Development of Fabric and Structures in Shear Zones

Abstract

Deformation in much of the crust is accommodated by discrete movement on faults or by ductile deformation concentrated in shear zones. Faults dominate deformation in the upper part of the crust and are readily recognized where exposed by offset of rock units, slickensided fracture surfaces, and effects of brecciation or cataclasis. Ductile shear zones occur deeper in the crust, often as downward extensions of normal, reverse or strike-slip faults. There is every gradation from zones in which deformation is entirely brittle to zones in which deformation is entirely ductile. Individual shear zones are often quite planar when viewed in outcrop or on a map, and they commonly form subparallel arrays (with a single sense of shear) or conjugate arrays (with opposing sense of shear on shear zones of different orientation) (Choukroune and Gapais 1983), reflecting different patterns of bulk strain. Islands or lozenges of undeformed or weakly deformed rock are commonly found within shear zones or help define shear zone boundaries. There is often an anastomosing character to shear zones in a region (e.g. Ramsay and Allison 1979). A characteristic set of mesoscopic and microscopic structures occurs that can be used to help identify shear zones and also to determine the sense and amount of shear within shear zones. The critical feature of all these structures is an asymmetry that is either intrinsic to the structure or results from the orientation of the structure with respect to the foliation or shear zone boundaries. The sense of asymmetry can be related to the sense of shear or vorticity of the deformation. It must be emphasized that establishing a local sense of shear does not by itself establish that the deformation is one of simple shear or of any other particular shear regime. To determine shear regime one needs to be able to observe undeformed shear zone walls or determine the vorticity of the flow. There are, in fact, a number of structural features that may allow vorticity to be determined (e.g. Passchier 1987). In the remainder of the abstract I describe some of the best structures for identifying shear zones and indicating sense of movement. Emphasis is given to those structures that can be used as field criteria for sense of shear. For more details the reader is referred to an excellent review of shear geometry and kinematics (Ramsay 1980), the theory of more general non-coaxial flows (Ghosh and Ramberg 1976) and several treatments of sense of shear criteria (Simpson and Schmid 1983; Simpson 1986; Hanmer and Passchier 1991). The classic signature of a ductile shear zone is the presence of a foliation that increases in intensity from the walls to the interior of the zone, with the angle between foliation and the walls decreasing towards the interior, thus tracing out a sigmoidal pattern (Ramsay 1980; Simpson 1986). This phenomenon is best developed in initially isotropic or weakly anisotropic rocks such as granites, gabbros or coarse-grained gneisses. A strong linear fabric, parallel to the shear direction, is commonly produced in highly sheared rocks. In weakly deformed rocks an intersection lineation may form perpendicular to the shear direction. S-C structures are common in crystalline rocks at early stages of deformation. Closely-spaced (mm-cm) bands of differential shear (C) develop parallel to the shear zone 355