Page 104 - Microtectonics

92 4 · Foliations, Lineations and Lattice Preferred Orientation
4.2.9 The intersection lineation of foliation and a folded sur-
Geological Context of Foliation Development face is usually parallel to the fold axis if folds and foliation
are of the same age. If the intersection lineation is oblique
4.2.9.1 to the fold axis, the structures are known as foliation-tran-
Foliations and Folds sected folds (Johnson 1991). Foliation transected folds may
form if the vorticity vector of non-coaxial flow was oblique
Commonly, secondary foliations are referred to as axial pla- to the fold axis, or if folds and foliation are of different age.
nar foliations (e.g. Hobbs et al. 1976), i.e. they show a con- Some foliations, such as diagenetic foliation and foliations
sistent geometrical relationship with the axial planes of folds in shear zones, need not be associated with folds at all.
(Figs. 4.4, 4.29, 4.30, ×Photo 4.30). This relation was recog-
nised as early as Sedgwick (1835) and Darwin (1846), and
is generally accepted to indicate that folds and foliation de-
veloped during the same deformation phase. Commonly,
foliations are not perfectly parallel to axial planes of folds,
but symmetrically arranged with respect to the axial plane
(Fig. 4.30a). This effect is known as foliation fanning. A fo-
liation may also refract where it passes from one lithology
to another. Foliation fanning and refraction can be due to
strain partitioning generally related to viscosity contrast
(Treagus 1983, 1999), or to passive rotation of relatively com-
petent layers in the limbs of folds after initial bedding par-
allel shortening. In some cases, a foliation may even be per-
pendicular to the axial plane (Fig. 4.30b). This is due to Fig. 4.30. a Refraction of foliation in competent layers. Apart from
buckle folding where the outer arc of the folded layer is ex- the refraction defined by a change in orientation, a change in mor-
tended in the fold hinge normal to the fold axis but parallel phology commonly occurs: in the psammitic layers the foliation is
to the bedding plane; locally this leads to shortening in the usually disjunctive whereas in the pelitic ones it may be continuous.
(After Fig. 5.3 in Hobbs et al. 1976). b Highly variable foliation ori-
fold hinge normal to the bedding plane and foliation seg- entation in a sequence of rocks with strong competency contrast.
ments parallel to bedding (Fig. 4.30b). (After Roberts and Strömgård 1972, and Fig. 5.16 in Hobbs et al. 1976)

Fig. 4.29. Secondary foliation (subhorizontal) defined by preferred orientation of micas parallel to the axial plane of folds. Quartz mica
schist. São Felix de Cavalcante, Goiás, Brazil. Width of view 17 mm. PPL