Brittle‐Ductile Rheological Behavior in Subduction Zones: Effects of Strength Ratio Between Strong and Weak Phases in a Bi‐Phase System

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. Plain Language Summary In subduction zones, brittle‐ductile behavior is commonly proposed to explain deep transient slips at the subduction interface. This particular behavior is generally characterized by the fracturing of strong pods located in a weak fluid‐like material. In this study, we observe this mixed rheological behavior at the mineral‐scale in oceanic rocks under deep transient slip conditions. We carry out petrological and microstructural observations that show micro‐fracturing of strong magmatic clasts and ductile deformation of a weak metamorphic hydrated matrix. Numerical experiments, inspired by these observations, are used to constrain the physical conditions for this brittle‐ductile behavior. Numerical results show four types of behavior: (a) both matrix and clast are brittle and fractured; (b) the clast is brittle and fractured and the matrix is ductilely deformed; (c) only the clast is brittle and fractures are localized at clast edges; and (d) both matrix and clast are ductile. This study demonstrates that the behavior of this bi‐phase material is controlled by the strength ratio between the brittle strong clast and the ductile weak matrix. These physical conditions significantly differ from the theoretical rheological prediction and may be the key to a better understanding of the mechanics of deep transient slips. Key Points The modeled brittle‐ductile rheological behavior occurs in a more limi