Journal of Structural Geology, Vol. 14, No. 8/9, pp. 1079to 1100, 1992 0191-8141/92 $05.00+0.00 Printed in Great Britain © 1992Pergamon Press Ltd Microstructural and crystal fabric evolution during shear zone formation G. E. LLOYD Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, U.K. R. D. LAW Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, U.S.A. D. MAINPRICE Laboratoire de Tectonophysique, USTL, Place Eugene Bataillon, 34095 Montpellier C6dex 05, France and J. WHEELER Department of Geological Sciences, University of Liverpool, Liverpool L69 3BX, U.K. (Received 6 September 1991; accepted in revisedform 28 April 1992) Abstract--The microstructures and crystal fabrics associated with the development of an amphibolite facies quartzo-feldspathic mylonitic shear zone (Torridon, NW Scotland) have been investigated using SEM electron channelling. Our results illustrate a variety of microstructures and fabrics which attest to a complex shear zone deformation history. Microstructural variation is particularly pronounced at low shear strains: significant intragranular deformation occurs via a domino-faulting style process, whilst mechanical incompatibilities between individual grains result in characteristic grain boundary deformation accommodation microstructures. A sudden reduction in grain size defines the transition to medium shear strains, but many of the boundaries inherited from the original and low shear strain regions can still be recognized and define distinctive bands oriented at low angles to the shear zone margin. Grains within these bands have somewhat steeper preferred dimensional orientations. These domains persist into the high shear strain mylonitic region, where they are oriented subparallel to the shear zone margin and consist of sub-20/~m grains. The microstructures suggest that the principal deformation mechanism was intracrystalline plasticity (with contributions from grain size reduction via dynamic recrystallization, grain boundary migration and grain boundary sliding). Crystal fabrics measured from the shear zone vary with position depending on the shear strain involved, and are consistent with the operation of several crystal slip systems (e.g. prism, basal, rhomb and acute rhomb planes) in a consistent direction (probably parallel to a and/or m). They also reveal the presence of Dauphine twinning and suggest that this may be a significant process in quartz deformation. A single crystal fabric evolution path linking the shear zone margin fabric with the mylonitic fabric was not observed. Rather, the mylonitic fabric reflects the instantaneous fabric which developed at a particular location for a particular shear strain and original parental grain orientation. The mature shear zone therefore consists of a series of deformed original grains stacked on top of each other in a manner which preserves original grain boundaries and intragranular features which develop during shear zone evolution. The stability of some microstructures to higher shear strains,the exploitation of others at lower shear strains, and a continuously evolving crystal fabric, mean that the strain gradient observed across many shear zones is unlikely to be equivalent to a time gradient. INTRODUCTION THE crystallographic fabrics of shear zones which display the classic geometrical relationships between finite strain features (e.g. tectonic foliation, lineation, etc.) and shear zone boundaries with increasing hetero- geneous shear strain due to bulk simple shear (Ramsay & Graham 1970, Ramsay 1980) should also be interpret- able in terms of simple shear deformation. Previous work (e.g. Etchecopar 1977, Bouchez 1978, Burg & Laurent 1978, Bouchez et al. 1979) suggests that the dominant quartz slip direction (a) becomes aligned with the direction of bulk simple shear. The a-axis point maximum therefore indicates the shear direction, towards which the lineation (X) within the foliation plane (XY) progressively rotates. It also occupies the pole to a straight single girdle c-axis distribution. At very high shear strains, the a-axis maximum lies within the XZ plane at a very low angle to X and the c-axis fabric is oriented almost perpendicular to X. Consequently, the crystal slip plane should be parallel to the macroscopic shear plane. Studies of some shear zones (e.g. Burg & Laurent 1978, Simpson 1981, Schmid & Casey 1986) support these observations, although the angle between the a- axis maximum and the lineation is often greater than expected whilst the c-axis girdle is often kinked. Other, perhaps most, shear zones (e.g. Law et al. 1984, 1986, Platt & Behrmann 1986, Schmid & Casey 1986, Law 1987) depart from these simple relationships, which 1079