Mechanisms of Strain Localization: Implications for Lithospheric Strength and the Structure of Plate-Boundary Shear Zones

Abstract:

Deformation mechanism switches caused by grain-size reduction are a likely principal cause of strain localization, assisted by development of crystallographic preferred orientation, interconnected weak-phase layering, phase changes, and on longer time-scales, shear heating. Grain-size reduction results in a marked increase in the rate of grain-size sensitive creep mechanisms, which include grain-boundary diffusion creep, pressure solution, diffusion- or dislocation-accommodated grain-boundary sliding, and dislocation creep in which grain-boundary migration is the main recovery mechanism (DRX creep). In polyphase systems, grain-boundary sliding plus grain-boundary diffusion may allow the development of a well-mixed fine-grained aggregate, in which the grain size is controlled by the phase showing the greatest degree of grain-size reduction by dynamic recrystallization. The resulting fine-grained mixture resists grain growth, and may be up to three orders of magnitude weaker than the undeformed rock.

Microstructural weakening requires a minimum level of stress to produce the required deformation. This buffers the stress, even when there is a velocity boundary condition to the system, along a plate boundary, for example. Shear zones therefore evolve at constant stress, and the cumulative width w of a plate boundary shear zone is a function of the strength of the undeformed rock, the imposed velocity difference, and the rheology of the shear zone material. This allows a first order prediction of w as a function of depth in the lithosphere.