Shape fabric formation by cataclasis in a quartzite from the Moine thrust zone, northwest Scotland

Abstract

A grain-shape fabric measured in quartzite from the Moine thrust belt cannot be accounted for by deformation distributed homogeneously over the bedding unit. Oblate grains, aligned subparallel to bed ding, are formed without equivalent thinning of the layer as a whole. The fabric shows evidence of crystal plastic strain and pressure solution. Healed microfractures aligned subparallel to bedding both predate and postdate lattice strain and indicate the importance of cataclasis throughout deformation. Shear offsets on these fractures split original detrital grains into platy fragments. If the fragments are used as original detrital grains for grain-shape strain analysis, they will give artificially high strain ratios. The shapes of quartz grains in deformed rock are often used for strain analysis by ellipse fitting or center-to-center methods. It is generally assumed that individual quartz grains in the deformed state represent original detrital grains with negligible or simple initial shape fabrics. These results emphasize the importance of recognizing localized deformation, especially cataclasis, which may affect the validity of these methods.