Page 264 - Microtectonics

256 9 · Natural Microgauges
9.6.3 9.6.4
Twins in Calcite and Dolomite Twins in Pyroxenes

The number and volume of twins in calcite has been pro- Deformation twins in clinopyroxene may also be used
posed as a microgauge for differential stress (Jamison as palaeopiezometers (Tullis 1980; Kollé and Blacic 1982;
and Spang 1976; Laurent et al. 1990) and has also been Orzol et al. 2003). The critical resolved shear stress for
calibrated experimentally (Rowe and Rutter 1990; Burk- twinning has been determined as 140–150 MPa (Kollé
hard 1993; Ferrill 1998). These methods can give a rough and Blacic 1982; Orzol et al. 2003). Twinning in clino-
estimation of differential stress in a rock, but only if dif- pyroxene is probably restricted to low temperature and
ferent methods are used for calcite deformed at high and high strain rate (Godard and van Roermund 1995;
low temperature (Ferrill 1998). Laurent et al. 2000). A practical application is demon-
An alternative but more elaborate method is the in- strated by Küster and Stöckhert (1999) and Trepmann
verse Etchecopar analysis that combines slip data on a and Stöckhert (2001).
large number of measured twin planes to calculate dif-
ferential stress and stress orientation in a way similar to 9.6.5
that applied to brittle faults (Angelier 1984, 1989; Etche- Microboudins
copar 1984; Lacombe and Laurent 1996; Ferrill 1998;
Laurent et al. 2000). Flow of a ductile medium around an elongate rigid ob-
All methods are based on the fact, that twinning only ject can produce an internal stress field that can lead to
takes place if the critical resolved shear stress (CRSS) on tensile fracturing and boudinage (Figs. 6.32, 6.33).
potential twin planes is exceeded. The CRSS for calcite is Microboudins can be used to calculate differential stress
now estimated to vary between 5–15 MPa (Jamison and based on the fibre-loading principle (Lloyd and Ferguson
Spang 1976; Lacombe and Laurent 1996; Laurent et al. 1981; Lloyd et al. 1982; Ji and Zhao 1994). A rigid fibre
2000) (Sect. 3.12.3); it may decrease slightly with increas- parallel to the extension direction in ductile coaxial flow
ing temperature and it may increase with increasing fi- is though to experience a gradient in tensile stress that is
nite strain (Laurent et al. 2000). Grain size and porosity maximum at the centre. Consequently, the fibre will break
may also play a role (Rowe and Rutter 1990; Ferrill 1998). near the centre and this process will be repeated in the
Small crystals seem to have fewer twins than large grains remaining fragments until some critical limit is reached
(Schmid 1982; Newman 1994). Differential stress is twice that depends on mechanical properties of the fibre and
the maximum shear stress at any point, and for a differ- on differential stress. The aspect ratio of microboudin
ential stress of 10–30 MPa, twins can only form in the fragments can therefore be used to estimate differential
direction parallel to the plane of maximum resolved stress (Masuda and Kuriyama 1988; Masuda et al. 1989,
shear stress. If differential stress is larger, twins can also 1990, 2003, 2004; Ji and Zhao 1993). The principle can be
form in other orientations. In this way, the size and sharp- used in any rigid mineral, including feldspar at low-grade
ness of the principal shortening direction maximum, metamorphic conditions (White et al. 1980). The pres-
determined for a population of twins, is a function of ence of tensional fractures (without a component of shear
the differential stress (Jamison and Spang 1976; Laurent movement) is also useful because it restricts the possi-
et al. 1990). The methods are also applicable to dolomite, ble magnitude of the differential stress to four times the
where the critical resolved shear stress for twinning is tensile strength of the material under consideration (Eth-
expected to be higher than in calcite (Jamison and Spang eridge 1983). The fibre-loading principle explains why
1976; Rowe and Rutter 1990; Newman 1994). some gaps between fragments are wider than others; the
Unfortunately, the twin palaeopiezometer techniques wider ones are thought to have formed earlier (Ferguson
rely heavily on the assumption of homogenous stress 1981, 1987; Masuda and Kuriyama 1988).
distribution in a sample, which is probably not realistic. Although not strictly a boudinage mechanism, the
Inhomogeneous stress distribution can have a similar extent of fracturing in rigid grains like garnet at low de-
effect on the spread in orientation of inferred principal formation temperature may also carry information on
stress directions as differential stress magnitude. Finally, local differential stress.
since twin density increases with increasing strain, the
twin palaeopiezometer is likely to give best results at very 9.6.6
low strain values and in non-porous and relatively even- Deformation Lamellae
grained and coarse calcite and dolomite without an LPO
(Burkhard 1993; Newman 1994; Ferrill 1998). It should Deformation lamellae have been proposed as palaeo-
be applied with care (Burkhard 1993) and not be used piezometers (Drury and Humphreys 1987; Blenkinsop
for strongly deformed rocks. and Drury 1988; Trepmann and Stöckhert 2003). Subbasal