Fluid and deformation regime of an advancing subduction system at Marlborough, New Zealand

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Abstract

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 modes1,2. 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 Island3,4, 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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Figure 1: Geological terrane map of central New Zealand8.
Figure 2: Nonlinear 2D inversion model of electrical resistivity below the Marlborough–northern Westland district.
Figure 3: Interpretative geological/fluidized states over pertinent depth ranges for labelled major conductive zones imaged below the magnetotelluric transect.

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Acknowledgements

This research was supported by the Geophysics program of the US National Science Foundation (grant EAR0440050), and the Plate Boundary program of the New Zealand Foundation for Research, Science and Technology. W. Hales of Alpine Springs Helicopters provided an airborne transport service to remote locations of the Marlborough and Westland regions. We thank B. Freer, C. Davis and P. Thorton of the New Zealand Department of Conservation, and numerous private landholders, for permission to access site locations. Additional field assistance was given by students M. Burgess and P. Winther. Many discussions were held with D. Eberhart-Phillips, R. Sibson and P. Upton. Illustrations were finalized by D. Jensen.

Author Contributions P.E.W., T.G.C. and G.R.J. designed the experiment. The home institutions of T.G.C. and Y.O. supplied the instrumentation. T.G.C. and G.J.H. reduced the observed time series. V.M. derived the induction vectors. All authors were essential to the success of the field campaign and contributed to the interpretation and the manuscript.

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Correspondence to Philip E. Wannamaker.

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Wannamaker, P., Caldwell, T., Jiracek, G. et al. Fluid and deformation regime of an advancing subduction system at Marlborough, New Zealand. Nature 460, 733–736 (2009) doi:10.1038/nature08204

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