Abstract
Intermediate-depth (50–300 km) earthquakes commonly occur along convergent plate margins but their causes remain unclear. In the absence of pore-fluid pressures that are sufficiently high to counter the confining pressure in such settings, brittle failure is unlikely. In such conditions, the rocks could fail by the mechanism of progressively self-localizing thermal runaway1, whereby ductile deformation in shear zones leads to heating, thermal softening and weakening of rock1,2,3. Here we test this hypothesis by focusing on fault veins of glassy rock (pseudotachylyte) formed by fast melting during a seismic event, as well as associated ductile shear zones that occur in a Precambrian terrane in Norway. Our field observations suggest that the pseudotachylytes as well as shear zones have a single-event deformation history, and we also document mineralogical evidence for interaction of the rocks with external fluids. Using fully coupled thermal and viscoelastic models, we demonstrate that the simultaneous occurrence of brittle and ductile deformation patterns observed in the field can be explained by self-localizing thermal runaway at differential stresses lower than those required for brittle failure. Our results suggest that by perturbing rock properties, weakening by hydration also plays a key role in shear zone formation and seismic failure; however, thermal runaway enables the rocks to fail in the absence of a free fluid phase.
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Acknowledgements
We acknowledge discussions at PGP, particularly with S. Braeck, and proof reading by P. Meakin., G. Hirth and C. Marone provided constructive and helpful reviews. This study was supported by a Center of Excellence grant to PGP from the Norwegian Research Council.
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All authors participated in collecting the data, interpretation of results and developing the model. T.J., T.B.A. and H.A. focused on the fieldwork, sampling and petrology, and T.J., S.M., L.H.R. and Y.Y.P. focused on the numerical simulations.
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John, T., Medvedev, S., Rüpke, L. et al. Generation of intermediate-depth earthquakes by self-localizing thermal runaway. Nature Geosci 2, 137–140 (2009). https://doi.org/10.1038/ngeo419
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DOI: https://doi.org/10.1038/ngeo419
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