Imaging the deep source of the Rotorua and Waimangu geothermal fields, Taupo Volcanic Zone, New Zealand

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Magnetotelluric data were recorded in a 45×10km band crossing the Rotorua and Waimangu geothermal fields in the northern part of the Taupo Volcanic Zone in the central North Island of New Zealand. 3-D inverse modelling of these data show that beneath the low resistivity areas marking the near surface geothermal fields, localised electrically conductive zones are present in the crust below about 2.5 and 3.5km depth at Rotorua and Waimangu, respectively. At increasing depth these conductive zones broaden and appear to merge with a larger conductive zone at 8km depth situated between the geothermal systems. At Rotorua the top of the conductive zone is situated directly beneath the area of greatest surface heat and gas discharge. At Waimangu the uppermost part of the deeper conductive zone is situated beneath the western part of Lake Rotomahana, also an area of intense surface thermal activity and high heat flux. The localised conductive zones are interpreted to be high temperature (quasi-magmatic) fluids rising from a broader zone of partial melt at deeper levels.