ACTIVE DISPLACEMENT FIELD PARTITIONING AND ARC-PARALLEL EXTENSION AT LOW CONVERGENCE OBLIQUITY, UNALASKA, ALEUTIAN ISLANDS, ALASKA

Relative motion between the Pacific and North American plates along the Aleutian Island arc varies east to west between 68 mm/yr to 75 mm/yr, and the obliquity of convergence ranges from as little as <10° in the east near Unalaska Island to ~80° in far western parts of the arc. During oblique convergence, displacement field partitioning commonly results in arc-parallel migration of forearc and arc assemblages with respect to backarc regions. The obliquity of convergence is viewed as a primary control for the onset of displacement field partitioning, which occurs at values above 10 to 20°. In the Aleutian Islands, incremental strain axes of the maximum horizontal shortening recorded by earthquake focal mechanisms on the plate boundary systematically vary along the curved arc. Near Unalaska in the eastern Aleutian chain, incremental strain axes are essentially normal to the plate boundary. Taken at face value, seismicity does not suggest development of a significant arc-parallel component of displacement. A GPS site on Unalaska occupied annually for three to four weeks between 1996 to 2000 has a S81°W velocity of ~4 mm/yr. Using incremental strain axes associated with megathrust earthquakes to constrain the azimuth of potential elastic strain included in the GPS velocity, a boundary-parallel residual of ~3 mm/yr reflects a minimum permanent displacement. This residual velocity is about 30% of the boundary-parallel component of oblique convergence. Furthermore, differential westward migration of Unalaska relative to arc segments farther east is recorded in the structural history of the island. Analysis of brittle structures indicates an early history of arc-normal contraction followed by arc-parallel transcurrent faulting and kinematically coordinated arc-parallel extension. The kinematics of ENE trending faults that dissect the island indicate right lateral displacement consistent with ~2 mm/yr of differential motion between GPS sites spanning an ENE transcurrent fault. NNW to NW trending valleys are localized by extensional half grabens oriented normal to the plate boundary which together with NNW striking basaltic dikes record substantial margin-parallel extension. Taken together, GPS velocities and structures suggest that displacement field partitioning is active even at a low convergence obliquity.