Page 69 - Microtectonics

3.12 · Deformation of Some Rock-Forming Minerals 57
material are dominant deformation mechanisms (Dunlap of other slip systems is reduced. For example, at low tem-
et al. 1997; van Daalen et al. 1999; Stipp et al. 2002). Char- perature, with increasing differential stress the system
acteristic structures are fractures in grains, undulose ex- (c)<a> is followed by {m}<a> and finally {r}<a>. At high
tinction, kink bands (Nishikawa and Takeshita 1999) and temperature, the sequence is (m)<c>, {m}<a>, (c)<a> and
evidence for pressure solution and redeposition of mate- {r}<a> (Hobbs 1985).
rial, sometimes in veins. Healed fractures are common, usu-
ally aligned with fluid and solid inclusions. BLG recrystalli- 3.12.3
sation may locally occur at very low-grade conditions in Calcite and Dolomite
strongly deformed quartz (Wu and Groshong 1991a).
At low-grade conditions (300–400 °C) dislocation glide At very low-grade conditions calcite deforms by fractur-
and creep become important, mainly on basal glide planes ing and cataclastic flow (Kennedy and Logan 1998). The
in the (c)<a> direction. Characteristic structures are patchy coarser grained fragments are heavily twinned and show
and, at higher temperature, ‘sweeping’ undulose extinc- undulose extinction, and are cut by veins and stylolites
tion (Fig. 3.17) and deformation lamellae (Fig. 3.18) oc- while small matrix grains can be strain- and twin free.
cur. A dominant dynamic recrystallisation mechanism Brittle deformation is apparently assisted by solution
under these conditions is BLG recrystallisation (Stipp et al. transfer, twinning and, especially in the fine-grained ma-
2002). Dauphiné deformation twinning is possible in trix, dislocation glide and BLG recrystallisation (Wojtal
quartz at low-grade conditions but also at higher tem- and Mitra 1986; Kennedy and Logan 1998).
perature (Tullis 1970; Barber and Wenk 1991; Lloyd et al. At low-grade conditions and if water is present, pres-
1992; Heidelbach et al. 2000; Lloyd 2000). sure solution is dominant in calcite and leads to stylolite
At medium temperatures (400–500 °C), dislocation development (Box 4.3) although other mechanisms may
creep is dominant, and prism {m}<a> slip becomes im- also contribute (Burkhard 1990; Kennedy and Logan
portant. Characteristic are relatively strongly flattened old 1997, 1998). Calcite is special in that deformation twin-
crystals and abundant recovery and recrystallisation ning becomes important from diagenetic conditions on-
structures (Fig. 3.41). Pressure solution may still play a wards (Schmid et al. 1981; Sects. 9.6.2, 9.9). Twinning oc-
role under these conditions (den Brok 1992). The domi- curs along three {e}-planes inclined to the c-axis and
nant recrystallisation mechanism here is SGR recrystalli- is initiated at very low critical resolved shear stress (be-
sation (Lloyd and Freeman 1994; Stipp et al. 2002). Old tween 2 and 12 MPa, depending on temperature and
grains may be completely replaced by recrystallised ma- mean stress; Turner et al. 1954; Wenk et al. 1986a; Burk-
terial. (Hirth and Tullis 1992; Stipp et al. 2002). Oblique hard 1993). However, the amount of strain that can be
foliations (Box 4.2) probably develop mainly in the com- achieved by twinning is limited and must be accommo-
bined SGR and GBM recrystallisation regime. dated at grain boundaries by pressure solution, grain
At 500–700 °C, recrystallisation is mostly by GBM re- boundary migration or grain boundary sliding. Evidence
crystallisation, grain boundaries are lobate, and pinning- for the activity of these accommodating mechanisms in
or migration microstructures are common (Jessell 1987; thin section are partly dissolved twins at grain bounda-
Stipp et al. 2002) at lower temperature ranges. Above ries, or twins that end before the grain boundary is
700 °C, prism-slip {m}<c> becomes important (Blumen- reached, left behind by the migrating boundary. Twins
feld et al. 1986; Mainprice et al. 1986) and rapid recrystal- can be used as indicators of temperature, strain and stress
lisation and recovery cause most grains to have a strain- (Sects. 9.2, 9.5.1 and 9.6.3).
free appearance. Grain boundaries are lobate or amoe- At low- to medium-grade metamorphic conditions,
boid in shape (Fig. 4.9). A special type of approximately dislocation glide on r- and f-planes becomes important
square subgrain structure occurs at these high grade con- besides deformation twinning: {f}<r∩f> (six systems) at
ditions, known as chessboard extinction or chessboard low temperature and {f}<a∩f> (three systems) at higher
subgrains (Fig. 3.23) which may be due to combined ba- temperature (Takeshita et al. 1987; de Bresser and Spiers
sal <a> and prism <c> slip (Blumenfeld et al. 1986; Main- 1997). In addition, c<a> slip may become important at high
price et al. 1986; Stipp et al. 2002) or the α–β transition temperature (Schmid et al. 1987; de Bresser and Spiers 1993,
in quartz (Kruhl 1996). Under these metamorphic condi- 1997; Barnhoorn et al. 2004). BLG recrystallisation is ac-
tions strain-free monomineralic quartz ribbons can form tive under low-grade conditions and increases in impor-
(Box 4.2; Figs. 5.11, 5.12). tance with increasing temperature. SGR recrystallisation
Temperature is an important, but not unique factor is active under a range of conditions (de Bresser et al. 2002;
determining quartz deformation behaviour; this also de- Ulrich et al. 2002; Bestmann and Prior 2003). Grain bound-
pends strongly on strain rate, differential stress and the ary sliding and ‘superplastic’ behaviour may be impor-
presence of water in the lattice and along grain bounda- tant in calcite if the grain size is very small (Schmid 1982;
ries. With increasing differential stress, more slip systems Schmid et al. 1987; Walker et al. 1990; Casey et al. 1998;
may become active since the critical resolved shear stress Brodie and Rutter 2000; Bestmann and Prior 2003).