Page 139 - Microtectonics

128 5 · Shear Zones
the layering. Such oblique foliations result from the fact C'-type shear band cleavage is oblique to shear zone
that some fabric elements (elongated new quartz grains, boundaries and to the older foliation in micaceous
micas) rotated through a smaller angle than the mylonitic mylonites (Figs. 5.15, 5.16, ×Photo 5.15; White 1979b;
foliation during non-coaxial progressive deformation. Platt and Vissers 1980). The angle between the shear
Another type of strain-insensitive foliation that can bands and the shear zone margin is 15–35° (Dennis and
occur in mylonites is a domain shape preferred orienta- Secor 1987; Passchier 1991b; Blenkinsop and Treloar
tion of grains that share a certain crystallographic pre- 1995). C'-type shear band cleavage develops mainly in
ferred orientation (Box 4.2). strongly foliated mylonites such as phyllonites and
mylonitic micaschists; usually, shear bands fail to con-
5.6.3 tinue into more weakly foliated layers (e.g. quartz lay-
Shear Band Cleavage ers) in such rocks (Fig. 5.15). The shear bands are usu-
ally anastomosing, short and wavy (Figs. 5.14, 5.15,
A mica-preferred orientation or compositional layering ×Photo 5.15). An older mylonitic foliation is cut by the
may be transected at a small angle by sets of subparallel shear bands and is deflected in the bands in the same
minor shear zones (Figs. 5.10d,e, 5.14–5.17, ×Photo 5.15). way as foliation curvature in a large-scale shear zone
Such minor shear zones are known as shear bands and (Sect. 5.5.3). Normally, the intersection of the older fo-
the complete structure is a shear band cleavage (Roper liation and the shear bands is normal to aggregate or
1972; White 1979b; Gapais and White 1982). Shear band grain lineations on both these surfaces. This is taken to
cleavage may superficially resemble crenulation cleav- indicate that both the older foliation and the shear band
age but develops by extension along the older foliation cleavage formed during the same event of mylonitic de-
rather than shortening. Some authors (e.g. Platt and formation. However, the C'-type shear band cleavage may
Vissers 1980) therefore use the terms ‘compressional’ have a deviant mineral composition, usually indicative
crenulation cleavage (as treated in Chap. 4), and exten- of retrograde metamorphic conditions (McCaig 1987;
sional crenulation cleavage for shear band cleavage. Geo- Norrell et al. 1989). Commonly, only one set of C'-type
metric differences between these two groups of struc- shear bands is developed in fabrics but occasionally a
tures are given in Box 5.3. second, less developed set may be present almost ortho-
Two types of shear band cleavage are distinguished: gonal to the main set (Fig. 5.14; Harris and Cobbold 1985;
C-type and C'-type (Figs. 5.10d,e, 5.14). In the literature, Behrmann 1987). Also, a younger set of shear bands may
this distinction is not always made. Our C- and C'-type overprint an older set that has a more gentle inclination
shear band cleavage correspond to C- and C'-bands of to the shear zone boundary (Platt and Vissers 1980;
Berthé et al. (1979a,b). C'-type equals extensional crenu- Passchier 1991b).
lation cleavage of Platt and Vissers (1980); we favour the Development of C'-type shear band cleavage is only
term C'-type because it is older and non-genetic. partly understood. It seems to develop late during shear

Fig. 5.14.
Three types of foliation pairs
that are common in ductile shear
zones. The shear zone is shown
(top) with typical foliation cur-
vature. The main differences in
geometry between the foliation
pairs are shown in the centre.
Elements used to determine
sense of shear are shown below.
Further explanation in text