Crust and mantle thickening beneath the southern portion of the Southern Alps, New Zealand

SUMMARY An 8.54 ± 0.20 km s −1 Pn speed is estimated on a line oriented ca. N5°E from the Alpine Fault beneath the Southern Alps of South Island based on a refraction experiment that uses the Fiordland earthquake of August 2003 as a source. This high Pn speed results from both strong anisotropy in the mantle lid of 7–13 per cent and a high Pn speed average of 8.3 ± 0.3 km s −1 . A maximum crustal thickness of 48 ± 4 km is calculated for the southern South Island near the town of Wanaka. This represents a crustal root of about 18 km, compared to measured crustal thicknesses at the east and west coasts of the South Island. The average topography in the southern Southern Alps is of the order of ∼1000 m, which is less than half that predicted by Airy isostasy for an 18 km crustal root. As recently proposed for the central South Island ∼120 km to the north, we propose that thickened cold, and therefore more dense, mantle lithosphere exists beneath southern South Island, and that this excess of mass is an effective load that pulls down the overlying crust. The load is similar to that beneath the central Southern Alps, despite the predicted convergence across the Alpine Fault there being nearly twice that at Wanaka. Gravity modelling of crustal structure along a profile through Wanaka suggests that this mass excess has a minimum density contrast of 35 ± 5 kg m −3 between thickened mantle and asthenosphere, assuming an across‐Moho density contrast of −300 kg m −3 . We speculate that the reason for the enhanced thickening beneath Wanaka is that the subducted Australian Plate at the southwestern corner of the South Island acts like a backstop onto which Pacific mantle collides at ∼26 mm yr −1 , ca. 3/4 the full plate speed.