NEOGENE MAGMATISM AND BLOCK MOTIONS IN THE PACIFIC NORTHWEST

Paleomagnetic and geodetic evidence documents Cenozoic clockwise rotation of the Pacific Northwest about a pole in the backarc at about 1°/Ma. Patterns of late Cenozoic magmatism appear to reflect the rotation and are compared to model GPS velocities after removing elastic strain due to subduction. The ancestral Cascade arc axis inferred from Miocene plutons lies west of the active arc in Oregon, crosses the active arc north of Mt Rainier, and lies east of the active arc along the Pemberton Belt in Canada. Westward displacement of the Miocene arc in Oregon is consistent with the northwest GPS velocity of the Miocene arc with respect to North America, and eastward displacement of the Pemberton Belt is consistent with its northeast GPS velocity. There, Mount Baker, Kulshan caldera, and Hannegan caldera form a mini-hotspot track over the last 4 Ma, migrating at 6 mm/yr, consistent to a first order with the N42°E plate motion at 3.3 mm/yr. Miocene plutons in the southern Cascades are offset in the direction of plate motion from Mount St. Helens, Jefferson, Three Sisters, and Mazama. Inferred rates of magmatic progression are about half the GPS rate of 6.5 mm/yr, indicating the active arc axis is also migrating west. The crossing pattern of the Miocene massif with respect to the present arc is consistent with the large-scale rotation of the upper plate above the subduction zone and suggests that long-lived magma sources may be moving laterally more slowly than the upper plate. GPS rotation rates are similar to Neogene paleomagnetic rates, suggesting 150+ km of northwest motion of the arc and substantial N-S shortening in the Columbia embayment since 18 Ma. As the Oregon fore-arc block rotated, the active arc followed on the block's trailing edge. Behind the Oregon arc, the well-documented migration of bimodal volcanism and normal faulting followed the rotation of the forearc massif. Isochrons derived from rhyolite ages in SE OR become younger toward the west, from 10 Ma near the Steens to 0 at Newberry. The isochrons are nearly perpendicular to the GPS velocity field, suggesting correlation with displacement direction and northwest migration of the arc. Thus, constrainable Neogene geologic strain rates appear to show remarkable consistency with present-day geodetic rates.