Foliations, Lineations and Lattice Preferred Orientation

Abstract

Foliations are subdivided into primary and secondary ones, the first being of sedimentary or igneous origin and the second formed by deformational processes. They are described according to their morphological characteristics, and classified into two main groups: continuous foliations and spaced foliations. Several mechanisms thought to be responsible for foliation development are explained and discussed. These are: mechanical rotation of pre-existing grains, solution transfer, crystalplastic deformation, dynamic recrystallisation, static recrys-tallisation, mimetic growth, oriented growth in a differential stress field and mi-cro folding. Subsequently, the relation between secondary foliations and axial planes of folds, the XY-plane of tectonic strain and volume change is discussed. The dependence of foliation development from lithotype and metamorphic conditions is also treated. Special attention is given to the practical use of foliations. Since they are present in the large majority of metamorphic rocks they are important reference structures that can commonly be traced between outcrops. They are especially useful for the establishment of overprinting relations of successive deformation phases. Lineations are another fabric element of major interest. A new subdivision into object and trace lineations is followed in this book. Object lineations can be further subdivided into grain and aggregate lineations, and trace lineations include crenulation and intersection lineations. The development of lineations is to a large extent similar to that of foliations, but some differences are discussed. The third fabric element treated in this chapter is lattice preferred orientation (LPO) of minerals. Especially in quartz and calcite this fabric element is not always obvious and special techniques may be required to determine whether an LPO is present. Several factors that influence the LPO pattern are discussed. LPO patterns in quartz are treated in detail and then compared to LPO patterns in other minerals. T he main fabric elements, present in most deformed metamorphic rocks are discussed in this chapter.