Deformation Mechanisms

Abstract

Chapter 3 deals with deformation structures on the scale of individual grains. Grain scale brittle deformation and cataclastic flow occur in the upper crust or at high strain rate. At deeper crustal levels, rocks deform by ductile flow through a range of mechanisms of ductile grain scale deformation such as dissolutionprecipitation, intracrystalline deformation by dislocation glide and creep, diffusion creep, twinning and kinking. Ductile deformation in rocks could not lead to high strain if it was not accompanied by mechanisms that reduce the damage imposed during the deformation process. There are two main groups of such mechanisms; recovery, which removes dislocations inside the crystal lattice, and recrystallisation that operates by migration of grain boundaries. Three main types of dynamic recrystallisation are treated; subgrain rotation, bulging and high temperature grain boundary migration. After deformation slows down or stops, grain boundary migration can continue by grain boundary area reduction, and so-called foam textures can develop. In absence of deformation this process is know as static recrystallisation.

The second part of this chapter discusses grain scale deformation processes for a number of rock-forming minerals. This is necessarily a short description of what is presently known, with a large number of references for further reading. Treated are quartz, calcite, dolomite, feldspars, micas, olivine, pyroxenes, garnet and amphibole. Finally, a short outline is given of the deformation of polymineralic rocks with quartz-feldspar aggregates as an example. The final section of this chapter treats flow laws and deformation mechanism maps.