Global Positioning System geodesy and structural analysis of parts of the Aleutian island chain support the interpretation of forearc migration westward along right-lateral transcurrent faults. Measured displacements are essentially arc parallel in the eastern (Unalaska Island) and western (Attu Island) parts of the arc and vary from 3.1 mm/yr to 31.4 mm/yr, respectively. At the center of the arc (Adak) subduction of the Amlia fracture zone disrupts the pattern of arc-parallel displacement and records an upper plate velocity of 9.6 mm/yr parallel to plate motion. Where active, displacement partitioning accommodates 30%–50% of the strike-slip component of plate convergence. In the area of impeded subduction partitioning ceases, suggesting that the process is sensitive to the degree of coupling across the plate-boundary megathrust. The differential arc-parallel displacements observed at the ends of the arc together with structural relations indicate substantial arc-parallel extension, which may play an important role in exhumation of high-pressure–low-temperature metamorphic rocks in ancient arc systems.