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Receiver function analysis reveals a 7 km step-like change in crustal thickness across the Taranaki-Ruapehu Line (TR-line) of North Island, New Zealand. The TR-line runs east-west between the active andesite volcanoes of Mt Taranaki and Ruapehu and marks the southern-most extent of subduction zone volcanism in New Zealand. North of the TR-line receiver functions show a strong and sharp P-to- S ( Ps) conversion at 25 ± 1.5 km depth, which is interpreted as a shallow Moho. At the TR-line the Moho Ps conversion deepens across a step to ∼32 km depth and weakens. Further south the Moho deepens to >35 km. Most of the 7 km step-change in crustal thickness occurs over a lateral distance of ∼8 km, yet there is little surface or topographic manifestation of the line. Given Fresnel zone considerations, the dip of the Moho offset could vary between 90° and 45°. Gravity, seismic attenuation and electrical data all show that the TR-line is not only a step in crustal thickness but also a profound lithospheric boundary as mantle properties, such as attenuation ( Q), implied density, and electrical resistivity change abruptly across the line. An east-west oriented cluster of earthquakes with hypocentral depths of 20-40 km is centred on the Moho step. We propose that the Moho step, the earthquakes and the rapid change in mantle properties across the TR boundary are causally related. Processes that could be responsible for the phenomena described here include the rapid removal of mantle lithosphere and lower crust to the north of the TR-line. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Near vertical and wide-angle seismic data provide evidence for a gradational crust–mantle boundary in a depth range of 15–20 km beneath the Central Volcanic Region (CVR), New Zealand. This volcanic area includes the Taupo Volcanic Zone and is a direct extension of Tonga-Kermadec oceanic backarc spreading into continental lithosphere. Long-range seismic refraction data show velocities of 6 km s−1 and less within the top 15 km of the crust of the CVR. At a depth of 15 km compressional seismic velocities increase to 6.8 km s−1, and then to 7.4 ± 0.2 km s−1 at ∼20 km depth. These 7.4 km s−1 seismic wave speeds are interpreted as anomalous upper mantle as beneath this level passive seismic studies show similar Pn wave speeds that increase slowly to ∼7.8 km s−1 at about 80 km deep. We interpret rocks between 15 and 20 km to be a layer of new crust formed by underplating. The strongest reflection observed, and what might also be interpreted as a reflection Moho, is from the top of the proposed underplated layer at 15 km depth. At 20 km depth no such distinct reflection is observed. Rather, wide-angle reflection data show a continuum of low-level reflectivity between 15 and at least 35 km depth, indicating some heterogeneity and/or structure within the lower crust and upper mantle. Thus the transition from lower crust to upper mantle is broad, and a conventional reflection Moho does not exist beneath the CVR. Buoyancy force calculations based on rock uplift for the central North Island indicate that the subjacent mantle, to a depth of 80–100 km is ∼70 kg m−3 or 2 per cent less dense than normal mantle. Best estimates attribute half of this density anomaly to the effects of increased temperature with additional contributions from partial melt (∼1.2 per cent) and melt residuum. [ABSTRACT FROM AUTHOR]

Abstract: Australia and New Zealand share in part a history in the Gondwana supercontinent. Australia has a long and complex tectonic history with the last major accretion in the early Paleozoic, whereas New Zealand is still undergoing major plate boundary processes. The Australian continent is relatively well covered with both active and passive seismic techniques. Multiple sources of information are therefore available for building a model of Moho depth. Results from on-shore and off-shore refraction experiments are supplemented by receiver functions from a large number of portable stations and the recently augmented set of permanent stations. Moho picks from more than 10500km of full-crustal reflection profiles provide valuable additional constraints. The composite data set provides good sampling of much of Australia, though coverage remains low in some remote desert areas. The various datasets provide multiple estimates of the depth to Moho in many regions, and the consistency between the different techniques is high. Some of the thinnest crust lies beneath the Archean craton in the Pilbara, and in the neighbourhood of the Simpson desert. Thick crust is encountered beneath parts of the Proterozoic in Central Australia, and beneath the Paleozoic Lachlan fold belt in southeastern Australia. There are a number of zones of sharp contrast in depth to Moho, notably in the southern part of Central Australia. Despite most of the continental material around New Zealand being submerged, Moho data for this region is mainly onshore concentrating on the Australia–Pacific plate boundary. Two major wide-angle reflection transects provide the bulk of the active source data with just a few traditional reflection profiles offshore. The plate boundary provides an abundance of local earthquakes for tomographic imaging and this data is supplemented with receiver functions from both portable and permanent networks. Onshore the combined coverage is as dense as that of Australia, although it could be argued that a higher spatial resolution is required to capture the nature of the Moho of tectonically active New Zealand. Three regions of thickened crust can be identified, one beneath the Southern Alps, another beneath Fiordland, and below the Wanganui Basin between the North and South Island. Thin crust is identified west of the volcanic arc, with extensive underplating below the back-arc region. [Copyright &y& Elsevier]