Dynamic recrystallization in semi-brittle faults

Abstract

Experiments designed to investigate the microstructure and mechanical behavior of shear zones across the cataclastic faulting to dislocation creep transition indicate that some mylonites may be the products of frictional faulting under semi-brittle conditions. The experiments involve simple shear of thin layers of polycrystalline halite to strains (γ) of 5–8, at temperatures to 250°C, normal stresses to 90 MPa and at shear strain rates of 7 × 10 −3 s −1. Microstructural and mechanical data define a transitional field of semi-brittle flow. Steady-state stress in the semi-brittle field is a function of the normal stress and the temperature, and is significantly less than that predicted by the power-law equation used to describe dislocation creep, and by the friction law used to describe cataclastic faulting. Fracture and localized slip are characteristic processes in the fields of cataclastic faulting and semi-brittle flow, whereas intracrystalline plasticity, recovery and dynamic recrystallization are characteristic of semi-brittle flow and dislocation creep. For both semi-brittle flow and dislocation, creep migration recrystallization is activated only after a critical strain and temperature—stress condition are achieved. The recrystallized grain size is consistent with previously established piezometric relations for dislocation creep. These experiments indicate that the field of semi-brittle flow for halite is characterized by frictional behavior even though the corresponding microstructure primarily reflects microscopic plastic deformation and softening processes.