Large continental transform faults are thought to cross-cut the crust and extend into the lithospheric mantle. However, the strain distribution associated with these faults at mantle depths is not well understood. At the heart of this question is the rheology of the lithosphere: when stressed by displacement on a trans-crustal fault, does the uppermost mantle localize strain in a continuing narrow fault zone, or is that strain instead distributed in a shear zone that widens with increasing depth? This study uses harmonic decomposition of receiver functions to measure the spatial and depth distribution of seismic anisotropy, a proxy for viscous deformation, around the Alpine Fault in Aotearoa New Zealand. Anisotropy aligned with the fault is present in the lithosphere at least 100 km away from the fault trace, suggesting that the Alpine Fault shear zone widens at depth.