Page 265 - Microtectonics

9.9 · Temperature Gauges 257
deformation lamellae in quartz are thought to develop systems that are active in a mineral and the strain rate
at differential stresses between 170–420 MPa. Spacing of (Lister et al. 1978); however, since other parameters (tem-
deformation lamellae may also serve as a palaeopiezo- perature, water activity) also influence the active slip sys-
meter (Koch and Christie 1981). tems and because it is not always possible to determine
which slip systems were active in natural rocks, this
9.7 method cannot (yet) be used. The irregularity of recrys- 9.7
Pressure Gauges tallised quartz grain boundaries increases with increas-
ing strain rate and decreasing temperature and may be
Lithostatic pressure is usually determined by classical calibrated, if temperature is known independently, as a
petrological methods such as mineral composition of the strain rate gauge (Takahashi et al. 1998).
rock and fluid inclusion density (Sect. 10.5). However, The shape of fibres and elongate crystals in veins and
microstructural assemblages may help where older min- fringes is strongly dependent on the relation between
eral assemblages or fluid inclusions have been destroyed. crystal growth rate and opening rate of the vein or fringe;
At low pressure, ductile and brittle calcite microstruc- although the processes involved in the development of
tures can be used as a gauge (Ferrill and Groshong 1993). these structures are still incompletely understood, it is
The presence of gas bubbles or amygdales (gas bubbles clear that they contain information on strain rate in the
filled with crystalline material) in pseudotachylyte indi- host rock (Sect. 6.2.3).
cates that the fault rock formed at shallow crustal levels, One of the most promising methods is direct isotopic
the exact magnitude of which depends on the composi- dating of strain fringe increments in quartz fringes
tion of the rock. Another promising development is the (Müller et al. 2000) and the growth and rotation rate of
recognition of deformed pseudotachylyte in many duc- porphyroblasts (Christensen et al. 1989; Ridley 1986;
tile shear zones. Pseudotachylyte is a brittle fault rock Joesten and Fischer 1988; Paterson and Tobisch 1992).
that mostly forms in the upper crust, and whose geom- However, even cm-size porphyroblasts may grow in less
etry is not easily destroyed by later deformation or meta- than 1 Ma under some circumstances (Sect. 7.2; Burton
morphism (Sect. 5.2.5).The presence of pseudotachylyte and O’Nions 1991; Paterson and Tobisch 1992), and this
depends on several factors but its presence carries infor- is out of range for present direct dating methods.
mation on lithostatic pressure in the rock during its de- δ- and Θ-type mantled porphyroclasts have been pro-
velopment (Sect. 5.2.5; Passchier et al. 1990a). posed as indicators of relatively high shear strain rate in
ultramylonite since they indicate rotation with limited
9.8 dynamic recrystallisation (Passchier and Simpson 1986; 9.8
Strain Rate Gauges Whitmeyer and Simpson 2003). If dynamic recrystalli-
sation rate is a function of differential stress, enhanced
Geological strain rates are estimated to lie usually be- rotation may indicate a high shear strain rate; however,
s (Pfiffner and Ramsay 1982;
tween 10 –13 and 10 –15 –1 other factors may play a role as well so development of
Carter and Tsenn 1987; Paterson and Tobisch 1992) and these structures should be further investigated.
could theoretically be estimated in rocks if differential In conclusion, estimates of strain rate are not yet ac-
stress and temperature of deformation are known, using curate in practice, but promising progress is being made.
known flow laws derived from experimental data
(Sect. 3.14). Differential stress values can be obtained 9.9 9.9
using a palaeopiezometer as discussed above. This Temperature Gauges
method to estimate strain rates has been applied to peri-
dotite using flow laws for olivine (Karato et al. 1986; Suhr Any experienced student of microtectonics is aware that
1993) and for crustal rocks using flow laws for quartz there is a correlation between metamorphic grade dur-
(e.g. Stipp et al. 2002; Trepmann and Stöckhert 2003) and ing deformation and the presence and geometry of par-
calcite (Ulrich et al. 2002). The discrepancies between ticular microstructures. Unfortunately, few of these struc-
results of different experiments give ‘error bars’ of one, tures have been calibrated to date; geometric tempera-
or even two orders of magnitude. Sources of error are in ture gauges could give independent data on temperature
the flow laws and their extrapolation to geological strain besides the classical petrological geothermometers and
rates, in the estimate of differential stress if grain growth may be less easily modified by retrogression and later
is inhibited or if recrystallisation is important, and in deformation than mineral composition.
temperature estimates. For olivine, an error of 50 °C in One of the most promising temperature gauges is twin
the temperature results in an error of one order of mag- geometry in calcite. At temperatures below 400 °C, crys-
nitude in the strain rates. Another possible method is tal-plastic deformation in calcite is mostly by mechani-
the use of LPO fabrics; a relation exists between the slip cal e-twinning (Groshong 1988). The geometry of such