S U M M A R YThe phase relationships contained in the magnetotelluric (MT) impedance tensor are shown to be a second-rank tensor. This tensor expresses how the phase relationships change with polarization in the general case where the conductivity structure is 3-D. Where galvanic effects produced by heterogeneities in near-surface conductivity distort the regional MT response the phase tensor preserves the regional phase information. Calculation of the phase tensor requires no assumption about the dimensionality of the underlying conductivity distribution and is applicable where both the heterogeneity and regional structure are 3-D.For 1-D regional conductivity structures, the phase tensor is characterized by a single coordinate invariant phase equal to the 1-D impedance tensor phase. If the regional conductivity structure is 2-D, the phase tensor is symmetric with one of its principal axes aligned parallel to the strike axis of the regional structure. In the 2-D case, the principal values (coordinate invariants) of the phase tensor are the transverse electric and magnetic polarization phases. The orientation of the phase tensor's principal axes can be determined directly from the impedance tensor components in both 2-D and 3-D situations. In the 3-D case, the phase tensor is nonsymmetric and has a third coordinate invariant that is a distortion-free measure of the asymmetry of the regional MT response. The phase tensor can be depicted graphically as an ellipse, the major and minor axes representing the principal axes of the tensor. 3-D model studies show that the orientations of the phase tensor principal axes reflect lateral variations (gradients) in the underlying regional conductivity structure. Maps of the phase tensor ellipses provide a method of visualizing this variation.

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Newly forming subduction zones on Earth can provide insights into the evolution of major fault zone geometries from shallow levels to deep in the lithosphere and into the role of fluids in element transport and in promoting rock failure by several modes. The transpressional subduction regime of New Zealand, which is advancing laterally to the southwest below the Marlborough strike-slip fault system of the northern South Island, is an ideal setting in which to investigate these processes. Here we acquired a dense, high-quality transect of magnetotelluric soundings across the system, yielding an electrical resistivity cross-section to depths beyond 100 km. Our data imply three distinct processes connecting fluid generation along the upper mantle plate interface to rock deformation in the crust as the subduction zone develops. Massive fluid release just inland of the trench induces fault-fracture meshes through the crust above that undoubtedly weaken it as regional shear initiates. Narrow strike-slip faults in the shallow brittle regime of interior Marlborough diffuse in width upon entering the deeper ductile domain aided by fluids and do not project as narrow deformation zones. Deep subduction-generated fluids rise from 100 km or more and invade upper crustal seismogenic zones that have exhibited historic great earthquakes on high-angle thrusts that are poorly oriented for failure under dry conditions. The fluid-deformation connections described in our work emphasize the need to include metamorphic and fluid transport processes in geodynamic models.

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S U M M A R YGalvanic distortion has long been recognized as an obstacle in the interpretation of magnetotelluric (MT) data. One fundamental problem for distortion removal is that the equations that describe the effects of galvanic distortion on the impedance tensor are underdetermined. We have previously shown that an explicit solution for four of the parameters of the regional (undistorted) impedance tensor can be resolved without any assumptions. These determinable parameters are the components of a tensor (the phase tensor) representing the phase information contained in the impedance. The coordinate invariants of the phase tensor provide a simple and objective guide to the dimensionality of the regional impedance tensor at each measured frequency. Where the regional structure is 2-D, one of the principal axes of the phase tensor will be aligned parallel to the strike of the regional conductivity. The distortion tensor and the parameters of the regional impedance tensor that represent the amplitude information cannot be determined without assumptions. Where the phase tensor shows the regional impedance tensor to be 1-D, the distortion tensor and the regional impedance can be determined to within a single multiplicative constant. Where a 2-D regional structure is indicated, two assumptions are necessary to determine the regional impedance tensor but the solution is not unique, and any choice of assumptions could be made with equal validity. For 3-D structures, the phase tensor provides the direction of greatest inductive response, which is the closest equivalent of a strike direction. In this case four constraints are required for a solution. In practice, a MT sounding may contain sections that display different characteristic dimension and the distortion tensor can be determined from the section of the sounding with the lowest characteristic dimension. The greatest amount of information is determined from a 1-D section. The use of the information contained in the phase tensor overcomes some of the shortcomings of traditional distortion analysis. Illustrating the tensors using an elliptical representation aids the interpretation of the tensor data involved in this analysis.

Magmatic activity in regions of continental extension may result in huge (>400 km3) explosive eruptions of viscous, gas‐rich silicic‐magma. Geochemical and geological data suggest that the large volumes of magma erupted are produced by extracting interstitial liquid from a long‐lived ‘mush zone’ (a mixture of solid crystals and liquid melt) that accumulates in liquid‐dominated lenses at the top of a much thicker region of lower melt‐fraction mush. Such lenses will be highly electrically conductive compared with normal mid‐crustal rocks. Here we use results of 220 magnetotelluric (MT) soundings to construct a 3‐D electrical resistivity image of the northern (silicic) part of New Zealand's Taupo Volcanic Zone, a young continental rift associated with very high heat flow and intense silicic volcanism. The electrical resistivity image shows a plume‐like structure of high conductivity, interpreted to be a zone of interconnected melt, rising from depths >35 km beneath the axis of extension.

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Taupo Volcanic Zone (TVZ) is a zone of intense volcanism and rifting associated with the subduction of the Pacific Plate beneath the continental crust of New Zealand's North Island. An image of the conductivity structure beneath the central part of the TVZ has been constructed using 2‐D inverse modeling of magnetotelluric data. A rapid increase in conductivity at a depth of 10 km beneath the TVZ, ∼3 km beneath the base of the seismogenic zone but well above the base of the quartzo‐feldspathic crust (∼16 km), is interpreted to mark the presence of an interconnected melt fraction (<4%) within the lower crust. Beneath the quartzo‐feldspathic crust the model shows a zone of increased conductivity on the eastern side of the TVZ consistent with an increased concentration of melt. At deeper levels the Pacific Plate is resistive compared with the overlying mantle.