Page 75 - Microtectonics

3.14 · Flow Laws and Deformation Mechanism Maps 63
concentrated in the mantle, which grows in volume as the 2001; Mainprice et al. 2003). Some important and com-
core shrinks with progressive deformation (Dell’Angelo monly quoted types of flow laws are given in Box 3.11. In
and Tullis 1989). Diffusion processes may also play a role the equation for dislocation creep given here, strain rate
in the recrystallised mantles. In quartz, dislocation creep is independent of grain size but has a strong non-linear
is accommodated by dislocation climb and SGR recrys- (power law) dependence of strain rate on stress. In the
tallisation dominates. The new grains have the same dis- equations for diffusion creep, strain rate has a linear de-
location density as old subgrains, and the new aggregate pendence on stress but a non-linear dependence on grain
is equally strong as the old grains; consequently, no core size. Flow laws have been proposed on the basis of ex-
and mantle structure develops and quartz deforms rela- periments and theoretical considerations.
tively homogeneously. The parameters in the equations have been determined
experimentally for a range of conditions. In many cases,
3.13.3 these data are incomplete or difficult to compare because
Deformed Rhyolites – an Exception of differences in confining pressure, sample preparation
etc. However, if a suitable set of data on rheology of a par-
Some deformed rhyolites and ignimbrites are an inter- ticular mineral can be found, it is possible to integrate
esting exception to the rule that in quartz-feldspar ag- the data to determine which mechanisms are expected to
gregates, deformed at low- to medium-grade conditions, operate under particular conditions; in general, the
quartz is the weaker mineral. In these rocks, quartz mechanism that operates at the lowest differential stress
phenocrysts may survive as porphyroclasts (Figs. 3.7, 3.9, for a particular strain rate is thought to be dominant.
3.10; Williams and Burr 1994). Probably, the fine-grained Conditions at which specific deformation mechanisms are
polymineralic matrix of a rhyolite can deform by grain dominant can be shown in a deformation mechanism map.
boundary sliding, or pressure solution and precipitation Such a map shows fields in which certain deformation
at such a low differential stress that limited intracrystal- mechanisms are dominantly, although not exclusively, ac-
line deformation is induced in quartz. Deformed rhyolites tive. Also shown are projected curves for several strain rates,
and ignimbrites can be recognised by the presence of which give an indication of the relationship of stress and
euhedral to subhedral quartz phenocrysts with typical strain rate for a specific temperature. Cataclasis occurs only
wriggly embayments (Figs. 3.9, 3.10). Obviously, the be- above a certain differential stress level, which is dependent
haviour of quartz and feldspar in an aggregate is depend- on fluid pressure (Sibson 1977a) and temperature (Griggs
ent not only on external conditions, but also on the origi- et al. 1960). Since grain size plays a major role in deter-
nal geometry of the aggregate before deformation. mining which deformation mechanism will be active, sev-
eral maps for different grain sizes are usually given.
3.14 Figure 3.43 shows an example of a deformation 3.14
Flow Laws and Deformation Mechanism Maps mechanism map for quartz, and the way in which it is
constructed. Parameters that have been used are given
In order to establish under which conditions deforma- in the inset. Using the equations given in Box 3.11, graphs
tion mechanisms as described in this chapter are active, are first made which plot shear stress against tempera-
data from experimental deformation are used in combi- ture at given strain rates for each of the deformation
nation with observations on rocks deformed at known mechanisms (Fig. 3.43a–c). In such graphs, normalised
metamorphic conditions. Experimental deformation of units are plotted. This is done to obtain numbers that are
rocks at a range of pressure and temperature conditions dimensionless since this allows easy comparison of dif-
can give us some idea of the activity of deformation, recov- ferent materials (Box 2.5). For example, homologous tem-
ery and recrystallisation processes at specific conditions. perature T (T = T / T where T is the melting tempera-
m
h
h
m
One drawback of experimental work is that geologically ture of a mineral in K) is used on the horizontal scale
s can-
realistic strain rates in the order of 10 –12 to 10 –14 –1 instead of absolute temperature; T = 0 at 0 K and T =1
h h
not be reproduced in experiments. Nearly all our data on at the melting temperature of the mineral. In this way,
deformation mechanisms are from experiments at much deformation behaviour of ice can be compared with that
higher strain rate. However, for many deformation mecha- of steel if both are at the same T value. Similarly, nor-
h
nisms, increase in temperature has an effect similar to a malised shear stress (σ / µ) is used instead of shear stress.
decrease in strain rate. Therefore, extrapolation of experi- At any point in the graphs of Fig. 3.43a–c, a single strain
mental results to geologically realistic strain rates is pos- rate is defined at a certain stress and temperature if other
sible by ‘projection’ of data from experiments carried out parameters are constant. When these diagrams are com-
at higher temperature. bined in pairs (Fig. 3.43d,e), each point will be attributed
The rheological behaviour of minerals and rocks is two strain rate values, one for each of the possible defor-
usually expressed in flow laws (Poirier 1985; Hirth et al. mation mechanisms; the mechanism with the highest