decreased by a factor of 3 from the interval 35-17 Ma to 16.9-7.4 Ma, probably reflecting a decrease in convergence rate. During the period of high volcanic production rate from 35 to 17 Ma, our oxygen isotope data suggest that the topography of the range was also high. On the other hand, the estimated topography was low between 16.9 and 7.4 Ma. In addition, the Oregon Cascades volcanic production rate has doubled since 7.4 Ma, while the topography increased about 700-900 m since 7 Ma. The δ18O values of authigenic and pedogenic minerals from north-central Oregon record the lowering and rising of the range since the beginning of the volcanic activity of the Cascades arc.