The junction of the Aleutian Island and the Kamchatka peninsula defines a s
harp turn in the boundary of the Pacific and North American plates, termina
ting the subduction zones of the northwest Pacific. The regional pattern of
shear-wave birefringence near the junction indicates that trench-parallel
strain follows the seismogenic Benioff zone, but rotates to trench-normal b
eyond the slab edge. Asthenospheric mantle is inferred to flow around and b
eneath the disrupted slab edge, and may influence the shallowing dip of the
Benioff zone at the Aleutian junction.