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

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.

Evidence for brittle behaviour in eclogites  - (a) Eclogitized mylonitic mafic rock from the Lago Superiore unit (Monviso ophiolite, Western Alps) showing a strong foliation, underlined by dark rutile beds and parallel omphacite-rich layers. Estimated modal proportion of garnet is between 30% and 40%.
(b) Garnet crystal (scanning electron microscope) from a mylonitized eclogite from the Lago Superiore unit showing darker and healed fracture networks crosscutting brighter cores formed during burial of the oceanic lithosphere down to eclogite-facies conditions.
c) Electronic microprobe image showing Magnesium composition.
 - © Philippe Yamato
Results - On the left, diagram showing the effective strength of an eclogite as a function of the strain rate and the proportion of garnet. On the right are presented the 3 different deformation modes (results extracted at different strain rates for 30% of garnets)

 - © Philippe Yamato

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) / @
Thibault Duretz (Géosciences Rennes) / @
Alain-Hervé Le Gall (OSUR multiCOM) / @