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.

Active and passive seismic experiments show that the southern Sierra, despite standing 1.8 to 2.8 kilometers above its surroundings, is underlain by crust of similar seismic thickness, about 30 to 40 kilometers. Thermobarometry of xenolith suites and magnetotelluric profiles indicate that the upper mantle is eclogitic to depths of 60 kilometers beneath the western and central parts of the range, but little subcrustal lithosphere is present beneath the eastern High Sierra and adjacent Basin and Range. These and other data imply the crust of both the High Sierra and Basin and Range thinned by a factor of 2 since 20 million years ago, at odds with purported late Cenozoic regional uplift of some 2 kilometers.

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A wideband (0.01–1000 s) magnetotelluric survey across the southern Sierra Nevada has identified zones of enhanced conductivity in the lower crust and upper mantle that underlie the resistive batholithic rocks at depths greater than 10–20 km. The eastern zone underlies the highest topography of the range and extends eastward. This eastern conductive zone extends to depths in excess of 100 km, based on model sensitivities, and lies well below the Moho depth of 32–38 km from seismological studies. Therefore the enhanced conductivity in the mantle cannot be attributed to conventional explanations for conductive continental crust and is instead likely due to partial melt. Such an interpretation is consistent with gravitational, seismological, and geologic evidence. Estimates of the partial melt fraction range from 2 to 5% at depths of 40–70 km, which are consistent with fractions of melt inclusions observed in xenoliths from the eastern Sierra Nevada. This partial melt accounts for approximately one third of the density decrease required in the mantle for support of the present high elevations. The other two thirds of the density decrease could be due to thermal expansion of the upper mantle or due to mineralogical changes. The western conductive zone also straddles the Moho and extends into the mantle beneath western foothills of the Sierra Nevada and the Great Valley sediments to the west. However, xenoliths from this region indicate high‐velocity crustal rocks to depths in excess of 60 km, and we therefore attribute the enhanced conductivity to graphitic metasediments and/or dehydration of metaserpentinite emplaced by downward return flow of the host rocks during intrusion of the Sierran plutons.

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A three‐dimensional finite‐element computer algorithm which can accommodate arbitrarily complex topography and subsurface structure has been developed to model the resistivity response of the earth. The algorithm has undergone extensive evaluation and is believed to provide accurate results for realistic earth models. Testing included comparison to scale‐model measurements, analytically calculated solutions, and results calculated numerically by other independent means. Computer modeling experiments have demonstrated that it is possible to remove the effect of topography on resistivity data even under conditions where such effects are large with respect to the subsurface responses. This can be done without resorting to lengthy and costly trial‐and‐error computer modeling. After correction, the data can be interpreted as if the anomalies are due only to subsurface structure. The results of case studies on field data measured in high‐relief topography indicate the following. (1) Pre‐ or postsurvey 3-D computer analyses of topographic effects are worthwhile and can be done for a small fraction of the survey cost. (2) Three‐dimensional terrain corrections can significantly improve the quality of subsurface interpretation in terrain geometries which cannot be modeled with a 2-D algorithm. (3) Terrain effects on collinear electrode arrays can be minimized by alignment of the arrays parallel to the strike of the topography (that is, along streamlines and ridgelines). The computing cost of performing 3-D analyses is relatively small compared to the man‐hour costs required to implement them. The total cost of a 3-D terrain correction is typically small compared to the survey cost, especially if one uses the new generation of large main‐frame computers.

Seismicity in both compressional and extensional settings is a function of local and regional stresses, rheological contrasts, and the distribution of fluids. The influence of these factors can be illustrated through their effects on electrical geophysical structure, since this structure reflects fluid composition, porosity, interconnection and pathways. In the compressional, amagmatic New Zealand South Island, magnetotelluric (MT) data imply a concave-upward ("U"-shaped), middle to lower crustal conductive zone beneath the west-central portion of the island due to fluids generated from prograde metamorphism within a thickening crust. Change of the conductor to near-vertical orientation at middle-upper crustal depths is interpreted to occur as fluids cross the brittle-ductile transition during uplift, and approach the surface through induced hydrofractures. The central South Island is relatively weak in seismicity compared to its more subduction-related northern and southern ends, and the production of deep crustal fluids through metamorphism may promote slip before high stresses are built up. The deep crustal conductivity is highly anisotropic, with the greater conductivity along strike, consistent with fault zone models of long-range interconnection versus degree of deformation. The central Great Basin province of the western U.S. by contrast is extensional at present although it has experienced diverse tectonic events throughout the Paleozoic. MT profiling throughout the province reveals a quasi one-dimensional conductor spanning the lower half of the crust which is interpreted to reflect high temperature fluids and perhaps melting caused ultimately by exsolution from crystallizing underplated basalts. The brittle, upper half of the crust is generally resistive, but also characterized by numerous steep, narrow conductors extending from near-surface to the middle crust where they contact the deep crustal conductive layer. These are suggested to represent fluidised/altered fault zones, with at least some fluids contributed from the deeper magmatic exsolution. The best-known faults imaged geophysically before this have been the listric normal faults bounding graben sediments as imaged by reflection seismology. However, the major damaging earthquakes of the Great Basin appear to nucleate near mid-crustal depths on near-vertical fault planes, which we suggest are being imaged with the MT transect data, and where triggering fluids from the ductile lower crust are available. In both compressional and extensional examples, the fluidised fault zones are hypothesized to act to concentrate slip, with major earthquakes resulting in asperities along the fault surface.