We examine shear wave splitting in teleseismic phases to observe seismic anisotropy in the South American subduction zone. Data is from the CHARGE network, which traversed Chile and western Argentina across two transects between 30¿S and 36¿S. Beneath the southern and northwestern parts of the network, fast polarization direction ($varphi$) is consistently trench-parallel, while in the northeast $varphi$ is trench-normal; the transition between these two zones is gradual. We infer that anisotropy sampled by teleseismic phases is localized within or below the subducting slab. We explain our observations with a model in which eastward, Nazca-entrained asthenospheric flow is deflected by retrograde motion of the subducting Nazca plate. Resulting southward flow through this area produces N-S $varphi$ observed in the south and northwest; E-W $varphi$ result from interaction of this flow with the local slab geometry producing eastward mantle flow under the actively flattening part of the slab.