Page 141 - Microtectonics

130 5 · Shear Zones

Fig. 5.17. Polished hand specimen showing a C/S fabric in granite. C-planes are horizontal, S-planes trend upper right to lower left between
C-planes. Shear sense is dextral. Section parallel to the aggregate lineation and normal to the foliation. South Armorican Shear zone, France

Box 5.3 Difference between compressional (normal) crenulation cleavage
and shear band cleavage (extensional crenulation cleavage)
Shear band cleavage (Fig. 5.14) and compressional crenu- they have a different morphology and kinematic significance.
lation cleavage (Figs. 4.12, 4.15, 4.18) have a similar geometry We outline some morphological and kinematic differences
and could be confused by a casual observer. Nevertheless, below.

zone activity after a strong mineral preferred orienta- fabric attractor, one of these potential shear band orien-
tion has already been established, and represents prob- tations makes a smaller angle with the foliation than the
ably an energetically favourable flow partitioning in other. Strongly foliated mylonites may have a strong
strongly anisotropic materials (Fig. 5.19, ×Video 11.7b; mechanical anisotropy, which inhibits development of
Platt and Vissers 1980; Platt 1984; Dennis and Secor 1987; the steeper set (Fig. 5.19) and causes a decrease in the
Passchier 1991b; Hafner and Passchier 2000). In an iso- angle between the main set and the older foliation (Cobbold
tropic rock, two sets of conjugate shear bands would be et al. 1971; Cobbold 1976; Platt and Vissers 1980).
expected around the shortening ISA; since the foliation The geometry of shear bands imposes a geometric limit
in a mylonite lies between the extensional ISA and the on the possible flow regimes in mylonites during their