Page 136 - Microtectonics

5.4 · Complex Fault Rocks 125
grained, striped gneiss (Fig. 5.11, ×Photo 5.11). In grani- 1998) but can be difficult to recognise if the ductile over-
toid rocks, monocrystalline quartz ribbons and polycrys- print is strong. Some pseudotachylyte veins have under-
talline feldspar ribbons are common in such striped gone ductile deformation after their solidification; these
gneisses (Fig. 5.12). Some high grade mylonites contain are easier to recognise, even after strong ductile over-
elongate porphyroclasts of pyroxene or garnet (Hanmer print. In fact, pseudotachylyte veins seem to act as the
2000) which form by intracrystalline deformation (Chap. 3) preferred nucleation sites of mylonite zones in many lo-
and possibly by fracturing of kinked pyroxene crystals cations (Fig. 5.4b; Allen 1979; Sibson 1980; Passchier
(Hanmer 2000). 1982b, 1984; Passchier et al. 1990a; Takagi et al. 2000). The
Even at high metamorphic grade, ultramylonites recognition of ductilely deformed pseudotachylyte is
may still form, probably at high strain rate (Whitmeyer important, since the presence of a brittle deformation
and Simpson 2003). Such ultramylonites may contain phase is an indication for either deformation at shallow
porphyroclasts and notably mineral fish (Fig. 5.33) crustal depth, or unusually high strain rates (Passchier
with thin or no mantles, possibly due to limited cohe- et al. 1990a).
sion between clasts and matrix (Kenkmann 2000), or to Evidence for weak ductile deformation of pseudo-
low differential stresses and high recovery rate which tachylyte is the presence of flattened inclusions and a
limit recrystallisation in such clasts (Pennacchioni et al. mica-preferred orientation in the matrix (Passchier
2001). 1982b, 1984). Strongly deformed pseudotachylyte veins
The fabric gradient sketched above is generally valid are difficult to distinguish from thin ultramylonite zones,
for polymineralic rocks but metamorphic conditions of which lack a brittle predecessor. Indications may be an
transitions depend on mineral composition of the par- unusual ultra fine-grained (<5 µm) homogeneous ma-
ent rock. However, fabric is only a rough indicator and trix of biotite, quartz and feldspar with isolated porphy-
cannot be used alone to determine metamorphic grade roclasts of quartz, but fewer or no clasts of other miner-
in mylonites; this should be done using minerals, which als, the presence of sulphide aggregates in quartz inclu-
have grown or recrystallised during the deformation. sions, and sharp boundaries of ultramylonite with the
Since mylonite zones may have a long history of re- wall rock (Passchier 1982a, 1984). Ductile deformation
activation, relicts of older fabrics may be present in low of pseudotachylyte is usually restricted to the main fault
strain lenses. It is tempting to use these low strain lenses veins, and injection veins may be less deformed and still
to determine the metamorphic conditions of mylonite recognisable; if a suspicion exists that a mylonite may
genesis because of the large, weakly deformed crystals have a pseudotachylyte predecessor, the presence of in-
they contain, but the results may indicate metamorphic jection veins should be investigated in the field or in hand
conditions prior to mylonitisation. Another factor that specimen. Any narrow, dark mylonite zone in metamor-
has to be taken into account is static recrystallisation, phic rocks should be checked for relicts of pseudotachy-
which may re-equilibrate minerals in mylonites after de- lyte structures.
formation. Ductile deformation of pseudotachylyte may be
caused by a separate tectonic event after increase in meta-
5.4 morphic conditions to the depth where the rock deforms 5.4
Complex Fault Rocks ductilely (Passchier et al. 1990a), or by ductile deforma-
tion at the level where the pseudotachylyte formed; the
Since many shear zones have a long period of activity or latter could happen in the transition zone between domi-
can be reactivated, several fault rock types can overprint nant ductile deformation and brittle fracturing (Fig. 5.2;
each other in a single shear zone. Most common are brit- Sibson 1980; Passchier 1982b, 1984). A pseudotachylyte
tle fault rocks, which transect mylonite, since mylonite vein may rapidly crystallise into a fine-grained aggre-
forms at depth and has to pass the field of brittle frac- gate under such conditions. At the high differential stress
turing before it reaches the surface (Grocott 1977; Streh- level sustained at these conditions, a fine-grained aggre-
lau 1986; Scholz 1988; Passchier et al. 1990a). Such over- gate such as a crystallised pseudotachylyte may deform
printing by brittle structures is usually easy to recognise. ductilely by diffusion-assisted grain boundary sliding
However, it may be difficult to differentiate a low-grade while the coarse-grained wall rock is rigid (Sect. 3.14;
mylonite where some minerals were deformed by brittle Sibson 1980; Passchier 1982b, 1984).
fracturing from a mylonite overprinted by cataclasite It has to be kept in mind that mylonites, like other
formation. These situations can be distinguished because metamorphic rocks, record mainly peak and retrograde
cataclasite will transect all minerals in the mylonite, usu- metamorphic conditions. Brittle faulting or early mylo-
ally along narrow zones. nitisation that predates these conditions may be com-
A less common type of superposition is ductile de- pletely obliterated by recrystallisation. Important early
formation of brittle fault rocks. Ductilely deformed cata- thrust structures in many metamorphic terrains do not
clasite or breccia does occur (Guermani and Pennacchioni contain any brittle fault rocks due to this process.