Page 55 - Microtectonics

3.8 · Solid-State Diffusion Creep, Granular Flow and Superplasticity 43
3.7.3 3.8 3.8
Subgrain Rotation (SGR) Recrystallisation Solid-State Diffusion Creep, Granular Flow
and Superplasticity
A special recrystallisation process occurs when dislocations
are continuously added to subgrain boundaries. This hap- If the temperature in a deforming rock is relatively high
pens only if dislocations are relatively free to climb from with respect to the melting temperature of constituent
one lattice plane to another. The process is known as climb- minerals, crystals deform almost exclusively by migra-
accommodated dislocation creep. In such cases, the angle tion of vacancies through the lattice. This process is
between the crystal lattice on both sides of the subgrain known as grain-scale diffusive mass transfer. There are
boundary increases until gradually the subgrain can no two basic types: Coble creep and Nabarro-Herring creep.
longer be classified as part of the same grain (Fig. 3.26b; The former operates by diffusion of vacancies in the crys-
Box 3.6); a new grain has developed by progressive tal lattice along grain boundaries; the latter by diffusion
misorientation of subgrains or subgrain rotation. This proc- of vacancies throughout the crystal lattice (Knipe 1989;
ess is known as subgrain rotation-recrystallisation (abbre- Wheeler 1992).
viated SGR recrystallisation) and generally occurs at higher Especially in fine-grained aggregates, crystals can
temperature than BLG recrystallisation. SGR recrystalli- slide past each other in a process known as grain bound-
sation corresponds to Regime 2 of Hirth and Tullis (1992). ary sliding while the development of voids between the
Old grains tend to be ductilely deformed and elongate or crystals is prevented by solid state diffusive mass trans-
ribbon-shaped, with numerous subgrains. Core-and-man- fer, locally enhanced crystalplastic deformation, or solu-
tle structures form at low temperature and low strain, but tion and precipitation trough a grain boundary fluid.
generally subgrains and new grains occur in “sheets” be- This deformation process is referred to as granular flow
tween old grain relicts, or old grains may be entirely re- (Boullier and Gueguen 1975; Gueguen and Boullier 1975;
placed by subgrains and new grain networks. All gradations Stünitz and Fitz Gerald 1993; Kruse and Stünitz 1999;
between subgrains and grains of the same shape and size Fliervoet and White 1995; Paterson 1995; Fliervoet et al.
occur (Nishikawa and Takeshita 2000; Nishikawa et al. 1997). Since grain boundary sliding is rapid, it is the
2004). Subgrains and grains are commonly slightly elon- accommodation mechanism that normally determines
gate. Characteristic is a gradual transition from subgrain- the strain rate of granular flow (Mukherjee 1971; Pad-
(low angle-) to grain- (high angle-) boundaries (Fig. 3.25). manabhan and Davies 1980; Langdon 1995). In metal-
lurgy, some fine-grained alloys can be deformed up to
3.7.4 very high strain in tension without boudinage, a process
High-Temperature Grain Boundary Migration (GBM) known as superplastic deformation (Kaibyshev 1998;
Recrystallisation Zelin et al. 1994). The term superplasticity has also been
used in geology (Schmid 1982; Poirier 1985; Rutter et al.
At relatively high temperature, grain boundary mobility 1994; Boullier and Gueguen 1998b; Hoshikuma 1996) and
increases to an extent that grain boundaries can sweep refers to very fine-grained aggregates (1–10 µm) of equi-
through entire crystals to remove dislocations and possi- dimensional grains, which deformed to very high strain
bly subgrain boundaries in a process called high-tempera- without developing a strong shape- or lattice-preferred
ture grain boundary migration (GBM) recrystallisation orientation. Grain boundary sliding is thought to play a
(Figs. 3.25, 3.26; ×Video 11.6a,b,e; Guillopé and Poirier major role in such deformation (Boullier and Gueguen
1979; Urai et al. 1986; Stipp et al. 2002). GBM recrystalli- 1975; Allison et al. 1979; Schmid 1982; van der Pluijm
sation corresponds to Regime 3 of Hirth and Tullis (1992). 1991; Rutter et al. 1994). Grain size seems to be the ma-
Subgrain formation and rotation is normally active dur- jor parameter in determining whether an aggregate will
ing this process, but once grain boundaries are formed deform by dislocation creep or by solid state diffusive
by this process after a certain amount of rotation of former mass transfer and grain boundary sliding (Schmid et al.
subgrains (Lloyd and Freeman 1991, 1994), they can be- 1977; Behrmann 1983). Small grain size favours grain
come highly mobile. Grain boundaries are lobate and boundary sliding since diffusion paths are relatively
grain size is variable. New grains tend to be larger than short. Presence of a second mineral phase can also
coexisting subgrains. It is difficult to distinguish new enhance the process since it hampers grain growth
grains from relicts of old grains, except possibly by the (Kruse and Stünitz 1999; Newman et al. 1999; Krabben-
distribution of fluid and solid inclusions. If secondary dam et al. 2003).
phases are present in an aggregate, pinning or grain Many geologists use diffusion creep as a collective term
boundary mobility structures (Fig. 3.34) are common. At for Coble- or Nabarro-Herring creep and superplasticity
very high temperature, grains have highly loboid or amoe- or granular flow, since rheological flow laws (Box 3.11)
boid boundaries, but may be nearly “strain free”, i.e. de- for these processes are very similar. Box 3.7 lists evidence
void of undulose extinction and subgrains. for diffusion creep processes in thin section.