Page 261 - Microtectonics

9.5 · Gauges for the Orientation of Palaeostress Principal Axes 253
9.4 and low strain. In all cases, there will be a number of grains 9.4
The Concept of Palaeostress Gauges that do not fit in the determined directions of palaeostress
axes (Groshong et al. 1984; Pfiffner and Burkhard 1987).
The present state of stress in the lithosphere can be meas- The percentage of deviating grains can be taken as a meas-
ured directly in the crust (Harper and Szymanski 1991) or ure of the reliability of the method in any particular case.
assessed at greater depth using seismological or heat flow Attribution of the deviating grains to other phases of de-
data (Molnar and England 1990). For states of stress in the formation with a different orientation of the stress field is
geological past, microstructures can be used. Although sev- probably not realistic. Methods of determination of palaeo-
eral methods have been proposed to determine the ori- stress orientation using twins have also been proposed
entation and magnitude of stress in a rock during defor- for dolomite (Christie 1958), pyroxene (Raleigh and Talbot
mation (also referred to as palaeostress), it is important 1967; Trepmann and Stöckhert 2001), olivine (Carter and
to realise that stress is not preserved, only its effects. Moreo- Raleigh 1969) and plagioclase (Lawrence 1970).
ver, stress is only defined at a point, and usually varies in
magnitude and orientation from point to point in a rock, 9.5.2
and also changes strongly with time. Stress leaves traces Fractures and Fluid Inclusion Planes
in a rock only when permanent deformation is realised
and stress gauges are claimed to capture the properties Tensional fractures that lack displacement probably form
of the stress field during this time. However, in inhomo- parallel to the local σ –σ -plane of stress (Sects. 2.11, 3.2,
2
1
geneous materials such as rocks on the grain scale, stress 6.1). Although this orientation of stress in a crystal can be in-
was probably strongly variable from crystal to crystal and fluenced by crystallographic orientation, tensional fractures
even within individual crystals, and must have changed that cut several grains are probably close to the bulk stress
with time. Probably, stress gauges measure some kind of orientation. Tensional fractures, however, can form late dur-
‘mean value’ for stress and it is important to remain criti- ing the deformation history, mostly close to the surface or
cal about the meaningfulness of such mean values. even during sampling or thin section preparation and are
Microgauges to measure ‘palaeostress’ in deforming rocks therefore difficult to date. Planes of fluid inclusions are more
are divided into three main types; 1. gauges for the orien- reliable in this sense, since they form at some depth and the
tation of the principal stress axes; 2. gauges for differen- density and composition of fluid inclusions can give some
tial stress, and; 3. gauges for mean stress or pressure. indication of P-T conditions of their development (Sect. 10.5).
Also, the sequence of cross-cutting fluid inclusion planes
9.5 can usually be determined. In principle, planes of fluid in- 9.5
Gauges for the Orientation clusions can be used to determine the changes in orienta-
of Palaeostress Principal Axes tion of the σ –σ -plane in geological history and to link it
1
2
with metamorphic conditions (e.g. Boullier et al. 1991).
9.5.1
Twins in Calcite and Other Minerals 9.5.3
Deformation Lamellae
Calcite e-twins ({0112}) have been proposed as a tool for
determination of the orientation of palaeostress principal Quartz deformation lamellae (Box 3.3) have been inter-
axes (Turner 1953; Laurent et al. 1981; Dietrich and Song preted as planes of high resolved shear stress and can
1984; Borradaile and McArthur 1990; Shelley 1992; Burk- therefore be used to determine palaeostress directions
hard 1993; Marrett and Peacock 1999; Nemcok et al. 1999; in a way similar to calcite e-twins (Carter and Raleigh
Laurent et al. 2000; Fry 2001). Since movement on twins can 1969; Law 1990; Twiss and Moores 1992). Since calcite e-
take place in only one direction, data on the presence of twins can only accommodate movement in a single di-
twins, the orientation of twins and crystallographic c-axes rection, they are probably more reliable as microgauges
in a large number of grains can give an average value of than quartz deformation lamellae. The same objections
the orientation of principal shortening and extension di- and restrictions to application of the method mentioned
rections. Therefore, the quantity that is really measured for calcite twins are valid for deformation lamellae.
is strain accommodated by the twins. Only in isotropic
materials that undergo coaxial deformation will the ori- 9.5.4
entation of stress and strain axes coincide. Hence, calcite Flame Perthite
twins can only be used to measure the orientation of prin-
cipal axes of palaeostress in rocks that deform coaxially. Flame perthite occurs in K-feldspar and plagioclase bear-
Only straight twins can be used, which restricts use of ing rocks deformed at greenschist facies conditions at high
the method to low temperature deformation (Sect. 9.9) differential stress, especially in ductile shear zones (Fig. 7.28;