This study, carried out by Philippe Yamato and Thibault Duretz (Geosciences Rennes) and Samuel Angiboust (IPG Paris), shows that under conditions of 2.0 GPa and 550°C (typical of what can be found in subduction zones), metamorphic rocks (eclogites) can deform in different ways.
Their results (Figure 2), obtained using numerical models, indicate that if these rocks flow at strain rates lower than ~10-10 s-1, they are brittle at higher strain rates (i.e. at strain rate ~10-10 times lower than a seismic rate). This suggests that observation of brittle features in eclogites (i.e. faults) does not necessarily means that they underwent extreme strain rate. Care should therefore be taken when linking failure of eclogitic rocks to seismic deformation.
The authors also show that the effective stresses sustained by rocks can be high at these P-T conditions, and that it is possible to constrain the magnitudes of these stresses by analysing how they deformed. The effective stress can be as high as ~1 GPa in the case of entirely fractured eclogite and as high as ~500 MPa for rocks deformed in a ductile way but with garnet fracturing. It means that eclogites can be much more resistant than previously thought.
, , & (2019). Brittle/ductile deformation of eclogites: insights from numerical models. Geochemistry, Geophysics, Geosystems, 20. https://doi.org/10.1029/2019GC008249