Shear wave splitting measurements provide significant information about subduction zone mantle flow, which is closely tied to plate motions, lithospheric deformation, arc volcanism, and backarc spreading processes. We analyse the shear wave splitting of local S waves recorded by a large 2003–2004 deployment consisting of 58 ocean-bottom seismographs (OBSs) and 20 land stations and by nine OBSs from a smaller 2001–2002 deployment. We employ several methods and data processing schemes, including spatial averaging methods, to obtain stable and consistent shear wave splitting patterns throughout the arc–backarc system. Observed fast orientation solutions are dependent on event location and depth, suggesting that anisotropic fabric in the mantle wedge is highly heterogeneous. Shear waves sampling beneath the northern island arc (latitudes 17.5°–19°N) and between the arc and backarc spreading centre show arc-parallel fast orientations for events shallower than 250 km depth; whereas, fast orientations at the same stations are somewhat different for deeper events. Waves sampling beneath the central island arc stations (latitudes 15.5°–17.5°) show fast orientations subparallel to both the arc and absolute plate motion (APM) for events <250 km depth and APM-parallel for deeper events. Ray paths sampling west of the spreading centre show fast orientations ranging from arc-perpendicular to APM-parallel. Arc-parallel fast orientations characterize the southern part of the arc with variable orientations surrounding Guam. These results suggest that the typical interpretation of mantle wedge flow strongly coupled to the downgoing slab is valid only at depths greater than ∼250 km and at large distances from the trench. We conclude that the arc-parallel fast orientations are likely the result of physical arc-parallel mantle flow and are not due to recently proposed alternative lattice preferred orientation mechanisms and fabrics. This flow pattern may result from along-strike pressure gradients in the mantle wedge, possibly due to changes in slab dip and/or convergence angles.