Numerical modelling changes our understanding of brittle/ductile deformation of eclogites

Submitted by Isabelle Dubigeon on Tue, 05/21/2019 - 13:44
AN ARTICLE IN GEOCHEMISTRY, GEOPHYSICS, GEOSYSTEMS
Thibault Duretz & Philippe Yamato
Eclogite is a dense rock, composed by green omphacite and red garnet (usually 5-10 mm, Figure 1). This particular assemblage is possible because this rock, derived from a basic magmatic rock, was transformed at a very high pressure. Its presence in the field is therefore very often used as evidence of burial in a paleo-subduction zone. However, the way in which rocks deform at depth in convergence zones, as well as the magnitude of the stresses they can sustain during tectonic processes, remain enigmatic. Yet, it is what governs the behaviour of the lithosphere and the associated tectonic processes at all time scales, from short-term (such as seismicity) to long-term (such as the formation of mountain ranges).
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.

Reference
Yamato, P., Duretz, T., & Angiboust, S. (2019). Brittle/ductile deformation of eclogites: insights from numerical models. Geochemistry, Geophysics, Geosystems, 20. https://doi.org/10.1029/2019GC008249

Contact OSUR
Philippe Yamato (Géosciences Rennes) / philippe [dot] yamatoatuniv-rennes1 [dot] fr (@)
Thibault Duretz (Géosciences Rennes) / thibault [dot] duretzatuniv-rennes1 [dot] fr (@)
Alain-Hervé Le Gall (OSUR multiCOM) / ahlegallatuniv-rennes1 [dot] fr (@)

Isabelle Dubigeon
Mon, 04/20/2020 - 13:59