The cold and relatively dry nature of mantle forearcs in subduction zones

Abstract

Some of Earth's coldest mantle is found in subduction zones at the tip of the mantle wedge that lies between the subducting and overriding plates. This forearc mantle is isolated from the flow of hot material beneath the volcanic arc, and so is inferred to reach temperatures no more than 600 to 800 °C — conditions at which hydrous mantle minerals should be stable. The forearc mantle could therefore constitute a significant reservoir for water if sufficient water is released from the subducting slab into the mantle wedge. Such a reservoir could hydrate the plate interface and has been invoked to aid the genesis of megathrust earthquakes and slow slip events. Our synthesis of results from thermal models that simulate the conditions for subduction zones globally, however, indicates that dehydration of subducting plates is too slow over the life span of a typical subduction zone to hydrate the forearc mantle. Hot subduction zones, where slabs dehydrate rapidly, are an exception. The hottest, most buoyant forearcs are most likely to survive plate collisions and be exhumed to the surface, so probably dominate the metamorphic rock record. Analysis of global seismic data confirms the generally dry nature of mantle forearcs. We conclude that many subduction zones probably liberate insufficient water to hydrate the shallower plate boundary where great earthquakes and slow slip events nucleate. Thus, we suggest that it is solid-state processes and not hydration that leads to weakening of the plate interface in cold subduction zones.

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Figure 1: Schematic forearc showing water fluxes and potentially hydrated mantle.
Figure 2: Predicted hydration potential and seismic velocities for individual arcs.
Figure 3: Predicted per cent hydration of mantle wedges worldwide produced over last 50 Myrs.
Figure 4: Compilation of Vp observed in the shallowest parts of forearc mantle wedges versus slab thermal parameter.

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Acknowledgements

This work is funded by NSF awards OCE-1446970 to GAA, OCE-1249353 and OCE-1356132 to P.E.v.K., and EAR-1249703 to B.R.H.

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GA compiled and analysed seismic data, P.E.v.K. oversaw geodynamic modelling, B.R.H. oversaw petrology and thermodynamic modelling. All of the authors contributed to discussion and writing of the manuscript.

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Correspondence to G. A. Abers.

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The authors declare no competing financial interests.

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Supplementary information Discussions 1–4, Tables 1–4 and Figures 1–2 (PDF 1571 kb)

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Abers, G., van Keken, P. & Hacker, B. The cold and relatively dry nature of mantle forearcs in subduction zones. Nature Geosci 10, 333–337 (2017). https://doi.org/10.1038/ngeo2922

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