Page 107 - Microtectonics

4.2 · Foliations 95
ing or micro-shear zone development, and the new folia- importance at deeper crustal levels since large volumes
tion follows the axial planes of folds, or the shear zones of fluid would necessarily have to flux through the rock
(Hobbs et al. 1982; Fig. 4.31b). Mawer and Williams (1991) to remove material in solution (Engelder 1984; Bhagat and
describe a situation where fold hinges develop in a con- Marshak 1990), However, Goldstein et al. (1998) argue that
tinuous foliation deformed in non-coaxial progressive in accretionary complexes large volumes of water are pass-
deformation; new micas overgrow newly formed fold ing through the rocks and that in such settings large vol-
hinges, these become unrecognisable and a mixed folia- ume losses are to be expected. The difficulty is that vol-
tion is formed with an orientation oblique to the XY-plane ume loss during foliation development can rarely be di-
of tectonic strain (Fig. 4.31b; Mawer and Williams 1991). rectly measured in deformed rocks (Sect. 9.2). Well-pre-
Even ordinary slaty cleavage normally replaces a diagenetic
foliation and is therefore not necessarily exactly parallel
to the XY-plane of tectonic strain (Figs. B.4.5, 4.31a,
×Video B.4.5). In most of the cases mentioned above, the
foliation is oblique to the XY-plane of tectonic strain, ex-
cept in the case of very high strain values.
Some foliations are active as fold limbs or micro-shear
zones. These ‘active foliations’ will never be parallel to tec-
tonic strain axes, unless they become passive by rotation.
Examples are some constrictional crenulation cleavages
(Rajlich 1991), and shear band cleavages (Sect. 5.6.3). Care
is needed even in assessment of apparently ‘passive’
foliations because foliation planes, once formed, are eas-
ily mobilised as planes of shear movement (Bell 1986). In
many practical examples there is evidence of such ‘reac-
tivation’, resulting in shear movement along foliation
planes during deformation post-dating their formation.
Finally, there are ‘oblique foliations’ (Box 4.2; Figs. 5.10,
5.30), which represent only the last part of the tectonic
strain. These foliations are not normally parallel to the
XY-plane of tectonic strain (Fig. 4.31c; Ree 1991) but form
wherever some process such as recrystallisation or grain
boundary sliding resets the shape of elongated grains
formed by dislocation creep. As a result, the foliation will
only represent the last part of the deformation history
(Box 4.2; Fig. 4.31c).
4.2.9.3
Foliations, Strain and Volume Change

It is presently unclear to what extent solution transfer as-
sociated with foliation development leads to bulk volume Fig. 4.32. Two end-member models of crenulation cleavage develop-
ment in plane strain. The onset of crenulation cleavage development
change. Shortening values normal to the foliation up to
is shown in the squares at left. Schematic enlargements of an aggre-
70% are mentioned in the literature, but most observa- gate of four quartz grains (white), a pyrite cube (black) and a passive
tions are in the range of 30% (Gray 1979; Southwick 1987). marker circle are given. Deformed situations in cleavage domains (CD
Bulk volume loss of up to 80% has been reported, espe- grey) and microlithons (ML white) are shown in rectangles at right.
cially for slaty cleavage development at very low- and low- Local strain and volume loss in both situations are indicated schema-
tically (not to scale) by the elliptical shape of the deformed marker
grade metamorphic conditions (Ramsay and Wood 1973; circle and the outline of the original circle. a Significant volume loss in
Wright and Platt 1982; Etheridge et al. 1983; Beutner and
cleavage domains while microlithons are undeformed. Quartz grains
Charles 1985; Ellis 1986; Wright and Henderson 1992; are partly dissolved in cleavage domains but no fibres form near the
Goldstein et al. 1995, 1998). On the other hand, many stud- quartz or pyrite grains. b Volume-constant deformation where vol-
ies concluded that little or no significant bulk volume ume loss in cleavage domains is compensated by volume increase of
change accompanied cleavage formation (Waldron and microlithons. Quartz grains are partly dissolved in cleavage domains
Sandiford 1988; Wintsch et al. 1991; Tan et al. 1995; Saha but have fibrous overgrowths in cleavage domains and microlithons
(vertical striping); fibres also occur next to pyrite cubes. If no pyrite
1998; Davidson et al. 1998). On theoretical grounds, bulk cubes or similar objects are present, and if overgrowths on quartz are
volume loss on a large scale is expected to be of minor not clear, situations a and b are difficult to distinguish