Slip system domains. 2. Kinematic aspects of fabric development in polycrystalline aggregates

Abstract

In the first paper of the present series (Cobbold and Gapais, 1986), we develop theoretical models of plane strain domainal deformation by slip along inextensible and indestructible fibres. For such extreme mechanical conditions, we show that deformation is largely controlled by kinematic factors. We use our models to explain domainal patterns of natural slip systems, including bedding and foliation planes, faults, ductile shear zones and lattice planes. The present paper focusses on kinematic aspects of fabric development in polycrystalline aggregates. A three-dimensional kinematic model is developed. It reveals that common quartz fabrics may develop domainally by reorientation and selection of inextensible slip directions (〈a〉) axes) or slip planes with initially random orientation. Stable orientations of slip directions are close to both surfaces of no finite extension and orientations of large amount of shear. We explain 1. (1) common patterns of quartz 〈a〉 and c-axis fabrics (e.g., small circles and crossed girdles) 2. (2)characteristic differences between fabric patterns according to the shape of the finite strain ellipsoid 3. (3)development of asymmetric fabrics during non coaxial deformation histories. This last process involves selective destruction or replacement of certain orientation domains, so that a preferred orientation of slip lines develops, close to the bulk shear direction. Rotation recrystallization is shown to be an efficient natural selection mechanism. The models also account for many microstructural features found in oriented polycrystalline rocks, especially 1. (1)alternating band-like (or kink-like) domains of twinned preferred orientations 2. (2) individual grains or clusters of grains with retort-like shapes and extensive recrystallisation at boundaries 3. (3) small-scale shear bands, formed by strain partitioning and shear localization.