Ductile deformation and microstructures

Abstract

In contrast to the elastic deformation, which is reversible, usually neglected by field geologists but important for geophysicists working in seismology, ductile deformation is irreversible. This chapter is restricted to solid materials. Materials containing a melt fraction will be examined in Chapter 7. In the geological literature, ‘ductile’ is often used as a synonym for ‘plastic’. The latter is rather used, and will be used to specify deformation mechanisms that dominantly involve the action of dislocations. In contrast to brittle deformation, which by essence is discontinuous and highly localized (see Chapter 3), ductile deformation is generally continuous and affects large volumes of rock. However, ductile deformation may be concentrated into restricted rock volumes (or domains). Such localization is common in shear zones and/or when superplastic deformation mechanism is involved. Plastic deformation mechanisms naturally depend on temperature, magnitude of the applied stress, mineral nature and grain-size of the rocks. In upper parts of the crust, fluids are able to carry chemical elements over large distances and influence the deformation mechanisms. Micrographs of several microstructural types as well as deformation maps for olivine and calcite are given at the end of this chapter.