Lithospheric Structure of a Continental Back-Arc: Central Volcanic Region, New Zealand

Abstract

Seismic exploration data are used to interpret the lithospheric structure of central North Island, New Zealand. The prime focus of this study is the Central Volcanic Region, the apparent extension of Tonga-Kermadec oceanic back-arc spreading into continental lithosphere. Long-range seismic refraction data show velocities of 6 km/s or less within the top 15 km of the crust of the Central Volcanic Region. At a depth of 15 km compressional seismic velocities abruptly increase to 6.8 km/s, and then to 7.4 ± 0.2 km/s at ~20 km depth. These 7.4 km/s seismic velocities are interpreted as anomalous upper mantle as beneath this level passive seismic studies show similar Pn velocities that increase slowly to ~7.8 km/s at about 80 km depth. Seismic refraction data provide evidence for a gradational crust-mantle boundary in a depth range of 15-20 km beneath the Central Volcanic Region. These rocks are interpreted as 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 Central Volcanic Region. Low-velocity volcanics fill the ~2 km deep graben of the Central Volcanic Region. Hidden beneath this ash are two structures on the western margin of the Central Volcanic Region that record the early history of volcanic activity in the continental back-arc. These structures, the Tokoroa high and Mangakino Caldera, form a paired gravity high and low respectively. Interpretations from seismic refraction arrivals and residual gravity modelling indicate the the Mangakino gravity anomaly is produced by two separate, deep-reaching low-density bodies, whereas the Tokoroa gravity anomaly is due to a basement high coming, at most, to within ~650 metres of the surface. Causative bodies for these two structures are interpreted to be volcanic in origin. The -65 mgal residual gravity low of the Mangakino Caldera is modelled with low density bodies that extend to depths of around 7 km. These may be attributed to magma chambers that fed large ignimbrite eruptions and hence are remnants of caldera collapse events. An andesite volcano, or complex volcanic structure is the preferred interpretation for the Tokoroa gravity high. Strong reflections from beneath the Central Volcanic Region are interpreted to be from an interface at ~34 km depth within the upper mantle. Reflections from this interface (PmP2) are spatially confined to be directly beneath the eastern, and geothermally active, side of the Central Volcanic Region. Reflections amplitudes of PmP2 are ~50% greater than those for the crust-mantle transition zone (<6 to 6.8 km/s) reflector. 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 30-70% drop in S-wave speed (Vs) compared to the surrounding mantle. These strong reflections are inferred to be from a layer of ~4-9% partial melt. When the Amplitude Variation with Offset (AVO) response of the reflector and the Fresnel zone radius at wide incidence angles are taken into account, best estimates indicate a ~16 km width for the melt body. An unusual upper mantle beneath central North Island is also indicated by observed decreases in Vp and Vs velocities of up to 9% and 19%, respectively. Using derivatives of velocity with temperature, and partial melt percent, P-, and S-wave velocity reductions imply temperature anomalies in the mantle beneath the western North Island and Central Volcanic Region of 280°C and 300°C, and partial melts percents of 0.1% and 1.4%, respectively. Further evidence for a perturbed mantle comes from rock uplift studies, which indicate that beneath the western North Island and Central Volcanic Region the mantle has density anomalies of -35 and -66 kg/m3, respectively. Such density anomalies imply the mantle lid here is missing. Best estimates attribute half of the density anomaly beneath the central North Island to the effects of increased temperature (thermal expansion) with additional contributions from partial melt.