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Melt-rich channel observed at the lithosphere–asthenosphere boundary

Abstract

The lithosphere–asthenosphere boundary (LAB) separates rigid oceanic plates from the underlying warm ductile asthenosphere. Although a viscosity decrease beneath this boundary is essential for plate tectonics, a consensus on its origin remains elusive. Seismic studies identify a prominent velocity discontinuity at depths thought to coincide with the LAB but disagree on its cause1,2,3,4,5, generally invoking either partial melting6 or a mantle dehydration boundary7 as explanations. Here we use sea-floor magnetotelluric data to image the electrical conductivity of the LAB beneath the edge of the Cocos plate at the Middle America trench offshore of Nicaragua. Underneath the resistive oceanic lithosphere, the magnetotelluric data reveal a high-conductivity layer confined to depths of 45 to 70 kilometres. Because partial melts are stable at these depths in a warm damp mantle8, we interpret the conductor to be a partially molten layer capped by an impermeable frozen lid that is the base of the lithosphere. A conductivity anisotropy parallel to plate motion indicates that this melt has been sheared into flow-aligned tube-like structures9. We infer that the LAB beneath young plates consists of a thin, partially molten, channel of low viscosity that acts to decouple the overlying brittle lithosphere from the deeper convecting mantle. Because this boundary layer has the potential to behave as a lubricant to plate motion, its proximity to the trench may have implications for subduction dynamics.

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Figure 1: Regional tectonic map and location of the magnetotelluric survey.
Figure 2: Resistivity model obtained from anisotropic inversion of the sea-floor magnetotelluric data.
Figure 3: High asthenosphere conductivity explained by a thin partially molten layer.

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Acknowledgements

We thank the captain (M. Stein) and crew of R/V Melville and the governments of Nicaragua and Costa Rica for permission to work in their exclusive economic zones. The following are thanked for their participation in the research cruise: C. Armerding, C. Berger, E. Carruthers, B. Cohen, J. Elsenbeck, T. Matsuno, D. Myer, A. Orange, J. Perez, K. Shadle, J. Souders, K. Weitemeyer, B. Wheelock and S. Zipper; J. Lemire and A. Jacobs are thanked for their efforts with cruise planning, mobilization and demobilization. We thank B. Wheelock and D. Hasterok for discussions. This work was supported by the National Science Foundation (grants OCE-08411141 and OCE-0840894) and the Seafloor Electromagnetic Methods Consortium at Scripps Institution of Oceanography.

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K.K., R.L.E. and S.C. conceived the survey. K.K. and S.C. collected the data. S.N. and S.C. processed the data. S.N. analysed and inverted the data. S.N. and K.K. developed the interpretation and wrote the manuscript. All authors discussed the results and commented on the manuscript.

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Correspondence to S. Naif.

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Naif, S., Key, K., Constable, S. et al. Melt-rich channel observed at the lithosphere–asthenosphere boundary. Nature 495, 356–359 (2013). https://doi.org/10.1038/nature11939

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