Page 109 - Microtectonics

4.2 · Foliations 97
orientation in limestone at low temperature (Alvarez et al. distinguish cases where a pre-existing foliation has un-
1976; Mitra and Yonkee 1985; Kreutzberger and Peacor dergone an increase in metamorphic conditions after de-
1988). At higher temperature, crystalplastic flow and twin- formation, producing new minerals that may have grown
ning are important, and a foliation is mostly formed by mimetically over the existing foliation (Figs. 4.27, 4.28).
elongate grains. In all cases, foliations in limestones are Relicts of older mineral assemblages may help to recog-
less well developed than in pelites. nise these situations.
Metabasites deformed at low-grade conditions give rise In many areas where several overprinting foliations
to continuous or spaced foliations defined by preferred can be recognised, a “main foliation” may have formed
orientation of amphiboles, chlorite, epidote, micas and under peak metamorphic circumstances, whereas later
lenses of different composition. Mechanical rotation and events are characterised by deformation of this main fo-
oriented growth of new minerals is more important then liation to produce weaker and less penetrative foliations
solution transfer. At medium to high-grade conditions, under lower metamorphic or drier circumstances. This
oriented mineral growth and crystalplastic deformation may be related to the fact that during progressive meta-
are the main mechanisms of foliation development. Meta- morphism, water is generally released by mineral reac-
basites can be equally suitable to determine metamorphic tions favouring complete recrystallisation during defor-
grade as pelites, especially at low-grade metamorphic con- mation. After the peak of metamorphism, under retro-
ditions. grade circumstances when the rock has lost most of its
In many mountain belts the onset of cleavage is marked water, recrystallisation is difficult and deformation re-
by a cleavage front, that separates rocks with cleavage from sults mainly in folding, distortion or mylonitisation of
rocks without cleavage (Mattauer 1973; Holl and Anastasio earlier fabrics.
1995). Although this front is strongly dependent on de-
formation intensity and also lithologically controlled, a 4.2.10.2
minimum pressure of about 200 MPa corresponding to Overprinting Relations
an overburden of 5–7 km (Engelder and Marshak 1985)
and a minimum temperature of about 200 °C for pelitic Overprinting relations between foliations are probably
siliciclastic rocks and 175 °C for limestones (Groshong the most useful tools to furnish a reference frame for the
et al. 1984a) can be estimated. Criteria for incipient cleav- study of the tectonic evolution of a body of rock. They
age development are given in Kisch (1998). are particularly appropriate for study in thin section be-
cause of their penetrative nature and because of the usu-
4.2.10 ally small size of the fabric elements. The principle for
Practical Use of Foliations establishing a sequence of foliation planes is quite sim-
ple: if microfolds are visible the folded surfaces are al-
4.2.10.1 ways older than the fabric elements developed along the
Introduction axial surface, or cutting the folds. Any surface associated

Foliations can be used to obtain information on strain,
metamorphic conditions and overprinting relations. In
Sect. 4.2.9.2 it is shown how secondary ‘passive foliations’
can be used to find the approximate orientation of the
XY-plane of tectonic strain related to the formation of
that particular foliation (not the total strain), provided
the problems mentioned in Sect. 4.2.9.3 are kept in mind.
Especially continuous foliations may be used to a first
approximation to identify the XY-plane of strain. Foli-
ations, which are thought to have developed by mechani-
cal rotation of fabric elements only, can in principle be
used to estimate the magnitude of finite strain (Sect. 9.2).
If a foliation is defined by parallel minerals that show
a “growth fabric” (e.g. Fig. 4.8) without signs of post-crys-
talline deformation, the foliation must have formed un-
der metamorphic circumstances during which these min- Fig. 4.33. D fold, folding bedding (S ) with development of S foli-
1
0
1
ation along the axial surface of the fold. Later D deformation
2
erals were stable. A foliation defined by euhedral amphi- folded S 1 to produce an S 2 crenulation cleavage in pelitic layers that
boles must have formed under circumstances where these cut the D fold through both limbs. Note the deviation of S around
2
1
amphiboles were stable. However, care must be taken to the more resistant fold hinge