A Dislocation and a Grain Boundary Walk Into a Bar

Abstract:

Deformation at tectonic plate boundaries is governed by the mechanical properties of crustal and lithospheric rock, which evolve through changes in the microstructure of its constituent minerals. We present a theoretical model coupling the evolution of grain boundaries and dislocations to rock deformation and tectonic activity. In particular, changes in dislocation density are governed by the generation of dislocations at grain boundaries (for example, to ensure strain compatibility) and by the Frank-Read sources, and by their loss through absorption into the grain boundaries and by dipole annihilation. The kinetics of these sources and sinks depends on the dislocation glide and climb velocities, each of which depends differently on stress, temperature, and dislocation density itself. Our model predicts that the competition between microphysical processes, such as the motion of dislocations in the crystal lattice, and the growth of mineral grains, causes transitions in the deformational response of rocks to changes in stress. For example, at moderate stress, the accumulation of dislocations impedes grain-growth and the strengthening of rocks, and thus helps preserve plate boundary weakness. At high stress, oscillations in grain evolution and rock strength can ensue, with periods commensurate with the cycle of seismic stress release and recovery, therefore leading to nonmonotonic post-seismic creep. This model thus uses the microphysics of mineral grains to connect the evolution of plate boundaries over geological timescales to processes at the human timescale, such as earthquake cycles.