Page 138 - Microtectonics

5.6 · Microscopic Shear Sense Indicators in Mylonite 127
asymmetry we mean that the object itself has an asym- (Fig. 5.10a; Ramsay and Graham 1970; Simpson and
metry without the need to involve other fabric elements Schmid 1983). This external asymmetry is due to rota-
or a reference frame; with external asymmetry we mean tion of the foliation towards the fabric attractor in non-
that the orientation of the object with respect to other coaxial flow, and has the same external asymmetry as
fabric elements determines its asymmetry. This section the foliation gradient discussed in Sect. 5.5.2.
describes the most common monoclinic microstructures, Another type of foliation in mylonites is defined by
and explains how sense of shear can be derived from them. grain shape preferred orientations within monominer-
In order to determine shear sense in a mylonite zone, alic domains (Box 4.2). Aggregates of small grains in
thin sections should be properly oriented. Sections nor- mylonites (usually formed by dynamic recrystallisation)
mal to the symmetry axis of shear sense markers give best can be characterised by a slightly elongate shape of most
results. This symmetry axis will be near the vorticity vec- of the grains (Means 1981; Lister and Snoke 1984). This
tor for flow in the shear zone. The plane normal to the grain shape preferred orientation is usually oblique to
vorticity vector is known as the vorticity profile plane compositional layering or mica-preferred orientation in
(VPP). In most mylonite zones, the VPP lies normal to a mylonite (Fig. 5.10f). The relation between shear sense
the intermediate strain axis. This means that a hand speci- and geometry of such an oblique foliation is as shown
men should be sectioned parallel to the VPP, i.e. parallel schematically in Figs. B.4.2, 4.31c and 5.10f (see also
to the aggregate or grain lineation and normal to the com- Box 4.2). Examples are shown in Figs. 5.22, 5.24, 5.31.
positional layering or main foliation (Fig. 5.10). Problems Oblique foliations have been reported for quartz (Brunel
may arise when several lineations and foliations are 1980; Law et al. 1984; Law 1998; Knipe and Law 1987;
present: this is usually due to overprinting of several de- Lister and Snoke 1984; Dell’Angelo and Tullis 1989), car-
formation phases. In this case, it will be necessary to re- bonates (Schmid et al. 1987; de Bresser 1989; Barnhoorn
construct the deformation sequence first, then to decide et al. 2004), olivine (van der Wal et al. 1992; Zhang and
which phase is of interest: usually, sense of shear can only Karato 1995) and rock analogues such as ice (Burg et al.
be determined for the last phase or phases. Obviously, only 1986), octachloropropane (Jessell 1986; Ree 1991) and
oriented samples should be used to determine shear sense. norcamphor (Herwegh et al. 1997; Herwegh and Handy
In crustal mylonite zones, which developed under low- 1996, 1998). They are assumed to develop by an interplay
to medium-grade metamorphic conditions, a large of passive deformation and rotation of grains in non-
number of sense-of-shear markers are available, and coaxial flow resulting in increasingly elongate grains, and
most are empirically established. (Reviews in Bouchez processes such as grain boundary migration and fractur-
et al. 1983; Simpson and Schmid 1983; Passchier 1986a; ing or microshearzone development which produce more
Hanmer and Passchier 1991). The most important ones equidimensional grain shapes (Fig. 4.31c; Means 1981; Ree
visible in thin section are shown schematically in Fig. 5.10 1991; Herwegh and Handy 1998). In this way, the foliation
and are discussed below. will remain fixed in orientation with respect to the kin-
ematic frame of progressive deformation, usually at an
All observations are for sections parallel to the ag- angle of 20–40° to the fabric attractor (Dell’Angelo and
gregate lineation and normal to the foliation. In all Tullis 1989; Ree 1991; Fig. 4.31c). The actual angle prob-
other sections the structures either show a less pro- ably depends on the vorticity number of flow, the recrys-
nounced monoclinic symmetry, or orthorhombic and tallisation mechanism and on the efficiency of fabric
higher symmetry. developing and fabric destroying processes (Hanmer
1984a; Herwegh and Handy 1998). For olivine, an alter-
native mechanism of kinking and grain boundary mi-
5.6.2 gration has been proposed (van der Wal et al. 1992). Such
Foliation Orientation foliations are therefore to some extent strain-insensitive.
Oblique foliation occurs mostly as a grain shape pre-
Many shear zones do not show foliation curvature ferred orientation (Box 4.2) in monomineralic layers of
(Sect. 5.5.3) as in Fig. 5.10a, but have sharp boundaries quartz or calcite in layered low- to medium-grade mylo-
with the wall rock. The mylonite in the shear zone can nites (Fig. 5.31); examples of polymineralic strain-insen-
have several foliations, which make a small angle with sitive foliations are less common, and occur mainly in
the wall rock and with each other. Such foliations can be medium- to high-grade mylonites (Hanmer and Passchier
mica-preferred orientation, a layering or a shape pre- 1991); an example is the mica-preferred orientation (S ;
m
ferred orientation. If they develop during mylonite gen- Fig. 5.24) oblique to the mylonitic compositional layer-
esis, they can be good shear sense indicators. Mica pre- ing observed in some micaceous mylonites (Passchier
ferred orientations, and aggregate shape preferred ori- 1982a). In ultramylonites such an oblique foliation is
entation (Box 4.2) are ‘passive’ foliations and are com- common and visible under crossed polars as a preferen-
monly slightly oblique to the shear zone boundary. tial extinction of the matrix at an angle of less than 5° to