Pn anisotropy and distributed upper mantle deformation associated with a continental transform fault

Abstract

Differences in Pn speeds within continental lithosphere off the west coast of South Island, New Zealand, require anisotropy of at least 10 ± 3%. These data concur with SKS splitting and are inferred to show high strain of mantle lithosphere over a zone >200 km wide. An active source seismic experiment yields Pn speeds of 8.6 and 7.7 km/s along nearly perpendicular lines. The higher speed is sub‐parallel to the polarization of the faster quasi‐S wave measured from SKS splitting. Most of the observed ∼2 s of splitting in South Island can be accommodated in mantle lithosphere 150 km thick, if we assume a P to S anisotropy ratio of 1.4. The large magnitude of Pn anisotropy shows that anisotropy must occur in the relatively cold uppermost part of the mantle lithosphere and raises the possibility that dynamic recrystallization occurs in cold (∼500–600°C) olivine at geologic strain rates.