Geoscience Reference
In-Depth Information
throughout the rock as regional planar fabrics. Many regional
foliations form broadly perpendicular to σ
max
, in a response
to regional tectonic forces, and vary with rock type and depth
(i.e. increasing temperature, pressure). The following fl owchart
may help to discriminate between different foliations in the
fi eld.
8
Distinguishing
foliations
Are planes
spaced or
penetrative?
Spaced
Penetrative
Form of
planes?
Microfold
limbs
Dissolution
seams
Are matrix
grains visible to
naked eye?
Yes
SCHISTOSITY
PRESSURE
SOLUTION
CLEAVAGE
CRENULATION
CLEAVAGE
No
Fractures
Evidence of
very high
strain?
Yes
MYLONITE
DISJUNCTIVE
FOLIATION
No
SLATY
CLEAVAGE
Figure 8.15 (p. 178) shows examples of different foliations to
illustrate some of the differences between them, and Table 8.3
(p. 179) summarizes some characteristics of the main types of
tectonic fabric.
Some mylonitic fabrics can be mistaken for slaty cleavage (or
even fi ne-scale igneous fl ow banding). To confi rm a mylonitic
fabric, look for evidence of high strain, such as: strong
stretching lineation (Section 8.3.2); strongly wrapped
porphyroclasts with 'tails' (Section 8.3.3); boudinaged layers or
grains; and intrafolial folds (Figure 8.16, p. 180). Original
bedding structures are rarely preserved in mylonites, whereas
beds are commonly traceable as folded bands of different colour
and grain size in rocks with slaty cleavage. Slaty cleavage in
