ELSEVIER Tectonophysics 303 (1999) 147–158 On the relationship between the formation of shear zones and the form of the flow law for rocks undergoing dynamic recrystallization E.H. Rutter * Rock Deformation Laboratory, Department of Earth Sciences, University of Manchester, Manchester M13 9PL,UK Accepted 23 August 1998 Abstract 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 D0:8) and more positive values of n (e.g. n D 5 to 7). A further important factor is the critical strain, " 0 , for the onset of recrystallization (and hence strain weakening). Shear zone formation is favoured by smaller values for " 0 . 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.  1999 Elsevier Science B.V. All rights reserved. Keywords: experimental rock deformation; shear zones; strain-dependent rheology; dynamic recrystallization; constitutive flow law 1. Introduction Localization of deformation is a phenomenon ob- served for a wide variety of engineering materials and considerable attention has been given to under- standing the conditions that give rise to localized strain (e.g. Drucker, 1959; Hahn, 1962; Hart, 1967; Jonas et al., 1976). Shear localization is a phe- Ł Fax: C44-161-275-3947; E-mail: e.rutter@man.ac.uk nomenon ideally to be avoided in the deformation processing of metals. The deformation of the middle and lower part of the Earth’s crust and upper mantle frequently in- volves the formation of localized zones of plastic strain. From the evidence of plate tectonics, in which large tracts of relatively undeformed rocks are sep- arated by narrow zones of intense deformation, and evidence from within mountain belts, it is probable that the most important displacements of tectonics 0040-1951/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII:S0040-1951(98)00261-3