STEADY ROTATION OF THE CASCADE ARC IN OREGON AND WASHINGTON

Clockwise displacement of the 16 Ma ancestral Cascade volcanic arc from the presently active volcanic chain in the northwestern USA is in the same sense and at nearly the same rate as present block motions calculated from Global Positioning System (GPS) velocities in a North American reference frame. Migration of the ancestral arc over the past 16 Ma can be explained by clockwise rotation of upper plate blocks (about a pole in the backarc) at about 1.0°/Ma over a linear melting source moving westward 1 to 4.5 km/my due to slab rollback. Block motion and slab rollback are in opposite directions within the northern arc, but both are westerly within the southern extensional arc where rollback may be enhanced by proximity to the edge of the Juan de Fuca slab. From the GPS block motions, northwest-directed Basin-and-Range extension of 140 km is predicted behind the Oregon arc since 16 Ma, and 70 km of northward shortening is predicted in the Washington arc. The similarities between post–16 m.y. arc migration, paleomagnetic rotation, and modern GPS block motions suggest that decadal GPS–derived velocities match long-term crustal motions. Thus, crustal strain rates determined from GPS data may be useful in seismic hazard calculations for the northwestern U.S.