Page 101 - Microtectonics

4.2 · Foliations 89

Box 4.5 Fabric gradients certain minerals concentrate commonly in the fold hinges
(quartz, calcite, feldspar, chlorite) and others (biotite,
One of the problems in tectonics is that the evolution of struc- white mica, opaque minerals) in the limbs. This may be
tures cannot be directly observed in nature. As a result, there
has been a tendency to look for intermediate stages or gradients due to the high solubility of quartz and calcite, and the
in the geometry of structures, here referred to as fabric gradi- effect of enhanced permeability where micas are present
ents. Fabric gradients are gradual changes in the fabric of a rock (Gray and Durney 1979a,b; Engelder and Marshak 1985;
over a certain distance in the field (e.g. Fig. 1.5) or in thin section Schweitzer and Simpson 1986). As a consequence, differ-
(e.g. Fig. 4.15). Examples are increasing tightness of folds, a de- entiation is not common in pure mica phyllites.
creasing grain size in a mylonite (Fig. 5.9), a decrease in angle
between two foliations, an increase in amplitude of crenulations Examples are also known where ion exchange takes
and gradual appearance of a second foliation (Fig. 4.19). If such place between developing microlithons and cleavage do-
fabric gradients are associated with changes in strain or meta- mains. White mica and chlorite may be redistributed in
morphic grade, it is tentative to interpret them as evolutionary this way, chlorite concentrating in the microlithons, and
stages in the development of the most evolved fabric. As far as white mica in the cleavage domains (Waldron and Sandi-
can be determined with experiments, this assumption com- ford 1988; Price and Cosgrove 1990). High-resolution com-
monly holds. This is fortunate, since it allows us to reconstruct
and study fabric evolution processes, which would otherwise positional mapping of minerals in cleavage and microli-
remain inaccessible. It is dangerous, however, to assume that thon domains is a powerful tool to recognise newly grown
such fabric gradients always and in all aspects represent a se- minerals. Williams et al. (2001) give an example from the
quence of evolutionary stages. The simple fact that fabric gra- Moretown Formation, western Massachusetts, where most
dients are found at the surface implies that intermediate stages newly grown plagioclase grew in hinge (microlithon) do-
of the fabric gradient cannot be regarded as intermediate stages
on a P-T-t loop. For example, in a fabric gradient of increas- mains and a large amount of phengitic muscovite in limb
ingly complex foliations with euhedral micas, the grains may (cleavage) domains.
have been subhedral during the evolution of every part of the Some spaced foliations which have mainly formed by
fabric gradient, but micas across the gradient obtained a solution transfer processes may occur as cleavage bun-
euhedral shape by late static recrystallisation.
dles (Fig. 4.23a; Southwick 1987; Fueten and Robin 1992)
centred on thin parts of layers, fold closures or other ob-
jects that may have acted as stress concentrators, or as
continuous ‘mica films’ in psammites (Fig. 4.23b; Gregg
1985). Such foliations probably nucleated near the stress
concentration site, and grew out into the surrounding
medium normal to the shortening direction (Fletcher and
Pollard 1981; Gregg 1985; Tapp and Wickham 1987).

Fig. 4.24a,b. Schematic diagram of development of a foliation by crys-
talplastic deformation illustrating the role of lattice orientation. Trac-
Fig. 4.23. a Cleavage bundle nucleated on a gap in a bedding plane ing in grains indicates active slip planes for dislocations in quartz.
probably related to strain concentration. b Mica films developed in The grains with horizontal and vertical slip planes do not deform
psammite as a result of solution transfer because of their special orientation