Page 257 - Microtectonics

9.2 · Strain Gauges 249
inequant grains is thought to reflect the intensity of de- and hinges of folds as in some ptygmatic folds, or between
formation; the higher the strain, the more pronounced the centre and edge of boudins. If many deformed veins in
the preferred orientation becomes. This preferred orien- different orientations are present in a sample, they can be
tation can be measured by goniometer (Sect. 10.3.5). Some used to determine principal strain values, sense of shear
care should be taken since mineral grains are not passive (Sect. 5.6.12) and even the kinematic vorticity number
material lines; at low strain, the fabric in mica-rich ag- (Sect. 9.3.2). Care has to be taken, however, that area change
gregates may actually be stronger than that predicted by and volume change are not confused (Box 4.8) and that a
a model of passive rotation of material lines (Means et al. three-dimensional reconstruction of the strain is made
1984). This is probably due to folding of micas normal to wherever possible.
the shortening direction (Fig. 4.16(3)); material lines in An interesting method to determine principal stretch
this orientation will shorten but will hardly rotate values and thereby volume change was proposed by
(Fig. 4.16(1)). At high strain values, the fabric may be less Brandon et al. (1994) for sandstones which deformed by
intense than the theoretical prediction because micas do pressure solution and solution transfer, and where quartz
not stretch passively and rotate more slowly than passive grains with strain fringes show no effects of intracrys-
lines because of their low aspect ratio (Fig. 4.16(1, 3); talline deformation (Ring and Brandon 1999; Ring et al.
Means et al. 1984). Another point is that different miner- 2001). Thin sections are cut parallel to principal strain
als in a foliation may rotate at different velocities axes to show where fringes are developed (principal
(Kanagawa 1991). Fabric intensity in slates can therefore stretch > 1), and where grains have been dissolved (prin-
be used only as an approximate measure of strain (Eth- cipal stretch < 1). The mean original diameter of grains
eridge and Oertel 1979; Siddans 1977; Gapais and Brun can be determined in the direction of fringe growth since it
1981; Kanagawa 1991). The fabric of feldspar phenocrysts did not change in that direction. Principal stretches > 1 can
in granitoids may be more reliable (cf. Sect. 5.6.12). now be determined from the length of grains with their
Strain analysis is also possible using lattice preferred fringes, divided by their mean undeformed diameter; prin-
orientation patterns (LPO patterns). LPO patterns cannot cipal stretches < 1 are found from the mean diameter of
be used to determine individual strain ratios or volume dissolved grains in the direction of that principal strain
change, but carry information on the shape of the strain axis, divided by the mean undeformed diameter.
ellipsoid; the LPO pattern in a single thin section can give A method to determine finite strain in shear zones was
information on the full three-dimensional strain geometry devised by Ramsay and Graham (1970). They showed that
(Fig. 4.41; Lister and Hobbs 1980; Wenk 1985). An LPO pat- the curvature of a foliation into a shear zone reflects a
tern is easily re-equilibrated, however, and may reflect only gradient in finite strain from its peak in the core of a shear
the last part of the deformation history (Law 1990). Com- zone outwards; it develops because foliations rotate from
parison of the LPO pattern with another method of strain a position between the instantaneous extension axis and
analysis in the same rock may help to decide if strain ge- the fabric attractor towards the latter with increasing
ometry changed during the deformation history. strain (Sect. 5.5.3). The orientation of the foliation can be
One disadvantage of the determination of strain ra- used as a strain gauge, since the angle between the folia-
tios as described above is that even if ratios in three per- tion and the shear zone margin diminishes systematically
pendicular directions are known, the geometry of the strain with increasing strain. However, it is a function of W and
k
ellipsoid can be determined, but not its size. This means A of flow as well (Sect. 2.5.2). Only if W and A can be
k
k
k
that volume change cannot be determined in this way. estimated, e.g. if flow in a shear zone was by simple shear
Volume change is difficult to measure in rocks. Most tech- (W =1, A = 0), is it possible to calculate principal stretch
k
k
niques compare the chemistry of undeformed and de- values at any site in the zone from the orientation of the
formed rock volumes, assuming that they were originally foliation. The total displacement over the shear zone can
identical and that the undeformed volume did not change also be determined by integration of the strain profile
its composition (e.g. Mancktelow 1994). Unfortunately, (Ramsay and Graham 1970; Ramsay and Huber 1983). In
these assumptions are not always valid (Sect. 4.2.9.3). Al- practice, this method is reliable only at relatively low
ternatively, volume change can be determined if stretches strains. At high strain values the angles become very small
in several directions or a combination of stretches and and difficult to measure accurately.
strain ratios can be measured. Examples of structures that Finally, localised intragranular deformation can be used
can be used to determine stretch values in thin section to measure strain. Deformation bands in quartz grains can
are deformed microveins (Sect. 6.2), strain fringes be used to determine 3D strain using sections in several
(Sect. 6.3) and microboudinage of crystals (Sect. 6.6). Sets orientations and a U-stage (Wu and Groshong 1991b).
of folded or boudinaged microveins can be used to deter- Mechanical twins in calcite and other minerals record
mine stretch values provided that layer parallel shorten- small strains, and if twinning is the only mechanism of
ing or extension in the veins is minimal. This is the case if deformation, strain orientation and magnitude can be
little difference in layer thickness exists between limbs estimated in weakly deformed rocks with up to 15% strain