Crust and upper mantle structure of a continental backarc: central North Island, New Zealand

Abstract

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.