Fabric - Introducing Tectonics, Rock Structures and Mountain Belts - page 58

in Figure 4.8C, which have been sepa-
rated initially by extensional fractures.
Shear fracture cleavage is often found
in fault rocks, in which case it may
be a useful indicator of the sense of
movement on the fault ( see below).

orientation of planar minerals such as
clays or muscovite mica. There may
also be a parallel arrangement of elon-
gate particles or grain aggregates. The
origin of the slaty cleavage becomes
more obvious when the rock contains
objects of known initial shape such as
fossils, reduction spots, or pebbles, in
which case the cleavage plane is seen
to correspond to the plane of flattening
of the objects, as in Figure 7.1B. Slaty
cleavage will thus have a simple geo-
metric relationship to the maximum
compressive stress, and will usually
be either parallel to the axial planes of
related folds, as in Figure 5.1A, or have
a fan-like arrangement, either converg-
ing towards, or diverging away from,
the core of the fold (Figure 7.2A, B).

To understand why the fan arrange-
ment occurs, it is necessary to return
to the section on strain in folding in
Chapter 6. Assuming that the slaty
cleavage was initially parallel to the
XY strain axes, (i.e. it is a flattening
fabric) then buckling of a layer with
an initially parallel cleavage will result
in a convergent fan, as in Figure 7.2A.
However, initially parallel slaty cleavage
in a layer folded by flexural shear will
cause the cleavage to become diver-
gent (Figure 7.2B). Thus, bedded rocks
consisting of interlayered competent
and incompetent layers (sandstones
and shales, for example) may display
alternating divergent and convergent
cleavage fans, as shown in Figure 7.2C.

Slaty cleavage
This type of cleavage is confined to
fine-grained rocks, such as mudstones
or volcanic tuffs, which have been
deformed under low-grade metamor-
phism. It consists of closely-spaced
planes of weakness that make the rock
easily cleaved and suitable for roofing
slate (Figure 7.1A). The nature of the
cleavage planes is usually only appar-
ent under the microscope, when they
are seen to be caused by the parallel

Figure 7.1 Cleavage. A. Slaty cleavage
in slate; the cleavage is approximately
parallel to the axial plane of the fold.
B. Slaty cleavage in glacial mudstone
enclosing drop-stones. The bedding
(parallel to the black line) is inclined to
the right and the cleavage (parallel to the
red line) is vertical. Note the darker areas
(pressure shadows) at the sides of the
pale drop-stone showing where lighter
material has been dissolved by pressure
solution. C. Crenulation cleavage in
schist. The cleavage does not penetrate
the more competent bands at the top of
the picture, which are affected by open
folds only. D. Spaced cleavage in weakly
metamorphosed layered siltstone-
mudstone. The cleavage is accentuated
by darker bands formed by pressure
solution. B-D: note coins for scale.

A B

C D