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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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.

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.

[1] Deep seismic reflection data acquired as part of the SW-Iberia EUROPROBE project across the transpressional Variscan orogen sample three tectonic terranes: the South Portuguese Zone, the Ossa-Morena Zone, and the Central Iberian Zone. The seismic data reveal the existence of a mid-crustal reflective body 140 km long and of variable thickness (up to 5 km), the Iberian Reflective body. The conductivity image provided by coincident MT soundings, the amplitude characteristics of the seismics, mineralization studies related to magmatic ore deposits, and the surface geology suggest that the IRB is a mantle-derived mafic intrusion. The geophysical, geological and petrological data suggest that the IRB is most probably an Early Carboniferous (approximately at 350 -340 Ma) mantle-derived intrusion possibly linked to plume activity that took place in Europe in the Carboniferous and Permian.

S U M M A R YThe resistivity structure of the Rotokawa geothermal system in New Zealand's Taupo Volcanic Zone has been determined by 3-D modelling of data from a closely spaced (64 measurement sites) magnetotelluric (MT) survey. 3-D conductivity models were constructed using trial and error forward modelling of the phase-tensor data and 3-D inverse modelling of the impedance tensor data. Both the forward and the inverse resistivity models show good consistency. The most interesting feature of these models is a resistive (∼100 m) zone within the otherwise conductive material of the geothermal system. This zone coincides with the high temperature (300-335 • C) core of the geothermal system in which seismicity induced by fluid injection occurs and may mark the zone of fracture permeability that is feeding high temperature fluid into the geothermal system from deeper levels.

S U M M A R YMt Ruapehu is an active andesite cone volcano, which marks the southern termination of the Kermadec volcanic arc. Results from 40 broad-band magnetotelluric soundings have been analysed using the phase tensor. This approach provides a way of determining dimensionality, allowing for distortion removal, and visualizing data in a 3-D situation. The phase tensor analysis suggests that the shallow resistivity structure is largely 1-D in character, but that the deeper structure requires a 3-D interpretation. 1-D inversions show that at sites on Ruapehu a shallow conductive layer lies between a high resistivity layer, of a few hundred metres thickness, and higher resistivity layer corresponding to basement greywacke. The low resistivity layer is contiguous with the waters of the highly acidic Crater Lake, and thus is believed to be the hydraulically controlled upper limit of a zone of acid alteration overlain by dry volcanic rock and ash. To the southwest of the volcano the conductive layer merges with a surface conductor associated with Tertiary sediments. Following initial 2-D inversions, the deep resistivity structure has been derived through 3-D inversion of data from 38 sites. This indicates the existence of a dyke-like low resistivity zone that persists to at least 10 km depth and extends from beneath the summit of Ruapehu to the northeast where it appears to connect to a poorly constrained region of high conductivity, which lies outside the network of measurement sites. The low resistivity dyke-like feature may be identified with a volcanic feeder system, which also supplies the other volcanoes of the Tongariro Volcanic Centre and marks the conduit by which hot gases and (occasionally) magma reach the surface.

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