Page 118 - Microtectonics

106 4 · Foliations, Lineations and Lattice Preferred Orientation
Fig. 4.41.
Flinn diagram showing the rela-
tion of geometry of LPO pat-
terns of quartz c-axes (grey con-
tours) and a-axes (striped orna-
ment) with strain in the case of
coaxial progressive deformation.
An inset shows the orientation
of principal strain axes in the
pole diagrams. Horizontal solid
lines in pole diagrams indicate
reference foliation. Dots indicate
reference lineation. (After Lister
and Hobbs 1980)

Fig. 4.42.
Pole diagrams showing four
types of contoured LPO patterns
of quartz c-axes (grey) and
a-axes (striped) such as develop
with increasing metamorphic
grade in non-coaxial progressive
deformation. The variation is due
to a change in the dominant slip
systems. Explanation in text

tributes mainly to c-axes in the periphery of the diagram, by a single <a>-axes maximum parallel to the movement
slip on prism planes to those in the centre, and slip on direction (the fabric attractor) and a single girdle of c-axes
rhomb planes between both (Fig. 4.43a). Type II crossed- normal to the flow plane (Figs. 4.42, 4.43b; Sect. 2.9). The
girdle c-axis patterns probably develop in constriction c-axes from the periphery to the centre of the girdle stem
when rhomb slip is dominant over prism slip (Bouchez from c-axes of grains deformed by basal, rhomb and prism
1978; Schmid and Casey 1986). slip respectively (Fig. 4.43). At low temperature, basal <a>
In non-coaxial progressive deformation, domains of slip is most important and the girdles may have a strong
material line rotation are not of equal size as in coaxial cluster of c-axes in the periphery. With increasing tem-
progressive deformation (Sect. 2.7). As a result, one of the perature, prism <a> slip becomes more important (Wilson
<a>-axes maxima is favoured and the c-axis patterns may 1975; Bouchez 1977; Lister and Dornsiepen 1982; Law
be similar to those in Fig. 4.41 but one part will be better 1990) and the girdle tends to a maximum around the
developed than the other. Consequently, the pattern of Y-axis (Figs. 4.42, 4.43b). At very high temperature and
<a>- and c-axes obtains a monoclinic symmetry. For ex- hydrous conditions, prism <c> slip operates (Lister and
ample, at high strain accumulated by simple shear at low Dornsiepen 1982; Blumenfeld et al. 1985; Mainprice et al.
to medium-grade metamorphic conditions, the Type I 1986), and causes a c-axis maximum subparallel to the
crossed girdle and double <a>-axes maxima are replaced attractor (Figs. 4.42, 4.43), and <a> axes normal to it.