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Resistance to mantle flow inferred from the electromagnetic strike of the Australian upper mantle

Nature volume 412, pages 632–635 (2001)Cite this article

Abstract

Seismic anisotropy is thought to result from the strain-induced lattice-preferred orientation of mantle minerals, especially olivine1,2, owing to shear waves propagating faster along the a-axis of olivine crystals than along the other axes. This anisotropy results in birefringence, or ‘shear-wave splitting’3, which has been investigated in numerous studies1,4. Although olivine is also anisotropic with respect to electrical conductivity5 (with the a-axis being most conductive), few studies of the electrical anisotropy of the upper mantle have been undertaken, and these have been limited to relatively shallow depths in the lithospheric upper mantle6,7. Theoretical models of mantle flow have been used to infer that, for progressive simple shear imparted by the motion of an overriding tectonic plate, the a-axes of olivine crystals should align themselves parallel to the direction of plate motion8,9. Here, however, we show that a significant discrepancy exists between the electromagnetic strike of the mantle below Australia and the direction of present-day absolute plate motion10. We infer from this discrepancy that the a-axes of olivine crystals are not aligned with the direction of the present-day plate motion of Australia, indicating resistance to deformation of the mantle by plate motion.

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Figure 1: Apparent resistivity and impedance phases at station ASP before (x,y) and after rotation (x′,y′) and correction for static shifts.
Figure 2: Map showing locations of magnetotelluric stations.
Figure 3: Phase-sensitive electromagnetic strike for two stations, plotted as a function of period.
Figure 4: Impedance phases in the period range 103–104 s for three sites after rotation to the phase-sensitive strike.

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Acknowledgements

This research was supported by the Indian Institute of Geomagnetism, Bombay and the Geophysics Institute, University of Göttingen, via an initiative of Baldev Arora. I thank K. Bahr, A. Gatzemeier and P. Hahesy for help with fieldwork, G. Heinson for hospitality during my sojourn at the University of Adelaide and J. Booker for comments on an earlier version of this paper.

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  1. Geophysics Institute, University of Göttingen, Germany

    Fiona Simpson

  2. Institute of Geological & Nuclear Sciences, 69 Gracefield Road, PO Box 30-368, Lower Hutt, New Zealand

    Fiona Simpson

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Correspondence to Fiona Simpson.

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Simpson, F. Resistance to mantle flow inferred from the electromagnetic strike of the Australian upper mantle. Nature 412, 632–635 (2001). https://doi.org/10.1038/35088051

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