A sharp lithosphere–asthenosphere boundary imaged beneath eastern North America

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Abstract

Plate tectonic theory hinges on the concept of a relatively rigid lithosphere moving over a weaker asthenosphere, yet the nature of the lithosphere–asthenosphere boundary remains poorly understood. The gradient in seismic velocity that occurs at this boundary is central to constraining the physical and chemical properties that create differences in mechanical strength between the two layers. For example, if the lithosphere is simply a thermal boundary layer that is more rigid owing to colder temperatures, mantle flow models1,2 indicate that the velocity gradient at its base would occur over tens of kilometres. In contrast, if the asthenosphere is weak owing to volatile enrichment3,4,5,6 or the presence of partial melt7, the lithosphere–asthenosphere boundary could occur over a much smaller depth range. Here we use converted seismic phases in eastern North America to image a very sharp seismic velocity gradient at the base of the lithosphere—a 3–11 per cent drop in shear-wave velocity over a depth range of 11 km or less at 90–110 km depth. Such a strong, sharp boundary cannot be reconciled with a purely thermal gradient, but could be explained by an asthenosphere that contains a few per cent partial melt7 or that is enriched in volatiles relative to the lithosphere3,4,5,6.

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Figure 1: Three-dimensional view of the lithosphere–asthenosphere boundary and surface topography.
Figure 2: Imaging discontinuities with waveforms from individual stations.

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Acknowledgements

We thank L. Elkins-Tanton, D. W. Forsyth and G. Hirth for discussions. Data came from the IRIS Global Seismic Network, the US National Seismic Network, the Canadian National Seismic Network, and the Lamont Seismic Network. The National Science Foundation Geophysics Program provided support for this project.

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Correspondence to Catherine A. Rychert.

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Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

Supplementary information

Supplementary Figure S1

SV waveforms from station HRV are deconvolved and migrated in epicentral distance bins, illustrating one method we use to identify direct converted phases. (PDF 2746 kb)

Supplementary Figure S2

This demonstrates the trade-off in model parameters in our inversions. a) A range of models fits the phase from the base of the lithosphere at stations HRV and LMN. b) The range of acceptable models is limited by an independent constraint on the dominant period of the incident P-wave, which is determined by an auto-deconvolution test. (PDF 35 kb)

Supplementary Figure Legends

Full text to accompany the Supplementary Figures (DOC 21 kb)

Supplementary Methods

This describes the data included in our results, methods for identifying seismic discontinuities, ambiguous existence of the 61 km discontinuity, assumptions in our modelling, steps and methods of the inversions, auxiliary testing that ensures our assumptions do not affect the final results, error and parameter trade-offs, and the effects of temperature, hydration, grain size, and melt on seismic velocity. (DOC 81 kb)

Supplementary Table S1

This table summarises the parameters that were held fixed and those for which we inverted in each step of the inversions for HRV and LMN. (DOC 28 kb)

Supplementary Table S2

This table summarises the results of our inversions for velocities and thicknesses in the crust, lithosphere, and lithosphere-asthenosphere boundary at stations LMN and HRV. (DOC 32 kb)

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Rychert, C., Fischer, K. & Rondenay, S. A sharp lithosphere–asthenosphere boundary imaged beneath eastern North America. Nature 436, 542–545 (2005) doi:10.1038/nature03904

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