A dynamic model for aseismic ductile shear zones

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Scientific paper

A model for the dynamics of aseismic strike-slip shear zones is proposed under conditions of constant rate out-side the zone and with an almost perfectly rigid plastic rheology for the rock. It is shown that a necessary condition for the activation of the shear zone is the existence of an initial strength defect. Conditions in which shear heating can be effective are discussed. In the isothermal approximation the initial transient quickly disappears and gives way to a regime with enhanced shear rate. Adiabatic shear heating and/or structural strain softening lead to sharper shear zones by delaying the disappearance of the transient, but the shear zone will nevertheless operate eventually at constant shear rate. Migmatization in aseismic shear zones is unlikely to be due to shear heating but may be caused together with the activation of the shear zone by a local temperature increase due to an external source of heat. On propose un modèle dynamique pour l'activation de zones de cisaillement asismiques (failles ductiles) en supposant une rhéologie parfaitement rigide plastique et une vitesse de cisaillement constante imposée en dehors de la zone. On montre qu'une condition nécessaire pour l'activation de la zone est l'existence d'un affaiblissement local du matériau. On discute les conditions dans lesquelles l'échauffement plastique peut être efficace. Dans le cas isotherme, le transitoire initial disparait rapidement et est remplacé par un régime à vitesse de cisaillement constante mais plus forte dans la zone. L'échauffement plastique adiabatique et/ou un adoucissement structural donnent des zones plus tranchées en retardant la disparition du transitoire, mais la zone de cisaillement finira quand même par opérer à vitesse de cisaillement constante. Il est peu probable que l'échauffement plastique puisse causer une migmatisation dans les failles ductiles. Par contre une source de chaleur externe peut à la fois activer la faille ductile et causer une migmatisation.

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