Page 100 - Microtectonics

88 4 · Foliations, Lineations and Lattice Preferred Orientation
but in many cases solution transfer or oriented crystalli- fold hinges may have been erased (Fig. 4.18). Figures 4.18
sation or recrystallisation of new grains (Gray and Durney and 19 show a progressive sequence of development of
1979a,b) become important after the folds have reached crenulation cleavage with increasing pressure and tem-
a certain amplitude, and develop a spaced foliation along perature. This sequence can be understood as an exam-
limbs of microfolds (Figs. 4.12, 4.13, 4.18, 4.19; White and ple of progressive development of many spaced foliations
Johnston 1981; Williams et al. 2001). Spaced foliations can, (see also ×Photo 4.19b1–7 and Box 4.5).
however, also form without folding of the older fabric The efficiency of differentiation by solution transfer
(Fig. 4.20; Sect. 4.2.7.3; Durney 1972; Engelder and Marshak depends on the abundance of a fluid phase and is there-
1985). Besides harmonic microfolding of a foliation, fore most active under diagenetic and low-grade meta-
disharmonic microfolding or kinking of individual mi- morphic conditions. The mechanism is also dependent
cas can also increase mica-preferred orientation by rota- on the presence of one or more soluble minerals. Gray
tion of mica segments away from the shortening direc- and Durney (1979a) published the following mineral se-
tion (Fig. 4.16(1); Engelder and Marshak 1985). quence according to decreasing mobility by solution
transfer: calcite > quartz > feldspar > chlorite > biotite >
4.2.8 muscovite > opaques. In quartz- or carbonate-bearing
Development of Spaced Foliations phyllites, solution transfer seems to operate quite well:

Spaced foliations and tectonic layering have a marked
uniformity in the spacing between cleavage planes and
several ideas have been postulated on how this develops
(Williams 1990). In most cases, some form of dissolution-
precipitation and transport of material through a fluid
phase in combination with a mechanical interaction is
postulated for the development of spaced foliations. Three
groups of mechanisms have been postulated:
One option is that the periodicity develops spontane-
ously throughout a volume of slightly heterogeneous but
unfoliated rock in which compaction localizes by a self-
organisation mechanism due to the interaction of stress
and chemical gradients (Dewers and Ortoleva 1990). Mac-
roscopic patterns of alternating cementation and com-
paction result which represent cleavage seams or stylolites
and microlithons.
A second possibility is that foliation develops as single
cleavage plane “seeds”, which develop into cleavage planes,
while new planes are initiated on both sides at a regular
distance. This could happen if strong quartz rich domains
form next to developing cleavage domains. These quartz
rich domains will then initiate new cleavage domains at their
margins, thus gradually filling the rock volume with a spaced
fabric (Robin 1979; Fueten and Robin 1992; Fueten et al.
2002). Such mechanisms, however, are difficult to prove.
A third possible mechanism is through the develop-
ment of microfolds in an older foliation (Trouw 1973;
Cosgrove 1976; Gray 1979; Gray and Durney 1979a; Beut-
ner 1980; Wright and Platt 1982; Woodland 1985; South-
wick 1987; Ho et al. 1995, 1996; Worley et al. 1997; Stewart
1997; van der Pluym et al. 1998; Williams et al. 2001; Fueten
et al. 2002). The folding of an earlier foliation produces a
difference in orientation of planar elements, such as mica-
quartz contacts, with respect to the instantaneous short- Fig. 4.22. Progressive tightening of folds with formation of a differ-
ening direction. This may enhance preferred dissolution entiated crenulation cleavage (S ) by preferential dissolution of
2
in fold limbs, which produces a secondary foliation in the quartz in fold limbs caused by the orientation of quartz-mica con-
tacts with respect to the σ 1 direction; resolved normal stress over
form of a differentiated crenulation cleavage (Figs. 4.12, these contacts is higher in fold limbs than in hinges. a and b are two
4.22) and eventually a compositional layering in which stages in progressive deformation (cf. Figs. 4.12, 4.13)