Page 79 - Microtectonics

4

Foliations, Lineations

and Lattice Preferred Orientation

4.1 he main fabric elements, present in most deformed metamorphic rocks are
Introduction Tdiscussed in this chapter.
Foliations are subdivided into primary and secondary ones, the first being of
4.2 sedimentary or igneous origin and the second formed by deformational proc-
Foliations esses. They are described according to their morphological characteristics, and
classified into two main groups: continuous foliations and spaced foliations. Sev-
4.3 eral mechanisms thought to be responsible for foliation development are ex-
Lineations plained and discussed. These are: mechanical rotation of pre-existing grains, so-
lution transfer, crystalplastic deformation, dynamic recrystallisation, static recrys-
4.4 tallisation, mimetic growth, oriented growth in a differential stress field and mi-
Lattice-Preferred cro folding.
Orientation (LPO) Subsequently, the relation between secondary foliations and axial planes of
folds, the XY-plane of tectonic strain and volume change is discussed. The de-
pendence 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 in-
clude 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 al-
ways 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.