The direction of tectonic plate motion at the Earth s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally, suggesting large-scale coupling between the mantle and the surface plates(1,2). The fit is typically poor at subduction zones, however, where regional observations of seismic anisotropy suggest that the direction of mantle flow is not parallel to(3-7) and may be several times faster than(6) plate motions. Here we present three-dimensional numerical models of buoyancy-driven deformation with realistic slab geometry for the Alaska subduction-transform system and use them to determine the origin of this regional decoupling of flow. We find that near a subduction zone edge, mantle flow velocities can have magnitudes of more than ten times the surface plate motions, whereas surface plate velocities are consistent with plate motions(8) and the complex mantle flow field is consistent with observations from seismic anisotropy(5). The seismic anisotropy observations constrain the shape of the eastern slab edge and require non-Newtonian mantle rheology. The incorporation of the non-Newtonian viscosity(9,10) results in mantle viscosities of 10(17) to 10(18) Pa s in regions of high strain rate (10(-12) s(-1)), and this low viscosity enables the mantle flow field to decouple partially from the motion of the surface plates. These results imply local rapid transport of geochemical signatures through subduction zones and that the internal deformation of slabs decreases the slab-pull force available to drive subducting plates.