Strain variation between quartz grains of different crystallographic orientation in a naturally deformed metasiltstone

Abstract

Abstract The shapes of isolated quartz grains in a metasiltstone from the Fleurieu Peninsula, South Australia, vary markedly, the amount of strain undergone by individual grains being strongly influenced by crystallographic orientation relative to the finite strain axes. The least-deformed grains have c-axis orientations clustered about the principal strain directions; that is, they have orientations for which the resolved shear stress on the (0001) 〈a〉 and {1010} 〈c〉 slip systems would have been close to zero during coaxial deformation. A simple theoretical approach suggests that the geometry of the bulk strain should influence the observed c-axis fabric of these least-deformed grains. The fabrics should vary from (a) a pronounced maximum parallel to Z with a subsiduary distribution in the XY-plane for pure flattening strain, through (b) fabrics with maxima parallel to X and Z and a weak maximum parallel to Y, to (c) fabrics with a pronounced maximum parallel to X and a subsiduary distribution within the YZ-plane for pure constriction. The most elongate quartz grains in the metasiltstone show a strong maximum of c-axes parallel to the intermediate strain axis, Y. This suggests predominant prismatic slip in a direction at a large angle to the c-axis, probably on {1010} 〈a〉. The bulk strain undergone by the specimen is an integral of the variable strains in the component minerals, and the extremes (the least deformed or the most elongate grains) cannot be regarded as anomalous if a realistic strain estimate is to be obtained.