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Mechanisms and geologic significance of the mid-lithosphere discontinuity in the continents

Nature Geoscience volume 8, pages 509–514 (2015)Cite this article

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Abstract

The stable continents have a puzzling structure. Recent seismological studies have revealed a marked drop in seismic velocity at middle-lithosphere depths, but a generally small velocity drop at the lithosphere-asthenosphere boundary. The mid-lithosphere discontinuity has previously been attributed to changes in composition and/or crystal alignment (anisotropy) caused by metasomatic alteration, as well as to partial melting and/or accretion of intruded materials that occurred after the formation of the continents. We show that these models cannot easily explain the global presence of a large seismic velocity drop in the middle lithosphere and a small velocity change at the lithosphere-asthenosphere boundary. These models are also difficult to reconcile with long-term continental stability and, in particular, observations of nearly depth-invariant ages of rocks in the continental lithosphere that do not support the notion of late alteration events. Instead, we propose an elastically accommodated grain-boundary sliding model that predicts a substantial velocity drop at the mid-lithosphere discontinuity and a weak seismic signal at the lithosphere-asthenosphere boundary, as observed, without invoking late-stage modifications to the lithosphere. In this model, the mid-lithosphere discontinuity is a general feature of the stable continents, the precise depth of which depends primarily on temperature and water content. Consequently, the depth of the mid-lithosphere discontinuity may provide clues to the evolution of continents.

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Figure 1: Seismological observations of the MLD.
Figure 2: Schematic illustration of plausible evolution of continents that might cause the MLD signature.
Figure 3: Layered anisotropic structures and the corresponding seismic wave velocity-depth profiles.
Figure 4: Characteristics of the MLD based on the EAGBS model.

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Acknowledgements

This study is partly supported by the grants to S.K. and J.P. from the National Science Foundation. We thank I. Artemieva for providing us with the data on the temperature and age distribution in the continents, and R. Kind and F. Sodoudi for providing the data on the locations and the depths of lithospheric discontinuities in the South African Craton. Discussions with R. Dasgupta, H. Ford, I. Jackson, M. Long, S. Morris, K. Selway and S. Solomatov were helpful in preparing this paper. We thank U. Faul who provided critical comments on the earlier version of this paper.

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Authors and Affiliations

  1. Department of Geology & Geophysics, Yale University, New Haven, 06520, Connecticut, USA

    Shun-ichiro Karato, Tolulope Olugboji & Jeffrey Park

  2. Department of Geology, University of Maryland, College Park, 20742, Maryland, USA

    Tolulope Olugboji

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  1. Shun-ichiro Karato
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Contributions

S.K. planned and coordinated the whole project and wrote a majority of the paper. T.O. compiled the data on seismological observations and geotherms and prepared Fig. 1 (and Supplementary Information I), and evaluated the visibility of EAGBS in the receiver function observations. J.P. evaluated the seismological observations and assessed the visibility of velocity reduction caused by EAGBS in the receiver function observations (Supplementary Information III). All authors participated in the discussion.

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Correspondence to Shun-ichiro Karato.

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Karato, Si., Olugboji, T. & Park, J. Mechanisms and geologic significance of the mid-lithosphere discontinuity in the continents. Nature Geosci 8, 509–514 (2015). https://doi.org/10.1038/ngeo2462

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