Foundering of lower island-arc crust as an explanation for the origin of the continental Moho

Abstract

A long-standing theory for the genesis of continental crust is that it is formed in subduction zones1. However, the observed seismic properties of lower crust and upper mantle in oceanic island arcs2,3 differ significantly from those in the continental crust4. Accordingly, significant modifications of lower arc crust must occur, if continental crust is indeed formed from island arcs. Here we investigate how the seismic characteristics of arc crust are transformed into those of the continental crust by calculating the density and seismic structure of two exposed sections of island arc (Kohistan and Talkeetna). The Kohistan crustal section is negatively buoyant with respect to the underlying depleted upper mantle at depths exceeding 40 kilometres and is characterized by a steady increase in seismic velocity similar to that observed in active arcs. In contrast, the lower Talkeetna crust is density sorted, preserving only relicts (about ten to a hundred metres thick) of rock with density exceeding that of the underlying mantle. Specifically, the foundering of the lower Talkeetna crust resulted in the replacement of dense mafic and ultramafic cumulates by residual upper mantle, producing a sharp seismic discontinuity at depths of around 38 to 42 kilometres, characteristic of the continental Mohorovičić discontinuity (the Moho). Dynamic calculations indicate that foundering is an episodic process that occurs in most arcs with a periodicity of half a million to five million years. Moreover, because foundering will continue after arc magmatism ceases, this process ultimately results in the formation of the continental Moho.

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Figure 1: Seismic and petrological constraints on the thermal regime in arcs.
Figure 2: Detailed VP and density depth-structure of the exposed Kohistan arc section compared to the average continental crust and the Izu-Bonin arc crust.
Figure 3: Schematic illustrations of the lithological, seismic and density properties of the Kohistan and Talkeetna arc sections.
Figure 4: Modelled thickness of the density-unstable layer at the base of arc crust.

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Acknowledgements

The work was supported by NSF grant numbers EAR 0910644 (to O.J.) and EAR 1316333 (to M.D.B.). We thank N. Arndt for comments that helped to improve the manuscript. J. Connolly’s help in recalibrating the elastic property calculation of Perple_X is appreciated, as are discussions with P. Kelemen and B. Hacker.

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O.J. designed the project. Both authors conducted the calculations, contributed to the interpretation of the results and wrote the manuscript.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Seismic velocity and density along different geotherms for the Kohistan arc.

Plotted are the mean and range in VP and density as calculated along the 40, 60 and 80 mW m−2 geotherms.

Extended Data Figure 2 Seismic velocities of the lower arc and continental crust.

Histogram showing distribution of average seismic velocities directly above and below the Moho in continents (red, after ref. 4) and from active arcs (refs 6, 8, 32, 34, 35, 36, 37, 38, 39, 40). Also shown are the range of VP for density-stable and density-unstable rocks from the Kohistan and Talkeetna arcs, as dashed fields calculated from this study. In the arcs, sub-Moho rocks have on average a VP that is 0.5 km s−1 slower than do sub-Moho rocks in continents. The observed low velocities in the arcs agree with the velocities calculated for density-unstable crustal rocks from Kohistan.

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Jagoutz, O., Behn, M. Foundering of lower island-arc crust as an explanation for the origin of the continental Moho. Nature 504, 131–134 (2013). https://doi.org/10.1038/nature12758

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