Résumé
The brittle‐ductile rheological behavior in subduction zones is commonly proposed to explain deep transient slips. Generally observed at large scales in tectonic “mélanges”, here we show that it is also observed at the grain scale in exhumed blueschist metagabbros. In these rocks, petrologic and microstructural observations show a bi‐phase material constituted by strong microfractured magmatic pyroxene clasts located in a weak and ductile lawsonite‐rich metamorphic matrix. To constrain the mechanical conditions allowing the brittle deformation of a clast in a ductile matrix, we used two‐dimensional simple shear numerical experiments. Results show four behaviors: (a) entirely brittle; (b) brittle‐ductile with clast fracturing in a ductile matrix; (c) ductile‐dominant with limited plastic deformation at clast edges; and (d) entirely ductile. We propose that the conditions of the brittle‐ductile behavior, commonly associated with deep transient slips, are controlled by the strength ratio between the strong brittle phase and the weak ductile phase.