We evaluated shear wave splitting in local S and teleseismic sS phases from earthquakes that sample the Izu-Bonin subduction system backarc to examine the dynamics of slab deformation and mantle flow. Splitting times for local S phases range from 0.6 to 1.7 s and 0.9 to 3.2 s for teleseismic sS phases. Fast polarization directions range from ~NW-SE to ~ENE-WSW and suggest complex mantle deformation across the region. Mantle flow induced by both plate motion and trench-parallel flow likely combine with fossil slab fabric to control the shear wave splitting observations in the southern portion of Izu-Bonin. Strong variations in fast polarization directions beneath northern Izu-Bonin suggest a diversion to trench-parallel mantle flow behind the Philippine Sea plate. This study demonstrates that many regions of subduction previously inaccessible by conventional shear wave splitting techniques may now be feasibly examined through the use of new, complementary shear wave splitting data sets.