A major step in the continental Moho and its geodynamic consequences: the Taranaki-Ruapehu line, New Zealand

Abstract

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 (Qp−1), 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.