Seismic anisotropy and P-wave delays in New Zealand imply widespread deformation in the underlying mantle, not slip on a narrow fault zone, which is characteristic of plate boundaries in oceanic regions. Large magnitudes of shear-wave splitting and orientations of fast polarization parallel to the Alpine fault show that pervasive simple shear of the mantle lithosphere has accommodated the cumulative strike-slip plate motion. Variations in P-wave residuals across the Southern Alps rule out underthrusting of one slab of mantle lithosphere beneath another but permit continuous deformation of lithosphere shortened by about 100 kilometers since 6 to 7 million years ago.

We investigate anisotropy surrounding a continental back arc spreading region, the Central Volcanic Region (CVR), in the North Island of New Zealand and present a simple method for spatial averaging to examine heterogeneous anisotropy. S phases from local earthquakes yield consistent trench‐parallel fast directions (ϕ) in the southern, compressional region but varied directions in the northern, extensional region. In the forearc east of the CVR, ϕ is nearly trench‐parallel, suggesting shear in the mantle or trench‐parallel flow. In the western CVR, local phases from 50 to 80 km depth give ϕ parallel to extension (120°), and phases from 150 to 220 km depth yield similar ϕ (150°). These results suggest asthenospheric flow with olivine a axes oriented in the extension direction down to 100–150 km depth. In the eastern CVR, local phases show mixed ϕ, suggesting that it is a transition between the western CVR and the forearc. Trench‐parallel ϕ for shallow local events within the CVR may be caused by fluid‐filled cracks. Trench‐parallel ϕ for deeper local events and for SKS phases can be explained by standard olivine fabrics developed by lithospheric shearing, by trench‐parallel flow, and by fossil anisotropy in the subducting oceanic lithosphere. Alternatively, in the CVR they could be caused in part by hydrous minerals at the slab‐mantle upper interface, which could cause olivine a axes to orient parallel to the trench due to a maximum shear stress oriented perpendicular to the trench along the slab. We suggest that trench‐perpendicular flow in the extensional region helps to drive fluids away from the slab, while in the compressional region, trench‐parallel flow fails to distribute the fluids, explaining the location of changes in geophysical properties.

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Abstract. Teleseismic ScS and SKS events recorded on nine broadband seismograph stations have been used to investigate seismic anisotropy beneath the lower half of the North Island, New Zealand. This area lies above the Hikurangi subduction zone, and the array provides ray paths which sample the mantle both above and below the slab. Shear wave splitting measurements give similar fast polarizations and delay times at each station. The average SKS fast polarization is approximately NE-SW, subparallel to the strike of subduction and the major geological features, with an average SKS delay time of 1.6 + 0.1 s. This lack of.variation in splitting parameters suggests that similar fast polarizations are found in both the mantle wedge and the subslab mantle. The anisotropy in the lithospheric portion of the mantle wedge is most likely caused by the preferred orientation of olivine due to the shear deformation associated with oblique convergence. Any anisotropy in the slab is probably due to fossil mineral alignment. Anisotropy in the asthenosphere is most likely caused by the preferred orientation of olivine due to asthenospheric flow. The similar NE-SW fast polarizations found in the asthenosphere both above and below the slab suggest that the mantle flow is in a trench-parallel direction in both regions. IntroductionShear wave splitting of teleseismic phases such as ScS and SKS is now a common method used to investigate mantle anisotropy, which can in turn be related to deformation of the upper mantle [e.g., Silver, 1996]. This phenomenon occurs when a shear wave enters an anisotropic medium, upon which it is split into two orthogonally polarized waves which travel with different velocities [e.g., Crampin, 1981 ]. Shear wave splitting measurements can be characterized by two parameters; the polarization direction of the fast shear wave, q•, can be related to the symmetry of the anisotropic system, and the time separation between the two waves, fit, can be related to the strength of anisotropy and the path length through the anisotropic material. Thus, if the cause of the anisotropy is known, the splitting parameters can be related to the deformation and tectonic structure of a region. For example, mantle anisotropy is usually assumed to be due to straininduced lattice-preferred orientation of olivine. The polarization direction of the fast shear wave is then assumed to align parallel to the mantle flow direction, if it is in the form of