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Brittle‐ductile coupling: Role of ductile viscosity on brittle fracturing

Localized or distributed deformations in continental lithosphere are supposed to be triggered by rheological contrasts, and particularly by brittle‐ductile coupling. A plane‐strain 2D finite‐element model is used to investigate the mechanical role of a ductile layer in defining the transition from localized to distributed fracturing in a brittle layer. The coupling is performed through the shortening of a Von Mises elasto‐visco‐plastic layer rimed by two ductile layers. By increasing the viscosity of the ductile layers by only one order of magnitude, the fracturing mode in the brittle layer evolves from localized (few faults) to distributed (numerous faults), defining a viscosity‐dependent fracturing mode. This brittle‐ductile coupling can be explained by the viscous resistance of the ductile layer to fault motion, which limits the maximum displacement rate along any fault connected to the ductile interface. An increase of the viscosity will thus make necessary new faults nucleation to accommodate the boundary shortening rate.