Abstract

Deformation of the crust is believed to occur dominantly by cataclasis at low temperatures and/or effective confining pressure, by pressure solution at intermediate temperatures, and by dislocation creep at high temperatures. Each flow mechanism gives rise to distinctive microscopic and small scale structures. Brittle deformation with grain fracture leading to a reduction of particle diameter is characteristic of cataclastic flow. Pressure solution produces grain shape fabrics by intercrystalline diffusion assisted by the presence of water. Grains may change shape at constant mass, or decrease in mass (and therefore in size) by long range diffusion: mass is then not locally conserved. Reduction of grain diameter leads to increased rates of deformation (strain softening). Distinctive spaced cleavage zones form by pressure solution in which mineral species are redistributed due to different rates of deformation: the displacement field is discontinuous and deformation non-isochemical. Tectonic veins associated with pressure solution structures probably form by local mass transport; thus brittle and ductile mechanical behaviour coexist. Dislocation creep produces grain shape fabrics by intracrystalline deformation. and may cause grain size reduction by subgrain formation and recrystallization. Preferred crystallogra-phic orientations can arise from dislocation glide. Mass is conserved and deformation is believed to be essentially isochemical. Small scale structures formed by dislocation creep are ductile, with a continuous displacement field.