Peridotite fabrics from the sub-arc mantle beneath the accreted Talkeetna arc, Alaska, indicate dislocation creep of olivine with strain dominantly accommodated by slip on the (001)[100] slip system. Evidence for the dominance of this slip system has only rarely been observed, and implies deformation at temperatures of ~1000-1250°C, perhaps at elevated H₂O activity. Stretching lineations and olivine [100] axes are parallel to the inferred long axis of the Talkeetna arc, indicating that mantle flow and the fast S wave polarization direction were parallel to the arc axis. Thus, i) the fast S wave polarization directions observed parallel to several modern arcs now have an exposed geological analog; ii) arc-parallel fast polarization directions do not require the H₂O-induced (010)[001] slip system proposed by Jung and Karato (2001); iii) arc-parallel fast polarization directions can be caused by anisotropic peridotites and do not require the presence of fracture zones, fluid-filled pockets, or other features; and iv) the S-wave anisotropy observed at active arcs may reflect (001)[100] slip and a subhorizontal foliation and not (010)[100] slip and a vertical foliation as commonly assumed. Finally, the relatively low magnitude of shear wave splitting observed in some modern arcs may not be distinguishable from the anisotropy produced by randomly oriented olivine, and should be used with caution when drawing conclusions about tectonics.