Towards a more robust description of transient creep

Abstract:

Strain localization is common within crustal orogenic belts, and shear displacements of kilometers can be accommodated within zones less than ten meters wide. Strain localization is accompanied by major changes in grain size, lattice preferred orientation, major and accessory phase chemistry, pore geometry, phase dispersion, dislocation density, and twin geometry, suggesting that transients in strength have also occurred. High-strain experiments where creep dominates often show hardening up to strains of 1.0, followed by strength drops of 30-50%. In contrast with such observations, creep is often described by steady-state flow laws relying on simple descriptions of defect generation and motion. Most often, it is assumed that the kinetics of a single mechanism control deformation rate, or that the relative partitioning of strain amongst several mechanisms remains constant. But, when two or more mechanisms operate concurrently, an accurate flow law must account for kinetic interactions and changes in strain partitioning caused by the evolution of structure or changes in thermodynamic conditions. Data now at hand, strongly suggest that the evolution of structure variables including dislocation patterning, twin-boundary geometry, grain size, and LPO are coupled. The relative strain partitioning between mechanisms and the accumulation of damage leading to localization or failure is probably affected by changes in temperature, strain rate, stress, and chemical fugacity. Thus, better descriptions of strength transients will require improved theoretical and experimental constraints on the kinetics of the individual mechanisms. Importantly, whether load drops, instabilities, or seismicity are produced also depends on many additional parameters, including changes in loading conditions, the state of pore fluids, geometry of deformation, and temperature.