Page 133 - Microtectonics

122 5 · Shear Zones

Fig. 5.10. Schematic diagram showing the geometry of a mylonite zone and the nomenclature used. For thin sections parallel to the aggre-
gate lineation, the most common types of shear sense indicators are shown. Further explanation in text. This figure is schematic and does
not show all possible geometries. Other figures in this chapter show more detail

Figs. 5.7, 5.8) and rocks with over 90% matrix as ultra- 5.3.4
mylonites (Fig. 5.9 at left). The problem with this classifi- Dynamics of Mylonite Development
cation is that an arbitrary limit has to be defined between
matrix grain size and porphyroclast grain size. Another The relatively high finite strain values reached in mylo-
problem is that mylonites developed at high metamor- nites imply that strain rate in the mylonite zone must
phic grade or in fine-grained or monomineralic parent have exceeded that in the wall rock for some time, and
rocks do not normally develop porphyroclasts; for this that the material in the zone must have been ‘softer’ than
reason, ultramylonite does not necessarily represent a the wall rock. Nevertheless, many mylonites have the
higher strain than mylonite or protomylonite. Other com- same chemical and mineral composition as the wall rock.
monly used terminology is blastomylonite for a mylonite Apparently, changes occur in the rheology of material in
with significant static recrystallisation and phyllonite for a ductile shear zone after its nucleation. This effect is
a fine-grained mica-rich mylonite (resembling a phyllite). known as softening or strain-softening (Sect. 2.12). The
Some authors use the term phyllonite as a synonym for most important mechanisms that contribute to soften-
ultramylonite. ing are (White et al. 1980; Tullis et al. 1990):