WEAKENING, STRAIN LOCALIZATION, AND THE DEEP STRUCTURE OF LITHOSPHERIC SHEAR ZONES (Invited Presentation)

Strain localization requires weakening – which can be expressed as an increase in strain-rate at a given imposed deviatoric stress. Most weakening mechanisms require significant permanent strain before weakening can set in, so that the ambient deviatoric stress has to exceed the yield strength of the undeformed rock at any given depth. At the plate scale, this is likely to result in a self-organizing system of shear zones with a cumulative width w that can accommodate the imposed plate-velocity boundary conditions at the yield stress.

Dynamic recrystallization during dislocation creep can result in a switch to one of several grain-size sensitive rheologies. Because the grain-size is itself an inverse function of stress, these rheologies can have high effective stress exponents, which favors narrow shear zones in the strongest parts of the lithosphere. We examine the effect of different degrees of grain-size and stress sensitivity in shear zone rheology on w.

In polyphase materials, recrystallization of the stronger phase may lead to grain-boundary sliding accommodated by diffusion creep of the weaker phase, resulting in a fine-grained dispersed mixture with a composite rheology controlled by the volume concentration of the phases. We examine the implications of this process for shear zone rheology, and compare w for single-phase and polyphase systems.

Weakening resulting from microstructural changes has no inherent length scale, so it may produce a variety of shear zones with varying widths. Dissipative heating, however, is length-scale dependent, because of the effects of conductive heat loss, and it is relatively ineffective at initiating localization. We compare the implications of dissipative heating for the evolution of shear zones initiated by other mechanisms.