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The electric Moho

Nature volume 409, pages 331–333 (2001)Cite this article

Abstract

Since Mohorovičić1 discovered a dramatic increase in compressional seismic velocity at a depth of 54 km beneath the Kulpa Valley in Croatia, the ‘Moho’ has become arguably the most important seismological horizon in Earth owing to its role in defining the crust–mantle boundary. It is now known to be a ubiquitous feature of the Earth, being found beneath both the continents and the oceans, and is commonly assumed to separate lower-crustal mafic rocks from upper-mantle ultramafic rocks. Electromagnetic experiments conducted to date, however, have failed to detect a corresponding change in electrical conductivity at the base of the crust, although one might be expected on the basis of laboratory measurements2. Here we report electromagnetic data from the Slave craton, northern Canada, which show a step-change in conductivity at Moho depths. Such resolution is possible because the Slave craton is highly anomalous, exhibiting a total crustal conductance of less than 1 Siemens—more than an order of magnitude smaller than other Archaean cratons. We also found that the conductivity of the uppermost continental mantle directly beneath the Moho is two orders of magnitude more conducting than laboratory studies on olivine would suggest, inferring that there must be a connected conducting phase.

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Figure 1: Tectonic map of the Slave craton in the northwestern part of the Canadian Shield.
Figure 2: Contoured magnetotelluric phase responses for the seven sites from Yellowknife to the surface expression of the western boundary of the Slave craton.
Figure 3: Magnetotelluric response from a central site on the profile, site 106 (Fig. 2).
Figure 4: One-dimensional conductivity–depth models that fit the averaged impedances21 of the responses shown in Fig. 3.

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Acknowledgements

This work was performed under the auspices of Lithoprobe, Canada's national geoscience programme, funded by the Natural Sciences and Engineering Research Council of Canada and the Geological Survey of Canada. We thank the staff of Phoenix Geophysics Ltd for their attention to high-quality data acquisition.

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Authors and Affiliations

  1. Geological Survey of Canada, Ottawa, K1A 0E9, Ontario, Canada

    Alan G. Jones

  2. Department of Geological Sciences University of Manitoba, Winnipeg, R3T 2N2, Manitoba, Canada

    Ian J. Ferguson

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  1. Alan G. Jones
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  2. Ian J. Ferguson
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Correspondence to Alan G. Jones.

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Jones, A., Ferguson, I. The electric Moho. Nature 409, 331–333 (2001). https://doi.org/10.1038/35053053

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