Journalof StructuralGeology, Vol. 9, No. 5/6, pp. 635 to 646, 1987 0191-8141/87 $03.00+ 0.00 Printed in Great Britain © 1987Pergamon Journals Ltd. Bulk kinematics from shear zone patterns: some field examples DENIS GAPAIS, PASCAL BALE, PIERRE CHOUKROUNE, PETER R. COBBOLD, YAMINA MAHJOUB * and DIDIER MARQUER Laboratoire de Grologie Structurale, Centre Armoricain d'Etude Structurale des Socles (CNRS), Universit6 de Rennes, 35042 Rennes Cedex, France and "Drpartement de Grologie, U.S.T.H.B., Alger, Algrria (Received 23 June 1986; accepted in revisedform 10 February 1987) Abstract--Geological deformations which are statistically homogeneous at bulk scale (e.g. the macroscale) are often.localized into arrays of narrow shear zones at a smaller scale (e.g. the mesoscale). This paper shows that shear zone patterns can be used to estimate both a bulk finite strain ellipsoid and aspects of the bulk deformation history. We describe examples of heterogeneously deformed granitic rocks which reveal the following features. (1) Shear zones show preferred orientations. (2) There are correlations between shear zone orientations and directions and senses on shear. (3) For areas that have undergone coaxial deformation histories, shear zone patterns have orthorhombic symmetries directly related to strain ellipsoid shape. (4) For areas that have undergone non-coaxial deformation histories, shear zone patterns have a lower symmetry. (5) For areas that have undergone bulk simple shear, shear senses on shear zones are consistent with the bulk shear sense. Results are compared with predictions of kinematic models involving slip along inextensible fibres and sheets. According to these models, preferred orientations of slip surfaces track surfaces of no finite extension of the bulk strain ellipsoid, whereas slip directions track directions of large shear in the bulk strain ellipsoid. There are good correlations between preferred orientations of shear zones and fibre models. INTRODUCTION IN MANYdeformed rocks, especially those with an ini- tially homogeneous and isotropic structure (e.g. plutonic rocks), deformation is highly localized into band-like shear zones. These zones generally form anastomosing arrays, enclosing domains with smaller and more homogeneous strain. At the bulk scale, zones can be described as discontinuities in the displacement field, in other words, as slip surfaces; but, at a smaller scale, they are in fact coherent transition zones with continuous deformation gradients. Once developed, active shear zones must combine together, so as to accommodate most of the total deformation, irrespective of the defor- mation mechanisms that operate at the grain scale. Accordingly, one expects strong relationships between the geometry of a given shear zone pattern and the corresponding average (mean) deformation at bulk scale. By geometry we mean, in particular; (i) preferred orientations of shear zones; and (ii) changes in direction, sense and amount of shear according to shear zone orientation. Relationships between shear zone patterns and bulk strain have been examined by Mitra (1979) in a strain analysis of basement granitic rocks. His approach com- pares with those developed for intracrystalline slip sys- tems (Taylor 1938, Lister et al. 1978) or brittle faults (Oertel 1965, Freund 1974, Reches 1983). He deduced that five independent sets of shear zone orientations are required to accommodate a general bulk homogeneous strain. Thus, Mitra extended the von Mises criterion from lattice scale to rock scale and from discrete slip planes to ductile shear zones. Several kinematic models for accommodating a bulk plane strain have been derived from natural shear zone patterns (Ramsay 1980, Bell 1981). However, all these studies are mainly sup- ported by two-dimensional observations. They do not provide critical data to define: (i) relationships between shear zone preferred orientations and the bulk strain ellipsoid; and (ii) kinematic factors which can control the evolution of shear zone patterns during progressive deformation. Concerning the relationships between shear zone pat- terns and deformation history, few data are available and these remain essentially two-dimensional (see Col- lins & De Paor 1986). A coaxial history is expected to result in symmetric conjugate shear zones. In contrast, experiments (Hoeppener et al. 1969, Tchalenko 1970, Mandl et al. 1977, Logan et al. 1981), numerical models (Priour 1985) and field examples (Berth6 et al. 1979, Platt & Vissers 1980) all show that progressive simple shear results in the predominance of one set of shear zones over the conjugate set (e.g. C surfaces, Berth6 et al. 1979). Faulting experiments further emphasize that an imposed bulk simple shear can result in patterns of anastomosing zones which are locally complex, with several sets (e.g. Riedel experiments, see Tchalenko 1970). Comparable patterns are found in natural sheared rocks (Bell 1981, Simpson 1983, Gapais & Jegouzo 1985) and both their mechanical and kinematic signifi- cances are still unclear. The aim of the present paper is to describe how shear zone patterns can be used as qualitative shear criteria and strain markers at the bulk scale. We concentrate on their use as kinematic indicators, rather than discuss possible variations in geometry depending upon mechanical effects. The latter question will be left for further publications. 635