Page 169 - Microtectonics

158 5 · Shear Zones
hesive cataclasites are shear bands (Chester and Logan Porphyroclasts of minerals resistant to fracturing can
1987; Petit 1987); these can be interpreted in a similar form porphyroclasts with σ-type strain shadows of frag-
way as those in ductilely deformed rocks. Riedel shears mented grains or newly grown phases (Bos et al. 2000).
may also be present (Fig. 5.50). Care has to be taken, how- Foliation in the cataclasite can be deflected by C-type
ever, with R-shears; in many cases, these are deflected and C'-type shear bands on macro and micro scale (Lin
towards Y-shears in a sense as shown in Fig. 5.50 (top; 1999, 2001; Bos et al. 2000). These shear bands are filled
Logan et al. 1979; Strating and Vissers 1994). This deflec- with very fine-grained crushed material. Deformed in-
tion is probably due to a gradient in the orientation of herited micas can be used as kinematic indicators in
the stress field near Y-shears during development of some foliated cataclasites. Inherited biotite grains in a
R-shears, which therefore obtain a curved shape (Strating cataclased granite can form cleavage steps or biotite fish
and Vissers 1994). If many finely spaced R-shears are similar to those in mylonites (Kanaori et al. 1991; Lin
present in a non-foliated cataclasite, they might be con- 1999, 2001). Such fish have trails of fine-grained crushed
fused with foliation planes; in that case, use of the de- mica. Statistical analysis of the orientation of (001) in
flection towards Y-shears as a shear sense indicator would these micas against the main foliation trace allows de-
give the wrong result (Fig. 5.50). However, R-shears can termination of shear sense.
be recognised because their spacing is dependent on the
spacing of Y-shears, which is not the case for foliation 5.7.4
planes (Strating and Vissers 1994). Pseudotachylyte
Foliated cataclasite can resemble low-temperature
mylonite in many aspects. Although deformation in brit- In pseudotachylyte, shear sense can rarely be determined
tle fault rocks is by sliding on microfaults, fracturing and in thin section. Displacement of markers is the best in-
pressure solution, it creates a kind of coarse penetrative dicator, but this gives unreliable results (Fig. 5.13), espe-
flow that can have similar effects on the development of cially since no slickensides are present as for gouge and
asymmetric structures as ductile flow by dislocation cataclasite. In some pseudotachylyte veins, the orienta-
creep and recrystallisation. With some care and cross- tion of injection veins or of fracture sets in the wall rock
checking with other indicators, these asymmetric struc- can be used to determine shear sense. Particularly use-
tures can be used as shear sense indicators in a way simi- ful are main fault veins that grade into faults with asso-
lar to those discussed above for ductile flow. ciated Riedel fractures (Swanson 1998).