Mantle wedge anisotropy in the hikurangi subduction zone, central North Island, New Zealand

The anisotropic characteristics of the Hikurangi subduction zone in central North Island, New Zealand are studied using shear-wave splitting measurements from 55 local earthquakes recorded on a 200 km profile of 19 stations. The eastern fore-arc shows trench-parallel fast directions and a low average delay time of 0.2 s. The source of anisotropy is confined to the upper 60 km and is likely to be deformation induced within the overriding Australian plate. Central western North Island shows N-S trending fast anisotropy which may signify the presence of a viscous blanket of mantle material being entrained by the absolute motion of the Australian plate. Between these regions a dramatic switch to extension-parallel fast directions and delay times of 0.35 s are seen across the back-arc in the Taupo Volcanic Zone (TVZ). These results suggest asthenospheric flow beneath the overriding plate with the olivine a-axes oriented in the extension direction down to a maximum depth of 100 km.