Geoelectromagnetic Induction Studies in the North Island Volcanic Region, New Zealand

Abstract

Electromagnetic induction studies in the North Island Volcanic Region were undertaken to investigate the subsurface electrical conductivity regime associated with the Taupo Volcanic Zone. Field observations at 21 magnetovariation and 5 magnetotelluric stations were recorded. The application of the autoregressive (AR) spectral analysis technique to geoelectromagnetic data was tested by an exhaustive series of tests. The tests demonstrated the efficacy of the new technique. As compared to the conventional methods, the AR analysis yielded smoother estimates of the transfer parameters with improved frequency resolution over an extended bandwidth. The tests also highlighted the importance of pre-processing of data. High pass digital filtering instead of the commonly employed least-square linear detrending improved the repeatability of estimates of transfer parameters over different events at the same station. The vertical transfer functions indicated the presence of a high conductivity anomaly, the North Island Anomaly, running NW-SE, i.e. orthogonal to the trend of the present-day geologic features. Hypothetical event analysis helped in recognizing a low-response anomaly associated with the Taupo Volcanic Zone. A high conductivity feature correlating with the Mangakino gravity anomaly and the presence of, 'leaked' currents in the Hawke Bay were also detected. A limited two-dimensional modelling of the North Island Anomaly indicated the presence of a conductive mass (σ ≈ 1 mho/m) between 20 and 80 km depth. The southwestern edge of this structure was expected to lie close to Rotorua. In the NE, the structure would continue beyond the northern coastline of the North Island. The anomalous response associated with the Taupo Volcanic Zone was modelled by a near-surface 7 km thick, 15 km wide conductor, σ = 0.1 mho/m. Regarding the geologic development of the region, the North Island Anomaly was interpreted in terms of an old NW-SE trending subduction zone which retained this orientation between 18 and 3 my BP. The low response of the Taupo Volcanic Zone was related to the young age of the NE-SW trending subduction system. In its present early stage of development, it was conjectured that under the Taupo Volcanic Zone the zones of partial melt existed only in unconnected pockets.