On the relationship between the formation of shear zones and the form of the flow law for rocks undergoing dynamic recrystallization

The formation and maintenance of shear zones in rocks at high temperatures, when viscous flow takes place at constant volume, requires strain weakening. Several processes can lead to a strain weakening characteristic in the flow of such rocks to high strains, but conventional experimental rock mechanics techniques cannot normally provide high-quality mechanical data to high strains. Recent experimental data for calcite marble have demonstrated strain weakening accompanied by dynamic recrystallization. A power law with strain exponent r can be used to describe the dependence of the flow stress on strain, together with the conventional description of the dependence of strain rate on stress described by a stress exponent n. The stable localization of strain into shear zones is favoured by more negative values of r (e.g. r = -0.8) and more positive values of n (e.g. n = 5 to 7). A further important factor is the critical strain, ε0, for the onset of recrystallization (and hence strain weakening). Shear zone formation is favoured by smaller values for ε0. Shear zone formation weakens the crustal section in which they occur, but the amount of weakening may not be great, perhaps no more than 50% of the previous 'steady-state' flow stress. Certain rock types, such as mafic igneous rocks, may through their mechanical properties be more susceptible to shear zone formation than others, such as mica schists. It is emphasized that the analysis presented is based upon a strain-dependent flow law that describes single-phase dynamic recrystallization by grain boundary migration that leads to enhanced recovery, without any change in deformation mechanism. No consideration is given to possibly important effects arising from the activation of grain-size-sensitive flow, or to potential weakening effects arising from syntectonic metamorphic reactions.