Page 81 - Microtectonics

4.2 · Foliations 69
Fig. 4.2.
Diagrammatic representation of
various types of fabric elements
that may define a lineation

4.2 4.2
Foliations

In many areas several successive foliations can be distin-
guished in the field by overprinting relations (Sect. 1.1).
Their study in thin section, including the interpretation
of the metamorphic and deformational conditions dur-
ing their formation, is an important tool to unravel the
tectonic and metamorphic evolution of an area. Foliations
are also used as reference structures to establish the rela-
tive growth periods of metamorphic minerals, especially
porphyroblasts (Sect. 7.4). Foliations and lineations are
generally more penetratively developed in any volume of
rock than folds and are therefore better reference struc-
tures for the definition of deformation phases (Sect. 1.1).
Primary foliations are structures related to the origi-
nal rock-forming process. Bedding in a sedimentary rock
and magmatic layering in igneous rocks are the most com-
mon examples. A diagenetic foliation may be formed by
Fig. 4.3. a Foliations are visible on outcrop surfaces as foliation tracks;
diagenetic compaction. Secondary foliations are generated these should not be confused with lineations. Object lineations are
later (in the case of sediments: after lithification) as a re- visible on outcrop surfaces as lineation tracks; these should not be
sult of deformation and metamorphism. This group in- confused with lineations, since they generally have another orienta-
cludes cleavage, schistosity, differentiated compositional tion. b Section through a mylonite with mylonitic foliation (horizon-
layering, mylonitic foliation (Box 4.4), etc. tal) and shear band cleavage (inclined). The intersection lineation of
the two cleavages is normal to the aggregate lineation on the foliation
Development of secondary foliations is usually seen surfaces. This is an example of two lineations of approximately the
as evidence that the rock deformed in a ductile way, but same age that have different orientations and tectonic significance