GEOMAR - Helmholtz-Zentrum für Ozeanforschung Kiel

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 runaway, whereby ductile deformation in shear zones leads to heating, thermal softening and weakening of rock. 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.