Plate motion at the ridge-transform boundary of the south Cleft segment of the Juan de Fuca Ridge from GPS-Acoustic data

We measure the present-day plate velocity of the Juan de Fuca Ridge 25 km off-axis to be 63.6 +/- 3.6 mm/a at S67.2 degrees E +/- 7.9 degrees degrees (1-sigma) relative to the Pacific plate (PA). This velocity was derived from GPS-Acoustic (GPSA) measurements in 2000, 2001, 2002, and 2003 that observed the position of a seafloor array (44 degrees 43'N, 130 degrees 03'W, 2900 m depth) with a repeatability of +/- 4-6 mm. Three transient events at the Juan de Fuca Ridge and Blanco Transform account for similar to 10% of this motion in viscoelastic modeling, suggesting that the observed GPSA-PA velocity is due primarily to steady state plate dynamics. Subtracting the modeled transient motion gives a velocity of 57.3 +/- 3.9 mm/a at S72.9 degrees E +/- 12.1 degrees degrees (1-sigma), which is consistent at the 95% confidence level with the velocity calculated from the Wilson (1993) 0-0.78 Ma Euler pole. Therefore this site is interpreted to be in a region of continuous, full-rate plate motion, a robust result of this study which holds with and without correcting for transient motions. These results provide direct geodetic evidence that spreading occurs predominantly within 25 km of the axis at this intermediate spreading-rate ridge. Previously reported geodetic monitoring across the 1-km-wide axial valley from 1994-1999 and 2000-2003 shows no significant extension (Chadwell et al., 1999; Hildebrand et al., 1999; Chadwick and Stapp, 2002; W. W. Chadwick, personal communication, 2006) and seismic monitoring shows no activity. This suggests the crust between 0.5 and 25 km off-axis accommodates similar to 26 mm of aseismic deformation each year through some combination of near-axis fault motion and elastic strain accumulation.