Electrical resistivity image of the upper crust within the Taupo Volcanic Zone, New Zealand

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Abstract
The Taupo Volcanic Zone (TVZ) forms a region of back‐arc extension within the continental crust of New Zealand and is notable for its caldera volcanism, high rate of magmatic production, and high heat flow. On the eastern side of the TVZ, volcanism terminates abruptly along a linear margin. We present the interpretation of three multiple‐source bipole‐dipole resistivity surveys across this margin. The major features of the measured data can be reproduced by using two‐dimensional models of the resistivity in the upper 10 km of the crust. Along the margin the high‐resistivity graywacke basement (300 Ωm increasing to 1300 Ωm) is downfaulted to the west across a zone about 6 km wide, with a total vertical offset of approximately 2 km. The easternmost fault is overlain by an unbroken 0.24 Ma ignimbrite sheet, suggesting the fault has been inactive for this period. Further to the west, at depths greater than 2 km, a conductive zone about 15 km wide and of total conductance 180–280 S has been identified. This conductive zone can be modeled by using a range of resistivities (40–3 Ωm) and thicknesses (8–2 km). Models incorporating resistivities less than 15 Ωm require the low‐resistivity zone to be wedge shaped, so that conductance increases to the west. The low resistivity is believed to be caused by the presence of clay alteration within volcanoclastic rocks. The large volume of volcanic rocks at depth can be linked to a series of caldera‐forming eruptions that have taken place along the eastern side of the TVZ.

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