Page 111 - Microtectonics

4.2 · Foliations 99
with the axial planes of folds is genetically related with
those folds, but foliation planes that cut folds obliquely
are younger than the folds (Fig. 4.33).
A general outline of a common sequence of events in
slate and schist belts may serve to illustrate how the analy-
sis of overprinting relations works (Fig. 4.34; cf. Hobbs
et al. 1976, their Chap. 9; Williams 1985). During a first
deformation phase (D ), a penetrative slaty cleavage is
1
developed at varying angles with bedding, according to
the position in large D folds that are commonly asym-
1
metric. In the long limbs the angle between S and S may
1 0
become so small that it is not visible any more in the field
or even in thin sections (Fig. 4.34). The slaty cleavage (S )
1
may be spaced or continuous, but is generally not a crenu-
lation cleavage as analysed under normal microscopic
amplification. However, if analysed by SEM, it may show
crenulation cleavage features, folding a bedding-parallel
foliation of diagenetic origin.
A second phase of deformation (D ) commonly produces
2
a crenulation cleavage, folding S (Fig. 4.34). Various stages
1
or morphologies may be present depending on the inten-
sity of deformation (cf. Bell and Rubenach 1983) and ac-
cording to grain growth in response to metamorphic cir-
cumstances (Figs. 4.18, 4.19; ×Photo 4.19b1–7).
A third phase of deformation may be recognised by
folding of the S crenulation cleavage (Figs. 4.34, 4.35).
2
This may in some cases result in interesting structures,
since according to their orientation certain limbs may be
refolded and others straightened out (Figs. 4.36, 4.37,
×Photo 4.37a–c). Later phases of deformation may be
recognised in a similar way by overprinting (folding) of
earlier foliations.
The main problem of this analysis is to establish
how to correlate foliations from one thin section to an- Fig. 4.36. Sequence of events leading to selective refolding of a sec-
other, from one outcrop to another, or even from one ond foliation (S 2 ) by D 3 while the older foliation (S 1 ) seems unaf-
analysed area to another. This is a matter that is hard fected. a S is formed by vertical compression. b Oblique lateral com-
1
to solve with general rules, but the following sugges- pression by D caused a steep S differentiated crenulation cleavage.
2
2
tions may be of help (see also Williams 1985). Deforma- c Oblique D 3 compression is applied, resulting in selective refolding
of differentiated limbs of D folds because of their orientation. The
2
tion may be quite heterogeneously distributed through other limbs are progressively unfolded until S becomes approxi-
1
a rock body, especially the deformation that post-dates mately parallel to the axial plane of D 3 folds
peak metamorphic conditions. It is, for instance, com-
mon to find D or D deformation features concentrated not change much from one outcrop to the next, unless
4
3
in narrow zones, leaving other areas without visible ef- post-metamorphic faulting is involved. In rare cases
fects. Shear zones are, of course, the most spectacular (e.g. Lüneburg and Lebit 1998) successive deformation
example of this local concentration of deformation. On phases produced only a single cleavage, reflecting the
the other hand, foliations induced during peak metamor- total strain ellipsoid.
phic conditions are normally widespread and remark- Especially in the field, intrusive veins or dykes can be
ably continuous over large areas. These may, however, important to distinguish phases of deformation and their
vary abruptly because of lithological variation (e.g. strong associated foliations of different age. These bodies may
foliations may disappear abruptly at the contact of a calc- have intruded over a relatively short period of time and
silicate rock because of the lack of platy minerals to may be recognised over a large area by their similar com-
define a foliation). It is important in the correlation position and orientation. Structures cut by the veins are
of foliations to pay attention to their relation with meta- older, whereas younger structures affect the veins by fold-
morphism, since metamorphic conditions usually do ing, shearing or other deformation.