Page 256 - Microtectonics

248 9 · Natural Microgauges
9.1 9.1 YZ-, XY-and XZ-sections. In thin section, two-dimen-
Introduction sional finite strain can be completely described by three
numbers representing the orientation of the strain el-
Structural geologists have long used the macroscopic and lipse and strain magnitude (Sect. 2.6). Strain magnitude
microscopic geometry of the fabric of deformed rocks can be expressed as two principal stretch values, or a
to determine a sequence of tectonic and metamorphic strain ratio and area change. Methods to measure strain
events, finite strain, or sense of shear. However, deformed are mentioned in most structural geology textbooks.
rocks store a wealth of quantitative information that can Below, we give an outline of some methods that can be
be retrieved from the characteristic geometry of macro- applied in thin section to allow the reader to determine
and microstructures. Some microstructures can be used whether his material is suitable for strain analysis, and
to determine parameters such as stress, temperature etc., to make the best choice between different methods.
and we therefore introduce the term natural microgauges In some cases, strain ratio can be measured directly
for such features. This chapter gives some examples of using objects with known original shape such as spheru-
presently available microgauges and possible future de- lites and oolites. However, it is important that the meas-
velopments, and will hopefully stimulate readers into ured objects have the same rheology as the matrix in
research on the subject. Microgauges can only be cali- which they lie; for example, feldspar grains in a quartz-
brated if the effects of all parameters that cause their ite at low metamorphic grade cannot be used since they
geometric evolution are understood. At present, the study are stronger than quartz and therefore show only part of
of microgauges is in its infancy and much theoretical and the tectonic strain. If objects were not initially spherical,
experimental work remains to be done. Finally, we try to the R /φ method can be used; the elliptic ratio of ob-
f
indicate the limits and problems of the methods because jects (R ) is plotted against their orientation (φ) and the
f
the creation of numbers from rocks tends to give a (pos- geometry of the resulting graphs is compared with stand-
sibly) misplaced sense of confidence. ard patterns to determine strain (Ramsay and Huber
1983; Robin and Torrance 1987; Mulchrone et al. 2003;
9.2 9.2 Meere and Mulchrone 2003).
Strain Gauges If the exact original shape of objects is not known,
but if the statistic mean is assumed to approach a spheri-
In many tectonic applications it is desirable to determine cal shape as for detrital grains in a quartzite, measure-
finite or tectonic strain. In order to express strain in sim- ment of the dimensions of a large number of grains can
ple values, it must be homogeneous (Sect. 2.5). In prac- be used to determine the strain geometry (Dayan 1981;
tice, natural strain will always be inhomogeneous at most Law et al. 1984). Care must be taken in this case that the
scales of observation, but it may be considered homoge- original outline of the grains can still be seen, that no
neous in small volumes of fine-grained rock with a ho- sedimentary shape fabric was present, and that only old,
mogeneous fabric (Fig. 2.4). The presence of a straight, non-recrystallised grains are measured. In a passively
homogeneous (continuous) foliation is a good indication deformed grain aggregate without grain boundary mi-
for homogeneous strain at the scale of a thin section. gration, strain can also be determined from the orienta-
For a full description of three-dimensional homoge- tion of the deformed grain boundaries using the method
neous finite strain, six numbers are needed; three to de- of Panozzo (1984) explained in Sect. 10.6.2.
scribe the orientation of the strain ellipsoid, and three If the shape of objects such as grains in a sandstone
to describe strain magnitude. Strain magnitude can be has been affected by pressure solution, the Fry method
expressed by three principal stretch values, or by two (Fry 1979) or centre-to-centre method (Erslev 1988) should
strain ratios and volume change. In the second case, the be used. In these methods, the distances between grain
two strain ratios describe the shape of the strain ellip- centres measured in different directions in a deformed
soid (as given in a Flinn plot, e.g. Fig. 4.41), and volume aggregate are compared; if these distances were statisti-
change describes its size. cally equal before deformation (as in a well sorted sand-
Three-dimensional strain can be determined when stone), strain can be calculated (McNaught 2002). In re-
data from several thin sections in different orientations crystallised quartzite, however, it may be dangerous to
are combined. It is important to cut these thin sections use such techniques since the present centres of grains
parallel to principal strain axes X, Y and Z, if possible. do not normally coincide with the original centres (Box 4.2).
Many foliations are approximately parallel to the XY- Foliations which are thought to have developed by
plane of finite strain (Sect. 4.2.9.2), and stretching or mechanical rotation of fabric elements can, in principle,
grain lineations (Sects. 4.1, 4.3) are generally parallel to X. be used to estimate strain ratios and the geometry of fi-
If thin sections are cut normal to the lineation, parallel nite strain (Oertel 1983, 1985). This applies to feldspar
to the foliation, and parallel to the lineation but normal phenocrysts in a granite and mica grains in a slate (Oertel
to the foliation, this will approximately correspond to 1983, 1985). The degree of preferred orientation of