High frequency magnetotelluric survey of the volcanic cent and hydrothermal system on Mount Ruapehu New Zealand

Abstract

Previous magnetotelluric (MT) studies (Dravitzki, 2005; Cairns, 2006) carried out by Victoria University of Wellington (VUW) and GNS Science identified a shallow low resistivity anomaly on the summit plateau of Mount Ruapehu. It was inferred that the low resistivity was most likely to be related to the volcanic hydrothermal system (Dravitzki, 2005).

This current study builds on this earlier work with the aim of modelling the electrical resistivity structure of the volcanic vent and hydrothermal system of Mount Ruapehu. Data from high frequency MT measurements made at nine sites around the summit plateau, combined with data from previous studies were used to derive both one and three dimensional models.

The 3D models show two vertical pipe-like structures of relatively high resistivity compared to their surroundings. The first of these is located to the magnetic south of the centre of the model at depths of 150m to 500m. The second of these pipe-like structures is located to the magnetic north of the centre of the model, at depths of between 800m and 2,000m. The resistivity of these two structures is ~30Ωm and ~100Ωm respectively, with surrounding resistivities of 0.3Ωm and 10Ωm.

At these depths it is inferred that these pipe like structures indicate regions of resistive high temperature alteration mineral, which are possible candidates for a heat pipe structure as proposed by Hurst et. al. (1991). Additional three dimensional modelling of the wider area was carried out using MT data collected in the Tongariro Volcanic Centre during the previous VUW and GNS Science studies. These models displayed a region of low resistivity (0.3Ωm -10Ω2m) located to the magnetic north of Ruapehu, continuing northward for ~72km and ranging in depth from 1,500m to around 10,000m. Phase tensor analysis carried out in previous studies (Cairns, 2006) also indicated the existence of a deep conductive zone on the NE (with respect to true north) side of Ruapehu. In these studies it was suggested that this region was associated with the southern termination of the Taupo Volcanic Zone.