Page 156 - Microtectonics

5.6 · Microscopic Shear Sense Indicators in Mylonite 145

Box 5.5 Misinterpretation of fault rocks
Misinterpretation of fault rocks is relatively common. It is there- Foliated cataclasite can also strongly resemble low-grade mylo-
fore important to pay maximum attention to certain details that re- nite. Both have foliations and asymmetric structures such as por-
veal metamorphic conditions and strain rates. In this box, we present phyroclasts, mica fish and shear bands that can act as shear sense
a number of common errors and misconceptions and clues to avoid indicators. Foliated cataclasite is associated with polished fault
them. In thin section, the nature of fault rocks can usually be estab- planes (slickensides) and striations or fibres while low-grade my-
lished without problems, unless there is overprint of one type on lonites have penetrative aggregate lineations. In foliated cataclasite,
the other. In the field, however, the situation is more difficult. quartz is deformed to a fine fractured aggregate. In mylonite, quartz
Dark fine-grained rocks in a shear zone can be ultramylonite, shows evidence for dynamic (usually BLG) recrystallisation.
cataclasite or pseudotachylyte. A planar dark vein in a low poros- In high-grade gneisses, it is difficult to recognise ductile shear
ity rock such as a granite or gneiss with sharp boundaries to the zones. Usually, all rocks are deformed to some extent, and the dif-
wall rock and with injection veins is probably a pseudotachylyte. ference between deformed and more deformed is small. High
Characteristic of pseudotachylyte is also colour banding (Sect. 5.2.5) grade mylonites have stronger fabrics than their wall rocks, strong
with different coloured core and edges, which extend into the in- shape preferred orientations, grain lineations of constant orien-
jection veins. tation and often isoclinal folds. Shear sense indicators are, unfor-
If the dark fault material has a gradual transition to the wall tunately, rare in high-grade ductile shear zones.
rock, and occurs in an anastomosing geometry, or if it occurs in a Faults that overprint each other within a reactivated shear zone
sedimentary rock, it is more likely to be a cataclasite. An ultramy- are common; most common are brittle faults overprinting mylo-
lonite can have sharp boundaries as well, but lacks injection veins nites and occasionally mylonites overprinting a brittle fault rock.
and colour banding. Characteristic is a good aggregate lineation Different overprinting phases of brittle fault rocks can sometimes
and planar shape preferred orientation but this only develops in be recognised if they differ in direction of slip, and have striations
ultramylonite that is inhomogeneous, not in massive, homogene- or fibres in two superposed populations of different direction. My-
ous fine-grained ultramylonite (Box 4.4; Sect. 5.3). In thin section, lonites overprinting mylonites during a later deformation event can
the nature of vein-wall rock contacts and porphyroclasts can help be hard to distinguish. However, if they differ in metamorphic grade,
to make a distinction. Sharp boundaries with corrosion of micas low-grade mylonites tend to occupy a smaller rock volume than
are typical for pseudotachylyte; ultramylonite and cataclasite have high grade ones, so that the branches of low-grade mylonite will
less sharp boundaries usually with recrystallisation in the adja- separate lenses of high-grade mylonite. These are easy to distin-
cent large grains for ultramylonite, and fracturing and numerous guish if the shape preferred orientation in both mylonites has a
fluid and solid inclusions in adjacent grains for cataclasite. different orientation, especially the lineations; if two object line-
Coarser grained mylonite with porphyroclasts is occasionally con- ations of different orientation are found in one mylonite zone this
fused with porphyry, rhyolitic ignimbrite or even with some finely could be an indication for transpression (Passchier 1998), but it is
layered sedimentary rock. In such cases, mylonites can be recognised more likely that the zone formed in two phases. However, if these
in the field by the presence of aggregate lineations and in thin sec- are parallel, the two generations of mylonite can only be distin-
tion by the presence of mantled porphyroclasts, mica fish and by a guished by the microstructure, e.g. the brittle or ductile behaviour
finely grained recrystallised matrix with an LPO in quartz domains. of feldspar and the development of aggregate or grain lineations.

Fig. B.5.7. Mylonite zones may have several phases of activity. This image shows two shear zones with polyphase deformation. a An
older mylonite (I) is cut by a younger narrow shear zone (II) in which the lineation has a different orientation than in (I). Because of
this, the two phases can be distinguished in the field. b The younger low-grade mylonite (II) has a lineation with the same orientation
as high-grade mylonite (I). In this case, it is more difficult to distinguish between the two phases in the field, but the microstructure
can be used to resolve this problem. (I) was active at high-grade conditions, visible from ductile deformation of feldspar and GBM-
recrystallisation of quartz. (II) was active at low-grade conditions since feldspar is brittle and recrystallised, and quartz recrystallised
by SGR-recrystallisation. Grain size of quartz exaggerated