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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[1] Broadband MT (magnetotelluric) data were recorded that form an array of measurements at the south-eastern margin of the TVZ (Taupo Volcanic Zone), in the central North Island of New Zealand. These array data are used to investigate mechanisms by which the TVZ's extraordinarily high heat flux is transported to the surface. Taken together with seismological data, these MT data show compelling evidence that support a model of hydrothermal convection within the brittle (upper $6-7 km) part of the crust. Both 2-D and 3-D inversion models of these MT data show vertical low-resistivity zones that connect surface geothermal fields to an inferred magmatic heat source that lies below the brittle-ductile transition.

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The Transantarctic Mountains (TAM) are the world’s longest rift shoulder but the source of their high elevation is enigmatic. To discriminate the importance of mechanical vs. thermal sources of support, a 550 km-long transect of magnetotelluric geophysical soundings spanning the central TAM was acquired. These data reveal a lithosphere of high electrical resistivity to at least 150 km depth, implying a cold stable state well into the upper mantle. Here we find that the central TAM most likely are elevated by a non-thermal, flexural cantilever mechanism which is perhaps the most clearly expressed example anywhere. West Antarctica in this region exhibits a low resistivity, moderately hydrated asthenosphere, and concentrated extension (rift necking) near the central TAM range front but with negligible thermal encroachment into the TAM. Broader scale heat flow of east-central West Antarctica appears moderate, on the order of 60–70 mW m−2, lower than that of the U.S. Great Basin.

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Taiwan is the type example of an arc-continent collision. Numerous tectonic models have been proposed for this orogen, and include both thinskinned and thickskinned lithospheric deformation. These models predict very different structures at middle and lower crustal depths, but insuffi cient geophysical data exist to unequivocally distinguish between them. Long-period magnetotelluric (MT) data were collected in central Taiwan in 2006-2007 to constrain the crustal resistivity structure. A two-dimensional inversion of these MT data revealed a prominent electrical conductor that extends across the décollement predicted by the thinskinned model. This feature is interpreted to be due to 1%-2% saline fl uids, and is inconsistent with the thinskinned model. In contrast, the thickskinned model predicts this feature since fl uids are generated in the crustal root through metamorphism. Quantitative correlation of the resistivity and seismic velocity models supports small-volume, high-salinity fl uids in a thickened crust as the cause of this conductor.