Page 263 - Microtectonics

9.6 · Differential Stress Gauges (Palaeopiezometers) 255

Fig. 9.6. Relationship between recrystallised grain size and differ-
ential stress, expressed as flow stress for feldspar, olivine and cal-
cite. Data from indicated papers

Fig. 9.5. Relationship between recrystallised grain size and differ- cases, even stress differences in a single aggregate can be
ential stress, expressed as flow stress for quartz. BLG, SGR and GBM determined, such as near rigid porphyroclasts that can
are mechanisms of dynamic recrystallisation (after Stipp and Tullis
2003). Top part of graph based on unpublished data from Bishop cause a local increase in differential stress.
(1996) as quoted in Post and Tullis (1999). The older graph of Twiss Possible sources of error in the calculation of palaeo-
(1977) is shown for reference stress are:
Most available data on stable grain size and associated 1. the presence of old grain relicts that may be smaller
differential stress are related to quartz (Fig. 9.5; Twiss or (usually) larger than the recrystallised ones (Michi-
1977; Post 1977; Ross et al. 1980; Michibayashi 1993; Stipp bayashi 1993). Old grain relicts are recognisable by
and Tullis 2003) but there are also data for other minerals their irregular outline, deviant crystallographic ori-
(Fig. 9.6). Examples shown are for olivine (Karato 1984; entation and well-developed intracrystalline deforma-
van der Wal et al. 1993; Jung and Karato 2001a,b), calcite tion structures;
(Schmid et al. 1980; Rutter 1995; Barnhoorn et al. 2004) 2. misinterpretation of the active recrystallisation mecha-
and feldspar (Post and Tullis 1999). In olivine, water con- nism (Post and Tullis 1999);
tent may influence the size of recrystallised grains by 3. the presence of a second mineral that inhibits growth
GBM (Jung and Karato 2001a,b). Estimates for differen- of the mineral to be measured, e.g. mica in quartzite
tial stress in rocks based on grain-size palaeopiezometers (Krabbendam et al. 2003), or that causes local stress
range from a few MPa in high temperature deformation concentrations or strain shadows; therefore, only
to 100–300 MPa in some low-temperature mylonite zones monomineralic aggregates should be used, or interpre-
(Küster and Stöckhert 1999). tation should not be extended beyond the boundaries
Grain size reduction in mylonites is due to the fact of a monomineralic domain in a polymineralic rock;
that differential stress in active ductile shear zones can 4. static recrystallisation that may have affected grain
be high, especially at low temperature; consequently the size. Static recrystallisation can be recognised by the
stable recrystallised grain size is small. However, ex- presence of straight, polygonal grain boundaries and
tremely fine-grained rocks such as cherts may undergo ‘strain-free’ recrystallised grains (Box 3.10; compare
grain growth during dynamic recrystallisation in a shear Fig. 3.31 with Fig. 3.41), and evidence for grain bound-
zone to reach the stable grain size (Masuda and Fujimura ary adjustment to euhedral shape in strongly aniso-
1981). De Bresser et al. (2001) suggest that dynamic re- tropic crystals such as micas; these will show evidence
crystallisation leads to a balance between grain size re- for static recrystallisation before quartz and feldspar
duction and growth processes, set up in the neighbour- do. Static recrystallisation is commonly associated
hood of the boundary between the dislocation creep field with hydration and retrograde transformation of min-
and the (grain size sensitive) diffusion creep field. Fig- eral assemblages;
ures 3.30 and 3.31 show examples of typical fabrics in 5. the use of a piezometric relationship that has not fully
quartz mylonites where differential stress can be esti- been tested for possible effects of recrystallisation
mated from dynamically recrystallised grain size. In some mechanisms, influence of water or role of temperature.