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Fluid-rich subducting topography generates anomalous forearc porosity

Abstract

The role of subducting topography on the mode of fault slip—particularly whether it hinders or facilitates large megathrust earthquakes—remains a controversial topic in subduction dynamics1,2,3,4,5. Models have illustrated the potential for subducting topography to severely alter the structure, stress state and mechanics of subduction zones4,6; however, direct geophysical imaging of the complex fracture networks proposed and the hydrology of both the subducting topography and the associated upper plate damage zones remains elusive. Here we use passive and controlled-source seafloor electromagnetic data collected at the northern Hikurangi Margin, New Zealand, to constrain electrical resistivity in a region of active seamount subduction. We show that a seamount on the incoming plate contains a thin, low-porosity basaltic cap that traps a conductive matrix of porous volcaniclastics and altered material over a resistive core, which allows 3.2 to 4.7 times more water to subduct, compared with normal, unfaulted oceanic lithosphere. In the forearc, we image a sediment-starved plate interface above a subducting seamount with similar electrical structure to the incoming plate seamount. A sharp resistive peak within the subducting seamount lies directly beneath a prominent upper plate conductive anomaly. The coincidence of this upper plate anomaly with the location of burst-type repeating earthquakes and seismicity associated with a recent slow slip event7 directly links subducting topography to the creation of fluid-rich damage zones in the forearc that alter the effective normal stress at the plate interface by modulating the fluid overpressure. In addition to severely modifying the structure and physical conditions of the upper plate, subducting seamounts represent an underappreciated mechanism for transporting a considerable flux of water to the forearc and deeper mantle.

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Fig. 1: Tectonic setting and HT-RESIST survey area.
Fig. 2: Preferred resistivity model and porosity.
Fig. 3: Schematic of fluid transfer from subducting topography to the overthrusting forearc during a slow slip cycle.

Data availability

All electromagnetic data that were inverted and analysed in this study are available at https://doi.org/10.5281/zenodo.4721384 and as Source data provided with this paper. The seismic reflection data overlain on the resistivity models are available at https://doi.org/10.21420/62C1-GS40Source data are provided with this paper.

Code availability

A version of the MARE2DEM code used to invert the data is available at http://mare2dem.bitbucket.io.

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Acknowledgements

We acknowledge that this work was partially carried out on the lands of the Te Āti Awa. We thank Scripps Institution of Oceanography for providing the instrumentation necessary to collect the electromagnetic data used in this study, the New Zealand government for permission to work in their exclusive economic zone, and the captains (W. Hill and D. Murline) and crew of the R/V Revelle expeditions RR1817 and RR1903. We thank S. Constable and the Scripps Marine EM Lab (C. Armerding, J. Lemire, J. Perez and J. Souders) and the HT-RESIST science party (A. Adams, J. Alvarez-Aramberri, C. Armerding, E. Attias, E.A. Bertrand, D. Blatter, G. Boren, G. Franz, C. Gustafson, W. Heise, Y. Li, B. Oryan, N. Palmer, J. Perez, J. Sherman, K. Woods and A. Yates). We thank the National Institute of Water and Atmospheric Research (NIWA; https://niwa.co.nz/) for providing high-resolution bathymetry data. This work was supported by the National Science Foundation grant OCE-1737328. C.C. acknowledges funding support by the Department of Defense (DoD) through the National Defense Science and Engineering Graduate Fellowship (NDSEG) Program. D.B. was supported by a Royal Society of New Zealand Marsden Fund grant (MFP-GNS1902); by the MBIE Endeavour Grant: Diagnosing peril posed by the Hikurangi subduction zone; and by public research funding from the Government of New Zealand Strategic Science Investment Fund to GNS Science. We acknowledge computing resources from Columbia University’s Shared Research Computing Facility project, which is supported by NIH Research Facility Improvement Grant 1G20RR030893-01, and associated funds from the New York State Empire State Development, Division of Science Technology and Innovation (NYSTAR) Contract C090171, both awarded 15 April 2010.

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Authors

Contributions

S.N. and K.K. designed the experiment. S.N. and C.C. collected the data. C.C. and S.N. processed the data. C.C. modelled the data. All authors contributed to writing the manuscript.

Corresponding author

Correspondence to Christine Chesley.

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The authors declare no competing interests.

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Peer review information Nature thanks Martyn Unsworth and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Fig. 1 Preferred inversion model (vertical resistivity component ρv) with high-resolution bathymetry.

Seafloor receivers used in the inversion are grey cubes with station numbers. See Extended Data Fig. 6 for the horizontal resistivity and anisotropy. a, b, Northeast-facing (a) and southwest-facing (b) views of the bathymetry. High-resolution bathymetry were provided courtesy of the National Institute of Water and Atmospheric Research (https://niwa.co.nz/).

Extended Data Fig. 2 Porosity conversion of the preferred resistivity model using a range of Archie’s law cementation exponents.

a, m = 1.6. b, m = 2. c, m = 2.4 (same as Fig. 2b). d, m = 2.8.

Extended Data Fig. 3 Resolution test of key model features.

a, Model used to generate synthetic data. b, Model recovered from inversion of the synthetic data.

Source data

Extended Data Fig. 4 Example of CSEM data and model responses from this survey.

Amplitude (top) and phase (bottom) data (circles) and preferred model response (line) at 0.75 Hz for station 7 (green) and station 26 (blue) are shown. The rapid attenuation seen at station 7 and the much slower decay at station 26 are due to their respective locations on the conductive forearc and resistive Tūranganui Knoll.

Extended Data Fig. 5 MT impedance polar diagrams shown as a function of period and station ID.

Red and blue lines show the diagonal, |Zxx|, and off-diagonal, |Zxy|, components of the impedance tensor, respectively, as a function of geographic rotation, with north pointing up. The black arrow in the white circle is the strike direction for this survey. Grey shading masks the periods and stations where data are omitted from our 2D analysis due to 3D effects in the polar diagram shapes.

Extended Data Fig. 6 Vertical anisotropy of the preferred resistivity model.

a, Horizontal resistivity (ρh). b, Anisotropy ratio (ρv/ρh). The model has minimal anisotropy.

Extended Data Fig. 7 Preferred model r.m.s misfit breakdown.

a, b, Normalized r.m.s for CSEM data (a) and MT data (b). The blue dots and red dots in a are normalized residuals for all inline electric field amplitude and phase, respectively, at a given transmitter position. The bars in b are r.m.s. misfit for impedance tensor components of each MT receiver: blue, transverse electric (TE) mode apparent resistivity; green, TE phase; orange, transverse magnetic (TM) mode apparent resistivity; purple, TM phase.

Extended Data Fig. 8 CSEM data and response matrices.

CSEM data (top), model fits (middle) and residuals (bottom) for the highest power harmonics as a function of distance from the Hikurangi Margin and transmitter–receiver offset. The dashed box indicates data collected at 1/6 Hz. All other data were collected at 1/4 Hz. a, Fundamental frequency. b, Third harmonic. c, Seventh harmonic.

Extended Data Fig. 9 MT data and model responses.

Fit of the preferred resistivity model (lines) to all MT data (circles) used in this study. TE mode is blue and TM mode is red.

Extended Data Fig. 10 Sensitivity to forearc conductors C1f, C2f and C3f.

a, b, Change in model fit between the preferred model and forward models testing the sensitivity to the forearc conductors for the CSEM data (a) and the MT data (b). To generate the top row of each panel, the resistivity of each conductor was individually increased to 5 Ωm. The resistivity was increased to 10 Ωm in the bottom panel. The blue dots and red dots in a are the change in r.m.s. for all inline electric field amplitudes and phases, respectively, at a given transmitter position. In b, the bars are the change in r.m.s. misfit for impedance tensor components of each MT receiver: blue, TE apparent resistivity; green, TE phase; orange, TM apparent resistivity; purple, TM phase.

Source data

Extended Data Fig. 11 Sensitivity to subducting seamount, R1f.

Change in model fit between the preferred model and forward models testing the sensitivity to the subducting seamount for the MT data (CSEM data are insensitive to R1f). To generate the top, middle and bottom panels, the resistivity of the subducting seamount was decreased to 20 Ωm, 10 Ωm and 7 Ωm, respectively. The bars are the change in r.m.s. misfit for impedance tensor components of each MT receiver: blue, TE apparent resistivity; green, TE phase; orange, TM apparent resistivity; purple, TM phase.

Source data

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Chesley, C., Naif, S., Key, K. et al. Fluid-rich subducting topography generates anomalous forearc porosity. Nature 595, 255–260 (2021). https://doi.org/10.1038/s41586-021-03619-8

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