Grain size evolution and the rheology of ductile shear zones: from laboratory experiments to postseismic creep

Geologic, experimental, and geodetic studies indicate that ductile shear zones do not deform in steady state. We present a theoretical model for an elastically loaded ductile shear zone deforming by combined dislocation and diffusion creep, in which grain size is allowed to evolve toward its equilibrium (recrystallized) value. Several grain size evolution laws are considered. Simulations of laboratory experiments show that grain size evolution can lead to changes in the dominant deformation mechanism that are accompanied by transient strain-weakening or strain-strengthening episodes reminiscent of laboratory observations. Ductile shear zones can also produce postseismic creep as observed in geodetic data following large earthquakes. Non-steady-state ductile creep is difficult to differentiate from frictional sliding with velocity-strengthening friction. Hence, ductile creep may play a role in the transition from velocity-weakening to velocity-strengthening friction with increasing temperature.