Page 127 - Microtectonics

116 5 · Shear Zones
seismic activity on brittle faults in common tectonic set-
tings. Most pseudotachylytes therefore form in the upper
to middle crust. However, some occurrences from the deep
crust, which apparently formed at granulite or eclogite
facies, have been reported (Austrheim and Boundy 1994;
Boundy and Austrheim 1998; Clarke and Norman 1993).
Melting at temperatures between 750–1 600 °C is
thought to occur on the main fault vein of a pseudotachy-
lyte (Austrheim and Boundy 1994; Camacho et al. 1995;
Lin and Shimamoto 1998; O’Hara 2001; Di Toro and Pen-
nacchioni 2004). Some of the melt may intrude minor
faults, which branch from the main fault vein into the wall
rock, and form injection veins (Figs. 5.2, 5.4a, 5.5). The
small volume of melt formed in this way cools rapidly to
the temperature of the host rock. As a result, the melt
quenches to a glass or very fine-grained, aphanitic mate-
rial that occurs along fault planes and adjacent branch-
ing injection veins (Figs. 5.4a, 5.5). There is some evidence
that rock crushing may precede the melting stage in some
pseudotachylytes. Pseudotachylyte is normally not asso-
ciated with growth of quartz- or calcite veins and gener-
ally occurs in massive, dry, low-porosity rocks such as
granite, gneiss, granulite, gabbro and amphibolite. This is
because the fluid present in porous rocks lowers the ef-
fective normal stress over a fault plane upon heating; con-
sequently, not enough frictional heat can be produced to
Fig. 5.4. a Schematic drawing of a typical pseudotachylyte with main cause local melting. Therefore, pseudotachylyte is not
fault vein, injection vein, internal compositional banding and typi- normally found in porous sedimentary rocks (for a pos-
cal inclusions. The boundary with the wall rock is sharp. Mica grains sible exception see Killick 1990). It is not found in marble
in the wall rock show corrosion along the contact with pseudo- because of the dissociation of carbonates at high tempera-
tachylyte. b Pseudotachylyte in which the main fault vein has been
reactivated as a mylonite zone. The mylonite can be recognised as ture and the resulting decrease in normal stress over a
a former pseudotachylyte by its fine-grained homogeneous nature fault, and the ductile flow in carbonates, which inhibits
and the presence of injection vein relicts build-up of high differential stress. It may seem curious
that pseudotachylyte is a product of high temperature (melt
(for main fault veins) straight boundaries with the wall generation) related to low temperature brittle fault zones,
rock. They never show transitional zones of decreasing while such local melting is rare in higher-grade ductile shear
brittle deformation intensity towards the wall rock as is zones. In brittle fault zones, however, elastic strain energy
usually the case for cataclasite or breccia. The wall rock may be stored for a long period of time and is released in
can be cataclased or faulted, but these structures are nor- a matter of seconds in a small volume of rock along faults;
mally transected by the younger pseudotachylyte. Pseu- in ductile shear zones, heat is dissipated continuously over
dotachylyte is thought to form by local melting of the a larger volume of rock and is therefore usually insuffi-
rock along a brittle fault plane due to heat generated by cient to cause a significant rise in temperature.
–1
–2
rapid frictional sliding (10 to 1 m s ; Philpotts 1964; The matrix of pseudotachylyte is commonly black, dark
Sibson 1975, 1977a,b; Grocott 1981; Maddock 1986; Mad- brown, green or red and relatively homogeneous, but may
dock et al. 1987; Spray 1987, 1992, 1995, 1997; Shimamoto contain a compositional layering of irregular thickness,
and Nagahama 1992; O’Hara 1992; Swanson 1992; Lin 1994; which follows the contours of the vein (Fig. 5.4a). This lay-
Legros et al. 2000; Bjørnerud and Magloughlin 2004). ering is commonly of a different colour along the vein wall
Pseudotachylyte occurs associated with events such as and in the interior, and is interpreted to result from selec-
meteorite impact (Martini 1992; Thompson and Spray tive melting of the wall rock. The layering may be folded
1994; Spray et al. 1995; Hisada 2004), crater collapse, cal- and folds are interpreted to have formed by fluid flow in
dera collapse and giant landslides on superficial super- the melt. Even sheath folds (Sect. 5.3.2) parallel to the dis-
faults (Masch et al. 1985; Reimold 1995; Spray and Thomp- placement direction of the wall rock have been observed
son 1995; Spray 1997; Legros et al. 2000), in veins thicker in the layering (Berlenbach and Roering 1992). Amygdules
than 1 cm, but occurrences of veins less than 1 cm wide derived from gas bubbles are sometimes present in the
are more common and are thought to be associated with matrix (Maddock et al. 1987; Magloughlin 1989, 1998a,b).