An Introduction to Structural Geology and Tectonics
11. Fabrics: Foliations and Lineations
In everyday language, we use the word "fabric" frequently; we even wear it. When talking about fabrics that are used to make curtains, we mean a patterned cloth made by weaving fibers in some geometric arrangement. In a philosophical moment we might wonder about the "fabric of life," by which we mean the underlying organization of life. But, as we found to be the case with many terms, the word fabric has a related yet somewhat different meaning in geology. To a structural geologist, the fabric of a rock is the geometrical arrangement of component features in the rock, seen on a scale large enough to include many samples of each feature. The features themselves are called fabric elements. Examples of fabric elements include mineral grains, clasts, compositional layers, fold hinges, and planes of parting. Fabrics that form as a consequence of tectonic deformation of rock are called tectonic fabrics, and fabrics that form during the formation of the rock are called primary fabrics. It may sound picky, but some structural geologists also make a distinction between "fabric" and "texture." Whereas texture is sometimes used as a synonym for microstructure, for example in igneous texture, we restrict texture to crystallographic orientation patterns in an aggregate of grains (see Chapter 12). Tectonic fabrics provide clues to the strain state of the rock, the geometry of associated folding, the processes involved in deformation, the kinematics of deformation, the timing of deformation (if the fabric is defined by an arrangement of datable minerals), and ultimately about the tectonic evolution of a region. The purpose of this chapter is to describe two common fabric elements in rocks, foliations and lineations, and to introduce you to the outcrop characteristics and interpretation of these elements.