The seismic low-velocity zone (LVZ) of the upper mantle is generally associated with a low-viscosity asthenosphere that has a key role in decoupling tectonic plates from the mantle1. However, the origin of the LVZ remains unclear. Some studies attribute its low seismic velocities to a small amount of partial melt of minerals in the mantle2,3, whereas others attribute them to solid-state mechanisms near the solidus4,5,6 or the effect of its volatile contents6. Observations of shear attenuation provide additional constraints on the origin of the LVZ7. On the basis of the interpretation of global three-dimensional shear attenuation and velocity models, here we report partial melt occurring within the LVZ. We observe that partial melting down to 150–200 kilometres beneath mid-ocean ridges, major hotspots and back-arc regions feeds the asthenosphere. A small part of this melt (less than 0.30 per cent) remains trapped within the oceanic LVZ. Melt is mostly absent under continental regions. The amount of melt increases with plate velocity, increasing substantially for plate velocities of between 3 centimetres per year and 5 centimetres per year. This finding is consistent with previous observations of mantle crystal alignment underneath tectonic plates8. Our observations suggest that by reducing viscosity9 melt facilitates plate motion and large-scale crystal alignment in the asthenosphere.
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Numerical modelling codes related to this paper are available from https://doi.org/10.17611/dp/emc.2020.dbrdnature.1. Most figures were created using open software GMT 4.5.13.
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We thank the Iris and Geoscope data centres for providing seismological data. We thank J. P. Perrillat and M. Behn for discussions on mineralogy and attenuation models, and F. Dubuffet for preparing data for sharing as IRIS data products. The European Union Horizon 2020 research and innovation programme funds T.B. under grant agreement 716542. The LABEX Lyon Institute of Origins (LIO, ANR-10-LABX-0066) of the University of Lyon funded a beowulf cluster hosted and maintained at ENSL and used in this study. The world map figures were created using open software GMT 4.5.13.
The authors declare no competing interests.
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This file contains 1 Table (reference parameters for Eq. A.1) and 19 Figures. Figures S1-12 test the sensibility to radial anisotropy, EAGBS, composition or anelasticity model. Figures S14 and S19 depict the predicted temperatures. Other Figures illustrate the effect of changing sensitivity to melt content, shear modulus or grain size.
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Debayle, E., Bodin, T., Durand, S. et al. Seismic evidence for partial melt below tectonic plates. Nature 586, 555–559 (2020). https://doi.org/10.1038/s41586-020-2809-4
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