EAPS

Decline and Fall of Steady State: Playing to Your Strength and Avoiding Creep

Strain localization is common within crustal mountain belts, and shear displacements of kilometers can be accommodated within zones less than ten meters wide. Thus, accurate descriptions of the mechanics of orogenic events need to account for the strength, geometric spacing, and strain accommodation of these features. The formation of shear zones is accompanied by major changes in microstructure of the rocks: grain size, lattice preferred orientation, major and accessory phase chemistry, pore geometry, phase dispersion, dislocation density, and twin geometry, all show gradients from the undeformed wall rocks into the center of the shear zone. These changes in microstructure suggest 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 field and laboratory observations, geomechanical descriptions of creep often rely upon steady-state flow laws derived from simple models of defect generation and motion. Contained within these models are implicit assumptions, either 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 microstructural variables are coupled, and that 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.

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