Strong seismic reflections and melts in the mantle of a continental back‐arc basin

Abstract

A program of explosion seismology in central North Island, New Zealand, discovered a strong reflector within the upper mantle. Reflections from this (PmP 2 ) are spatially confined to come from an interface 35 km deep and directly beneath a 40 km‐wide, back‐arc extension zone with active volcanism, high heat flow, low Pn wave‐speeds and thinned crust. On the basis of relative reflection amplitudes, the mantle reflections are most readily explained by an interface with a negative seismic impedance contrast. A satisfactory fit is obtained for a layer with a 40–90% drop in S‐wave speed (Vs) compared to the surrounding mantle. We interpret this layer to be a 40 km‐wide reservoir of partial melt pooled at a thermal boundary layer within the upper mantle.