A Theory of Inter‐Granular Transient Dislocation Creep: Implications for the Geophysical Studies on Mantle Rheology

Abstract

AbstractTransient creep may play an important role in short‐term (small strain) time‐dependent deformation such as the post‐glacial rebound and the post‐seismic deformation. Among various mechanisms of transient creep, I focus on inter‐granular transient dislocation creep caused by plastic anisotropy in each grain. The essence of this process is the evolution of strain accommodation across grain‐boundaries from elastic to plastic (viscous) accommodation. Extending the previous studies, I show that evolution of strain accommodation leads to two asymptotic behaviors: at small strain, strain accommodation across grain‐boundaries is by elastic deformation and the strength is controlled largely by that of the soft slip system. In contrast, when strain exceeds a threshold value, strain accommodation occurs mostly by plastic deformation and the strength is largely controlled by that of the hard slip system. The model predicts that the threshold strain is on the order of elastic strain (depending also on the degree of plastic anisotropy) and the strength contrast between transient and steady‐state creep depends on the degree of plastic anisotropy of the crystal. Consequently, transient creep plays an important role in these short‐term time‐dependent processes. Since the soft and the hard slip systems have different properties, applications of rheological properties inferred from short‐term time‐dependent deformation to long‐term geodynamic processes needs to be made with a great care.