Fabric development in shear zones: theoretical controls and observed phenomena

Abstract

It is suggested that the kinematic framework controls the orientation of crystallographic fabrics developed in plastically deformed quartzites. Important directions in this framework are those of the instantaneous stretching axes, and the flow plane and flow direction if these can be uniquely defined. Rotation of the crystal axes takes place at any instant of time dependent on the orientation of the grain relative to the stretching axes. Because of this dependence the skeletal outline ∗ ∗ Defined on page 284 of a pattern of preferred orientation is sensitive to the closing stages of deformation. Thus fabrics measured in major movement zones cannot be related to early thrust or shear displacements without considering the effects of the geological history subsequent to those events. Nevertheless, asymmetric fabrics in movement zones may allow determination of the shear direction and sense of shear. Asymmetry in the intensity distribution is less susceptible to modification than asymmetry in the fabric skeleton, and may remain as a persistent measure of the sense of shear in mylonites subjected to coaxial deformation after non-coaxial events. However, fabric asymmetry need not always be related to the deformation history, and effects related to the population of initial grain-orientations must be considered, as well as the influence of recrystallization and grain growth. A problem of scale is involved in extrapolating the movement picture inferred from the behaviour of a few hundred crystal grains to larger dimensions. This question is also encountered when trying to specify deformation paths in mesoscopic shear zones. It is difficult to obtain simple shear experimentally because of the role discontinuities play in deformation. In certain cases in natural shear zones the quartz grains may be subjected to a coaxial deformation path while the bulk deformation is progressive simple shear. Caution must therefore be exercised when attempting to use quartz fabrics to infer characteristics of the bulk kinematics or movement picture applicable during deformation.