The geomagnetic field intensity in New Zealand: palaeointensities from Holocene lava flows of the Tongariro Volcanic Centre

Very few absolute palaeointensity data exist from Holocene-aged rocks in New Zealand. Here we present a new suite of high-quality palaeointensities, supported by detailed rock magnetic investigations. Samples from 23 sites representing 10 distinct eruptive units of the Tongariro Volcanic Centre, Taupo Volcanic Zone, New Zealand, were studied. Both traditional double heating and microwave palaeointensity methods were employed. The reliability of the palaeointensity data varies with rock magnetic properties of the samples, corresponding, in particular, to their positions within the lava flows. The highest success rates are from samples obtained from near the flow tops where a significant proportion of the remanence unblocked at intermediate temperatures (200-350 degrees C). By contrast, samples from flow centres, particularly the parts showing platey fracturing, have the lowest success rates. Reliable, high-quality palaeointensity results ranging from 32.4 +/- 5.1 mu T to 72.1 +/- 4.7 mu T were obtained from six flows with ages between c. 12 800 yr BP and the present. These correspond to virtual dipole moments that increase from 52 +/- 10 ZAm(2) in the early Holocene and peak at 112 +/- 14 ZAm(2) about 300 yr ago. The data agree well with calibrated relative palaeointensities from New Zealand lake sediments. The volcanic and sedimentary data together yield a Holocene virtual axial dipole moment curve that fits the global average variation well in the early Holocene, but which differs significantly in recent millennia. This difference is associated with recent migration of the southern high latitude core-mantle boundary flux lobe towards New Zealand, as is seen in global field models.