Page 115 - Microtectonics

4.4 · Lattice-Preferred Orientation (LPO) 103
and may even ‘overtake’ that of other slip systems. At low
differential stress, only one slip system may be active,
but at higher differential stress, several slip systems can
operate simultaneously. In fact, for maintenance of co-
hesion between grains, five independent slip systems
should be operating (Lister 1977). In silicates, however,
which usually have low crystal symmetry, fewer slip sys-
tems are active and space problems are accommodated
at low temperature by lattice bending, kinking, fractur-
ing and, at high temperature, by dynamic recrystallisa-
tion or grain boundary sliding.
The type of LPO pattern that is formed in a rock de-
pends on many factors, the most important of which are
(Schmid 1994):

1. The slip systems that are operating and the amount of
activity on each slip system.
2. The ratio of stretching rates along the ISA of the flow,
i.e. plane strain, flattening or constrictional flow. These
rates determine in which direction crystals rotate and
thereby the shape of the fabric (Fig. 4.41).
3. The finite strain. Usually, if the flow pattern does not
change during deformation, the LPO pattern increases
in strength and sharpness with increasing strain but
undergoes only slight changes in geometry (Sect. 4.4.4.2).
4. The kinematic vorticity number. In initially isotropic
materials, non-coaxial progressive deformation leads
to LPO patterns with monoclinic symmetry, and co-
axial progressive deformation to patterns with higher
symmetry.
5. The activity of bulging and grain boundary migra-
tion dynamic recrystallisation. Recrystallisation may
influence an LPO pattern in several ways but the ef-
fect is difficult to predict; it may weaken an existing
pattern by generation of new, randomly oriented
grains; or it may strengthen a pattern or part of a pat-
tern by removing (consuming) certain grains with a
relatively high dislocation density. Grains that are
unfavourably oriented for slip may be removed by this
process if they developed a high dislocation density
because of constriction by neighbours (Jessell 1987;
Ree 1990). However, the reverse is also possible; such
grains may have low dislocation density, since all
deformation is taken up in softer neighbours, and
therefore consume grains favourably oriented for slip
(Gleason et al. 1993). Evidence for both processes has
been found in experiments. Static recrystallisation
may also affect LPO patterns, but the effect is uncer-
Fig. 4.39. a Reorientation of a pile of books by slip: an axis normal to tain (Humphreys and Hatherley 1995; Heilbronner
the books (bold line) rotates towards the direction of gravity. Develop- and Tullis 2002; Park et al. 2001).
ment of LPO in crystals due to dislocation glide on slip systems oper- 6. Growth of grains from solution. The growth rate in
ates in a similar way. b Flattening of an aggregate of crystals with a many minerals is dependent on crystallographic di-
single slip system normal to a crystallographic axis (bold line). c All crys-
tal axes rotate towards the compression direction except those parallel rection, and growth of minerals from solution can
or normal to this direction. Those parallel to the compression direc- therefore produce a preferred orientation (Shelley 1979,
tion may deform by kinking or twinning with rotation of the segments 1989, 1994).