Page 167 - Microtectonics

156 5 · Shear Zones
changes host element geometry in this domain. Reliable cially similar but formed in different ways, commonly in
shear sense indicators are flanking folds without slip opposite sense of shear (e.g. Sect. 7.6.8). Examples of
along veins where some kind of alteration may have these complex shear sense indicators are treated in
caused forward rotation of the host element. This can Chap. 6 and 7.
be recognised since the crosscutting element is not
stretched, but the host element is (Fig. 5.47). Tensional and Constrictional Stepover Sites

Imbrication In mylonites with a strong mica foliation deformed at
low grade or high strain rate, approaching the brittle
The presence of a large number of rigid elongate crystals in deformation regime, small extensional fractures or
a deforming material may result in their interference caus- microfolds may occur between foliation-parallel but
ing tiling or imbrication of the objects (Fig. 5.48; Fernandez ‘overstepping’ micro-shear bands or faults (Fig. 5.49). If
et al. 1983; Arbaret et al. 1996). This type of structure seems such locations systematically show the same stair step-
to be common in phenocryst granites deformed in the liq- ping of faults for constrictional or tensional stepover
uid state (Blumenfeld and Bouchez 1988; Chap. 8), and has sites, they can be used as shear sense markers. Care
been described macroscopically by Blumenfeld (1983) for should be taken, however, since slip on micro-shear
feldspar grains in granitoid gneiss, and for porphyroclasts bands may not be the only deformation in the sample,
in ultramylonite (Brunel 1986; Blumenfeld and Bouchez and need not be representative for bulk deformation;
1988; Pennacchioni et al. 2001). The asymmetry of imbri- i.e. sense of shear on the micro-shear bands may be op-
cation can indicate shear sense, but a large number of ob- posite to bulk shear sense.
servations of tiled grains in a single location are needed for
a reliable analysis (Mulchrone et al. 2005). ‘V’-pull-apart Microstructures

Complex Indicators Hippertt (1993) introduced a new potential shear sense
indicator in the form of pull-apart structures that occur
Several microstructures with monoclinic shape symme- in the rim of feldspar porphyroclasts at low metamor-
try do not exclusively occur in mylonites. These struc- phic grade. Fractures in the edge of the porphyroclasts
tures may be reliable shear sense indicators, but need may open to a V-shape and are filled with quartz or an-
close observation and classification before they can be other mineral. In some mylonites these V-pull-apart
applied. The most common examples are inclusion pat- microstructures have a persistent asymmetry that can
terns in porphyroblasts (Sect. 7.6.8), tension gashes be used to determine sense of shear.
(Sect. 6.2), strain fringes and strain shadows (Sect. 6.3).
The main problem with these structures is that they con-
sist of several classes of geometries, which are superfi-

Fig. 5.49. Tensional and constrictional stepover sites in a mylonite.
Fig. 5.48. Tiling of feldspar grains in a granite deformed in the liq- The tensional site is filled with fibrous material (Sect. 6.2.5). The
uid state constrictional site shows microfolding of the foliation