The occurrence of seismic anisotropy in the Earth's upper mantle is a global phenomenon related to subcrustal deformation and flow processes. The shear wave splitting analysis method has led to a global set of anisotropy maps mainly derived from receiver-side analysis. Remote places with few seismometers deployed remain unexamined. Source-side splitting analysis allows mapping of mantle fabrics in these regions. Here, we investigate seismic anisotropy in the South Sandwich Islands subduction environment. Core-reflected ScS waves recorded at the Neumayer seismograph network are corrected for well constrained receiver anisotropy and then analysed for source anisotropy. Sub-slab mantle minerals are aligned horizontally almost parallel to the trench indicating a westward flow around the subducting slab. This is consistent with a model of horizontal mantle flow due to slab rollback that was previously inferred from marine and geochemical studies in the back-arc region. Copyright 2008 by the American Geophysical Union.