Page 269 - Microtectonics

9.10 · Rheology Gauges 261
LPO patterns can in many cases be used to determine
the active slip systems in crystals and indirectly tempera-
ture (Sects. 4.4.4, 4.4.5). The opening angle of quartz
c-axis fabrics has been proposed as a temperature gauge
with an accuracy of ±50 °C (Kruhl 1998; Law et al. 2004).
In the case of plane strain deformation, the angle between
girdles measured in the plane parallel to the lineation
and normal to the foliation shows a linear dependence
on temperature for most of its range (Fig. 9.12).
The ratio of the volume of clasts to melt-matrix in
pseudotachylyte can be used as a geothermometer to
estimate the initial temperature of the wall rock during
pseudotachylyte generation (O’Hara 2001).Other poten-
tial temperature gauges are the presence of flame-
perthite in K-feldspar and myrmekite in plagioclase Fig. 9.12. Effect of temperature on the opening angle of small-cir-
(Sects. 3.12.4, 7.8.3; Simpson and Wintsch 1989; Pryer cles of quartz-c-axes in an LPO diagram. (After Kruhl 1998 and
1993) and the geometry of foliations (Sect. 4.2.9.4). Law et al. 2004)

9.10 ness and viscosity contrast between the layer and the 9.10
Rheology Gauges matrix. The wavelength measured along the surface of a
layer can therefore be used to determine absolute vis-
9.10.1 cosity contrasts (e.g. Ramsay and Huber 1983). Boudins
Introduction and mullions can be used to the same purpose (Kenis
et al. 2004), but although buckle folding and boudinage
Deformation experiments have taught us much about the in single layers both occur in response to a viscosity con-
rheology of rocks under a range of metamorphic condi- trast, there is a difference; folding can occur in Newtonian
tions, but it would be useful to have more direct control and non-Newtonian materials, while boudinage is re-
from deformed rocks. There are indications that some stricted to non-Newtonian materials (Sect. 2.12; Neurath
fabric elements, which might be called rheology gauges, and Smith 1982).
can be used to determine rheological constants (Sects. 2.12,
3.14). Rheology gauges are a promising new subject in 9.10.3
geology, but few structures have been tested in practice Fish and Mantled Objects
yet. We restrict ourselves here to outline the potential of
methods with published examples. Rheology gauges can Fish and mantled porphyroclasts can have a complex
only be further developed with much theoretical work and shape, which depends on interaction with the deforming
experimental deformation of rocks and rock analogues. surrounding matrix (Sect. 5.6; Box 5.4). If metamorphic
conditions of deformation are known, it should be possi-
9.10.2 ble to derive information on the rheology of the matrix
Folding and Boudinage and possibly on strain rate from such assemblages
(Passchier et al. 1993; Pennacchioni et al. 2000; Bose and
The wavelength of buckle folds in isolated competent Marques 2004). However, much further research is needed
layers in a less competent matrix depends on layer thick- before rigid objects can be used as rheology gauges.