Teleseismic S-wave tomography of South Island, New Zealand upper mantle

An S-wave tomogram produced from finite-frequency tomography using teleseismic travel-time measurements made on and offshore the South Island of New Zealand shows four major features: high speeds in the upper mantle under the central portion of the island, low speeds along the eastern coast, and two high-speed regions in the northwest and southwest. The core of 7%-9% faster than average S-wave speeds in the central South Island is ∼150 km wide and reaches depths of ∼150 km. The structure is consistent with being either thickened lithosphere from the Cenozoic shortening across the island or subducted Hikurangi plateau; however, this Vs tomogram shows low speeds along the east coast reaching depths of ∼200 km and interpreted as Miocene or younger volcanism. This region of slow S-wave speeds beneath the east coast seems inconsistent with the presence of a cold, rigid Hikurangi plateau lying in the subsurface throughout much of the central South Island. High speeds in the mantle under the northwestern South Island reach depths of 400 km and are consistent with oblique subduction of Pacific lithosphere since 45 Ma, with high speeds in the southwest representative of subduction at the Puysegur trench. The ratio of S-wave travel-time residuals to P-wave travel-time residuals from an earlier study yields (partial differential)lnVs/(partial differential)lnVp ∼ 1.68, suggesting that variations in temperature primarily explain the seismic heterogeneity in the mantle under the South Island.