Deformation across the forearc of the Cascadia subduction zone at Cape Blanco, Oregon

Over the interval 1992-1999 the U.S. Geological Survey measured the deformation of a geodetic array extending N80 degrees E (approximate direction of plate convergence) from Cape Blanco on the Oregon coast to the volcanic are near Newberry Crater (55 and 350 km, respectively, from the deformation front). Within about 150 km from the deformation front, the forearc is being compressed arcward (N80 degrees E) by coupling to the subducting Juan de Fuca plate. Dislocation modeling of the observed N80 degrees E compression suggests that the main thrust zone (the locked portion of the Juan dr Fuca-forearc interface) is about 40 km wide in the downdip direction. The transverse (N10 degrees W) velocity component of the forearc measured with respect to the fixed interior of North America decreases with distance from the deformation front at a rate of about 0.03 mm yr(-1) km(-1). That gradient appears to be a consequence of rigid rotation of the forearc block relative to fixed interior North America (Euler vector of 43.4 degrees+/-0.1 degrees N, 120.0 degrees+/-0.4 degrees W, and -1.67+/-0.17 degrees(m.y.)(-1); quoted uncertainties are standard deviations). The rotation rate is similar to the paleomagnetically measured rotation rate (-1.0+/-0.2 degrees(m.y.)(-1)) of the 15 Ma lava flows along the Columbia River 250 km farther north. The back are does not appear to participate in this rotation but rather is migrating at a rate of about 3.6 mm yr(-1) northward with respect to fixed North America. That migration could be partly an artifact of an imperfect tie of our reference coordinate system to the interior of North America.