Page 94 - Microtectonics

82 4 · Foliations, Lineations and Lattice Preferred Orientation
development are rock composition, stress orientation and 4.2.7.2
magnitude, metamorphic conditions including tempera- Mechanical Rotation of Tabular or Elongate Grains
ture, lithostatic and fluid pressure, and fluid composition.
The relation between morphology and genetic processes During homogeneous ductile deformation, a set of ran-
is usually complex, and the description of foliation mor- domly oriented planes will tend to rotate in such a way
phology should therefore be separated from the interpre- that their mean orientation will trace the direction of the
tation of processes involved. Our knowledge of these proc- XY-plane of finite strain (Figs. 4.16(1), 4.17a; Jeffery 1922;
esses is still incomplete, although research over the last March 1932). A similar effect is thought to apply to tabu-
30 years has increased our understanding considerably lar or elongate grains with a high aspect ratio such as
(e.g. Siddans 1972; Wood 1974; Means 1977; Oertel 1983; micas or amphiboles in deforming rocks (Gay 1968; Oertel
Skrotzki 1994; Durney and Kisch 1994; Worley et al. 1997; 1970; Tullis and Wood 1975; Tullis 1976; Wood et al. 1976;
Ho et al. 1996, 2001; Williams et al. 2001; Fueten et al. 2002). Willis 1977; Wood and Oertel 1980; Means et al. 1984; Lee
This section gives a list of the main processes that are et al. 1986; Ho et al. 1995, 1996; Sintubin 1994b, 1996, 1998;
currently thought to play a role during formation of sec- Sintubin et al. 1995; Fig. 4.16(1)). If an earlier preferred
ondary foliations (Figs. 4.16, 4.17). In a number of cases, orientation was present, the foliation will not trace the
like the examples cited below, it may be possible to indi- XY-plane (Fig. 4.17b) in the case of bulk simple shear;
cate which processes have been important, but in general deformed originally equidimensional grains will trace the
several of the processes probably operate together. XY-plane in this case.
If deformation in a rock with random tabular or elon-
gate grains, such as a mica-bearing granite, occurs along
spaced shear zones, rotation of fabric elements in these
shear zones can develop a spaced foliation in a homoge-
neous parent rock (Wilson 1984); micas will tend to be-
come parallel and relatively closely spaced in the shear
zones, and less so in microlithons between shear zones
(Wilson 1984).

4.2.7.3
Solution Transfer

Pressure solution and solution transfer (Sect. 3.3) may
produce inequant grains that can help define a secondary
foliation (Figs. 4.16(2), 4.22). They also produce dark
seams of insoluble material along dissolution surfaces that
may have a stylolitic (Box 4.3) or planar appearance
(Engelder and Marshak 1985; Figs. 4.15, 4.20, 4.21). After
the foliation has developed, the resulting anisotropy of
diffusivity may enhance preferential grain growth in the
direction of the foliation.
Stress-induced solution transfer may also aid devel-
opment of foliations, either by increased rotation of elon-
gate minerals due to selective solution and redeposition
of material (Fig. 4.16(4)) or by truncation and preferen-
tial dissolution of micas which lie with (001) planes in
the shortening direction, coupled with preferential growth
of micas with (001) planes in the extension direction
(Fig. 4.16(5); Ishii 1988). The intrinsic growth rate of mi-
cas is anisotropic and fastest parallel to (001) planes (Eth-
eridge et al. 1974; Rosenfeld 1985). Solution transfer in-
cluding micas will therefore lead to a preferred orienta-
Fig. 4.17. Development of some foliations by progressive simple shear tion, even in the absence of rotation (Ishii 1988; see mi-
and pure shear of: a a random initial orientation of isolated elongate metic growth, Sect. 4.2.7.6).
or planar minerals; b an initial preferred orientation of isolated elon-
gate or planar minerals; c originally equidimensional grains. In c a Solution transfer is very important for the formation
grain shape preferred orientation is formed. Grey bars indicate the of spaced foliations, especially younger foliations over-
direction of the XY-plane of finite strain for the deformation shown printing older ones (Sect. 4.2.10.2).