Fossil intermediate-depth earthquakes in subducting slabs linked to differential stress release

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The cause of intermediate-depth (50–300 km) seismicity in subduction zones is uncertain. It is typically attributed either to rock embrittlement associated with fluid pressurization, or to thermal runaway instabilities. Here we document glassy pseudotachylyte fault rocks—the products of frictional melting during coseismic faulting—in the Lanzo Massif ophiolite in the Italian Western Alps. These pseudotachylytes formed at subduction-zone depths of 60–70 km in poorly hydrated to dry oceanic gabbro and mantle peridotite. This rock suite is a fossil analogue to an oceanic lithospheric mantle that undergoes present-day subduction. The pseudotachylytes locally preserve high-pressure minerals that indicate an intermediate-depth seismic environment. These pseudotachylytes are important because they are hosted in a near-anhydrous lithosphere free of coeval ductile deformation, which excludes an origin by dehydration embrittlement or thermal runaway processes. Instead, our observations indicate that seismicity in cold subducting slabs can be explained by the release of differential stresses accumulated in strong dry metastable rocks.

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We greatly benefitted from discussions with P. Agard, T. Ferrand, A. Schubnel, O. Onken, E. Cannaò and S. Poli and from constructive comments by T. B. Andersen. We thank M. Kendrick for revising the pre-submission manuscript, and A. Risplendente and L. Negretti for technical assistance during the SEM and wavelength-dispersive spectrometry microprobe work. M.S. and M.G. acknowledge funding by the People Programme (Marie Curie Actions, European Union’s Seventh Framework Programme FP7/2007—2013) to the Initial Training Network ZIP (Zooming In-between Plates, REA grant agreement no. 604713). Discussions within ZIP stimulated this work. M.S. also acknowledges support from the Italian MIUR and the University of Genova. G.P. acknowledges funding of the University of Padova.

Author information


  1. Dipartimento di Scienze della Terra, Ambiente e Vita, University of Genova, Genoa, Italy

    • Marco Scambelluri
    •  & Mattia Gilio
  2. Dipartimento di Geoscienze, University of Padova, Padua, Italy

    • Giorgio Pennacchioni
    •  & Fabrizio Nestola
  3. Dipartimento di Scienze della Terra e dell’Ambiente, University of Pavia, Pavia, Italy

    • Mattia Gilio
  4. GeoZentrum Nordbayern, Friedrich-Alexander University of Erlangen-Nürnberg, Erlangen, Germany

    • Michel Bestmann
  5. Department of Earth Sciences, Utrecht University, Utrecht, The Netherlands

    • Oliver Plümper


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M.S., G.P. and O.P. wrote the paper. M.S., G.P. and M.G. carried out the fieldwork, and the petrographic and microstructural study. M.B. did the field-emission SEM and EBSD work. O.P. did the TEM work. F.N. did the XRD and Raman analysis. M.S. and G.P. developed the concept.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Marco Scambelluri.

Supplementary Information

  1. Supplementary Information

    This file provides extended discussion and figures of: (1) the regional geological setting of the rocks studied (Supplementary Figure 1); (2) the microstructures of pseudotachylyte and cataclasite in gabbro (Supplementary Section 1); and (3) the microstructures of pseudotachylyte in eclogitic metagabbro and meta-peridotite (Supplementary Section 2)

  2. Supplementary dataset

    Experimental data