Residual mantle exposures in the accreted Talkeetna arc, Alaska, provide the first rock analog for the arc-parallel flow that is inferred from seismic anisotropy at several modern arcs. The peridotites exposed at the base of the Jurassic Talkeetna arc have a Moho-parallel foliation and indicate dislocation creep of olivine at temperatures of ~1000¿ to >1100¿C. Slip occurred chiefly on the (001)<100> slip system, which has only rarely been observed to be the dominant slip system in olivine. Stretching lineations and olivine <100> slip directions are subparallel to the long axis of the Talkeetna arc for over 200 km, indicating that mantle flow was parallel to the arc axis. The alignment of the olivine <100> axes yields a calculated S wave anisotropy with the fast polarization direction parallel to the arc. Thus (1) the fast polarization directions observed parallel to some modern arcs now have an exposed geological analog; (2) arc-parallel fast polarization directions can be caused by anisotropic peridotites and do not require the presence of fracture zones, fluid-filled pockets, or glide on the (010)<001> H2O-induced slip system; (3) seismic anisotropy beneath modern arcs may be due to slip on (001)<100> with a horizontal foliation rather than slip on (010)<100> with a vertical foliation; and (4) the observed dominance of the (001)<100> slip system may be due to high H2O concentrations, suggesting that strain in the oceanic upper mantle may be accommodated dominantly by (001)<100> olivine slip.