Page 142 - Microtectonics

5.6 · Microscopic Shear Sense Indicators in Mylonite 131

Fig. 5.18. Photomicrograph of mylonitic quartzite with mica fish arranged between C-type shear bands. Dextral shear sense. Varginha,
Minas Gerais, Brazil. Width of view 4 mm. CPL. (Sample courtesy Rodrigo Peternel)

growth; the segments between shear bands may have ro- grade shear zones and especially in deformed granites,
tated synthetically or antithetically, dependent on the vor- where C-type shear bands anastomose around feldspar
ticity and volume change in a shear zone (Passchier 1991b). porphyroclasts. C-type shear bands may nucleate at sites
If microlithons between C'-type shear bands were rigid of high differential stress adjacent to feldspar porphyro-
during C'-type cleavage development, the microlithons clasts, and subsequently propagate and join up (Hanmer
and shear bands must have rotated antithetically while and Passchier 1991; Ildefonse and Mancktelow 1993).
the shear zone wall rock must have extended parallel to C/S fabric probably reflects inhomogeneous simple
the zone (Fig. 5.19). There are indications that C'-type shear shear. Contrary to C'-type shear band cleavage it may
band cleavage is especially well developed in such ‘stretch- develop from the earliest stage of mylonite generation
ing’ shear zones (Passchier 1991b; Hafner and Passchier onward. The foliation in the microlithons probably con-
2000) and independent evidence from LPO analysis indi- tinues developing while the shear bands grow, contrary
cates that stretching shear zones may be common (Schmid to many C'-type shear band cleavages where the micro-
1994). If flow in a shear zone is simple shear when C'-type lithons are probably rigid. C/S fabrics can be overprinted
shear bands form, the bands must have rotated syntheti- by C'-type shear band cleavage (Berthé et al. 1979b).
cally and microlithons must have been deforming. Lister and Snoke (1984) proposed a modification of
C-type shear band cleavage is part of a so-called the nomenclature and distinguished Type I and Type II
C/S fabric (Figs. 5.14, 5.17) that consists of S-planes (from C-S mylonites. Type I corresponds to C/S fabrics as de-
French ‘schistosité’), transected by planar distinct C-type scribed above, while Type II refers to sets of parallel shear
shear bands or C-planes (from French ‘cisaillement’, bands without clear S-planes. The principle example of
meaning shear; Berthé et al. 1979a,b; Vernon et al. 1983; Type II C-S mylonites used by Lister and Snoke (1984), how-
Lister and Snoke 1984; Krohe 1990; Toyoshima 1998). ever, refers to a stair stepping of wings of small mica grains
C/S fabric is also written as S-C or C-S fabric in the lit- adjacent to mica-fish in quartzite mylonite (Sect. 5.6.7). They
erature. The C-type shear bands in C/S fabric are parallel envisage that the wings represent C-type shear bands
to shear zone boundaries (Figs. 5.10e, 5.14, 5.17, 5.18) and adjacent to the mica-fish. However, since such wings do
relatively straight and continuous, unlike C'-type shear not necessarily form by flow partitioning along shear
bands. C/S-fabric forms in weakly foliated mylonites with bands (Sect. 5.6.7), we discourage the use of Type II
a small percentage of micas. It is most common in medium- C-S mylonite terminology.