Linking metamorphic transformations and the brittle–ductile transition: Insights from numerical modeling of the granulite-to-eclogite transformation

Geophysical observations combined with detailed petro-structural analyses conducted in the field and in the laboratory indicate that « brittle » deformation occurs within subduction zones in rocks that are otherwise expected to deform in a « ductile » manner under the associated pressure–temperature conditions. These brittle events are most commonly localized in regions where metamorphic transformations are predicted to occur. Because such reactions may induce substantial changes in density and strength, they are frequently invoked as a primary mechanism driving the ductile-to-brittle switch in subducting rocks. However, the physical processes that link metamorphic transformations to changes in deformation style remain incompletely understood.

This contribution addresses this issue through the emblematic example of the granulite-to-eclogite transformation exposed at Holsnøy (Bergen Arcs, Norway). We combine field-based structural and petrological observations with numerical modeling developed over the past several years to investigate the mechanical and rheological consequences of this transformation.

We specifically examine whether eclogitization necessarily initiates along pre-existing brittle precursors or whether the reaction itself can trigger faulting, how the transformation propagates through the rock, and the extent to which the inherited granulitic foliation influences reaction localization. We further discuss the mechanisms leading to the formation of eclogitic shear zones as opposed to static eclogites (commonly referred to as eclogite fingers). Finally, we assess the relative roles of fluid availability and far field stress in controlling the spatial distribution and mechanical impact of the reaction.

By confronting field observations with numerical modeling, this presentation aims to show that the answers to these questions may not be unique, and that much remains to be done to fully understand the impact of metamorphic reactions on the rheological behavior of rocks.