The document discusses the concept of 'fabric' in geology, specifically focusing on tectonic fabrics and their elements, such as foliations and lineations. It distinguishes between different types of fabric, including random and preferred fabrics, and explains the impact of deformation on the characteristics of rocks, including the categories of tectonites and types of foliations such as cleavage and schistosity. Additionally, it addresses the terminology used by structural geologists to describe these fabrics and their formation processes under various metamorphic conditions.

2. INTRODUCTION
• In everyday language, we commonly use the word
“fabric.”
• When talking about fabrics that are used to
make garments, we mean a patterned cloth made by
weaving fibers in some geometric arrangement. But
the word “fabric” is not used only to refer to material
products.
• In a philosophical moment we might consider the “fabric
of life,” by which we mean the underlying organization
of life.

3. • As we found to be the case with many terms, the word fabric has a related yet
somewhat different meaning in geology.
• To a structural geologist, the fabric of a rock is the geometric arrangement of
component features in the rock, seen on a scale large enough to include many
samples of each feature.
• The features themselves are called fabric elements. Examples of fabric
elements include mineral grains, clasts, compositional layers, fold hinges,
and planes of parting.
• Fabrics that form as a consequence of tectonic deformation of rock are called
tectonic fabrics, and fabrics that form during the formation of the rock are
called primary fabrics
• It may sound picky, but structural geologists also make a distinction between
microstructure and texture.
• Although texture is sometimes used as a synonym for microstructure, for
example igneous texture, here we restrict the term texture to crystallographic
orientation patterns in an aggregate of and microstructure to their geometric
arrangement.

4. • Tectonic fabrics provide clues to the :
 strain state of the rock,
 the geometry of associated folding,
 The processes involved in deformation,
 the kinematics of deformation,
 the timing of deformation (if the fabric is defined by an
arrangement of datable minerals),
 and ultimately about the tectonic evolution of a region.
• The purpose of this lecture is to explore two common
tectonic fabric elements in rocks, foliations and lineations,
and to introduce you to the characteristics and
interpretation of these elements.

5. FABRIC TERMINOLOGY
• Let’s start by developing the inevitable vocabulary to
discuss tectonic fabrics (see Table 11.1).
• If there is no preferred orientation (i.e., alignment) of
the fabric elements, then we say that the rock has a
random fabric (Figure 11.1a). Undeformed sandstone,
granite, or basalt are rocks with random fabrics.
• Deformed rocks typically have a nonrandom fabric or
a preferred fabric, in which the fabric elements are
aligned in some manner and/or are repeated at an
approximately regular spacing (Figure 11.1b).

7. • There are two main classes of preferred fabrics in rock.
A planar fabric, or foliation (Figure 11.1c), is one in
which the fabric element is a planar or tabular feature
(meaning it is shorter in one dimension than in the other
two), and a linear fabric, or lineation (Figure 11.1d), is
one in which the fabric element is effectively a linear
feature (i.e., it is longer in one dimension relative to the
other dimensions).
• Structural geologists may use the word “fabric” alone
to imply the existence of a preferred fabric (as in, “that
rock has a strong fabric”), but you should use
appropriate modifiers if your meaning is not clear from
context alone.

8. • We’re not quite done with terminology yet! Fabrics are
complicated features, and there are lots of different
adjectives used by structural geologists to describe them.
• For example, if you can keep splitting the rock into smaller
and smaller pieces, right down to the size of the component
grains, and can still identify a preferred fabric, then we say
that the fabric is continuous.
• In practice, if the fabric elements are closer than 1 mm (that
is, below the resolution of the eye), the fabric is continuous.
• When there is an obvious spacing between fabric elements,
we say that the fabric is spaced. (Figure 11.2b)

10. Tectonites
• Rocks with a penetrative tectonic fabric are also called
tectonites.
• When linear fabric elements dominate, the rock is called an L-
tectonite, whereas a rock with dominantly planar fabrics is
called an S-tectonite, and, not surprisingly, rocks with both
types of fabric elements are called LS-tectonites.

11. • Typically, the deformation that leads to the
formation of a tectonite is accompanied by
metamorphism, so the fabric is defined by
grains that have been partially or totally
recrystallized , and/or by new minerals that
have grown during deformation (called
neomineralization).
• Because most rocks are deformed, tectonites
are among the most common rocks you will
see (Figure 11.4); some examples are slates,
schists, and mylonites

12. FOLIATIONS
• A foliation is any type of planar fabric in a rock. We are admittedly a bit loose
in our use of the term planar. Since, strictly speaking, a plane does not contain
any curves or changes in orientation, the terms curviplanaror surface would be
more appropriate.
• Although foliations are generally not perfectly planar, structural geologogists
nonetheless are in the habit of talking about planar fabrics. Thus, bedding,
cleavage, schistosity, and gneissosity all qualify as foliations.
• Fractures, however, are not considered to be foliations, because fractures are
breaks through a rock and are not a part of the rock itself.
• A rock may contain several foliations, especially if it has been deformed more
than once. To keep track of different foliations, geologists add subscripts to the
foliations: S0, S1, S2, and so on. S0 is used to refer to bedding, S1 is the first
foliation formed after bedding, and S2 forms after S1.
• The temporal sequence of foliation development is defined by cross-cutting
relationships, but in complexly deformed areas, it may be quite a challenge to
determine which foliation is which unless independent constraints on (relative)
time are available.

13. • There are many types of tectonic foliations that are
distinguished from one another simply on the basis of what
they look like.
• The physical appearance of a foliation reflects the process
by which it formed, and the process, in turn, is controlled
partly by the composition and grain size of the original
lithology, and partly by the metamorphic conditions under
which the foliation formed.
• Different names are used for the different types of
foliations. In Table 11.2 and the following discussion, we
will introduce different types of foliation roughly in
sequence of increasingly higher metamorphic conditions—
cleavage first, then schistosity, then gneissic layering.

15. What Is Cleavage?
• A secondary fabric element, formed under low-temperature conditions,
that imparts on the rock a tendency to split along planes.
The point of this definition is to emphasize
1. That cleavage is a feature of the rock that forms subsequent to the origin
of the rock.
2. That the term “cleavage” is used, in practice, for tectonic planar fabrics
formed at or below lower green-schist facies conditions (i.e., ≤ 300°C).
The term “cleavage” is not used when referring to the fabric in schists or
in gneiss.
3. That in a rock with cleavage, there are planes of weakness across which
the rock may later break when uplifted and exposed at the surface of the
earth, even though cleavage itself forms without loss of cohesion. By this
definition, an array of closely spaced fractures is not a cleavage.

16. • We recognize four main categories of cleavage
that are differentiated from one another by their
morphological characteristics (or, by how they
look in outcrop). These are disjunctive cleavage,
pencil cleavage, slaty cleavage, and crenulation
cleavage.

17. Disjunctive Cleavage
• Disjunctive cleavage is a foliation that forms mostly
in sedimentary rocks that have been subjected to a
tectonic differential stress under sub–greenschist
facies metamorphic conditions.
• It is defined by an array of subparallel fabric elements,
called cleavage domains, in which the original rock
fabric and composition have been markedly changed by
the process of pressure solution.
• Domains are separated from one another by intervals,
called microlithons, in which the original rock fabric
and composition are more or less preserved

18. • The adjective “disjunctive” implies that the
cleavage domains cut across a preexisting
foliation in the rock (usually bedding), without
affecting the orientation of the preexisting
foliation in the microlithons.
• Because pressure solution is always involved
in the formation of a disjunctive cleavage,
other terms such as (pressure-)solution
cleavage and stylolitic cleavage have been
used for this structure.

20. • If the context is clear, some geologists may refer to the
structure simply as spaced cleavage, though, as we see
later, crenulation cleavage is also a type of spaced
cleavage. Earlier in this century, many geologists
incorrectly considered cleavage domains to be brittle
fractures formed by loss of cohesion. The old term
“fracture cleavage” should therefore be avoided when
referring to disjunctive cleavage, because a cleavage
cannot be composed of fractures. Such arrays of
closely spaced fractures should be called a fracture or
joint array.

21. Pencil Cleavage
• If a fine-grained sedimentary rock (shale or mudstone)
breaks into elongate pencil-like shards because of its
internal fabric, we say that it has a pencil cleavage
(Figure 11.8).
• Typically, pencils are 5–10 cm long and 0.5–1 cm in width.
• In outcrop, pencil cleavage looks as if it results from the interaction
of two fracture sets (and in some locations, it is indeed merely a
consequence of the intersection between a fracture set and bedding),
but actually the parting reflects an internal alignment of clay grains
in the rock.
• Pencil cleavage forms because of the special characteristics of clay.
The strong shape anisotropy of clay
flakes creates a preferred orientation parallel to bedding when they
settle out of water and are compacted.

22. • This preferred orientation imparts the tendency
for clay-rich rocks to break on bedding planes
that is displayed by shale.
• Shale, mudstone, and slate are all names that
are used to describe clay-rich rocks; mudstone
is the general term for these rocks, whereas
shale and slate are foliated clay-rich rocks.

23. Slaty Cleavage
• Pencil cleavage may be considered a snapshot of an early
stage in the process by which slaty cleavage develops.
• This type of cleavage is best shown in fine grain rocks such
as mudstone that have been deformed under very low grade
metamorphic conditions.
• As shortening perpendicular to cleavage planes
accumulates, clay throughout the rock develops a preferred
orientation at an angle to the original sedimentary fabric and
this orientation dominates over the primary fabric (Figure
11.9c).

24. • Slaty cleavage is defined by
strong dimensionally preferred
orientation of phyllosilicates
in a very clay-rich rock, and
the resulting
rock, which is considered to
be a low-grade metamorphic
rock, is called a slate.

25. • Slaty cleavage tends to be smooth and planar.
This characteristic, coupled with the
penetrative nature of slaty cleavage, means
that slates can be split into thin sheets, which
made them popular roofing materials in the
nineteenth century and early parts of the
twentieth century.

26. Phyllitic Cleavage and Schistosity
• When metamorphic conditions reach the lower greenschist facies,
the clay and illite in a pelitic rock react to form white mica and
chlorite. If reaction occurs in an anisotropic stress field, these
phyllosilicates grow with a strong preferred orientation. Rock that is
composed of strongly aligned fine-grained white mica and/or
chlorite is called phyllite and the foliation that it contains is called
phyllitic cleavage.
• The mineralogy and fabric of phyllites give the rock a distinctive
silky luster. Phyllitic cleavage is intermediate between slaty
cleavage and schistosity.

27. Schistosity
• When metamorphic conditions get into the middle
greenschist facies, the minerals in a pelitic lithology react to
form coarser-grained mica and other minerals.
• When these reactions again take place in an anisotropic
stress field, the mica has a strong preferred orientation.
• The resulting rock is a schist, and the foliation it displays is
called schistosity.
• In schist that contains porphyroclasts (relict large crystals)
or porphyroblasts (newly grown large crystals), the
schistosity tends to be wavy, as the micas curve around the
large crystals.

29. Crenulation Cleavage
• Crenulation cleavage is caused as the name suggests by small scale
folding (crenulation) of very thin layers or laminations within a
rock.
• If the axial surface of such crenulations are closely spaced and
parallel, they produce a marked foliation.
• This foliation is often enhanced by the selective recrystallization,
leading to concentration of certain constituents in layers.
• Thus micas, for example, may become concentrated in one set of
limbs of asymmetric crenulation or in both sets of limbs of
symmetric crenulation as a result of migration of the quartz and
calcite into the limbs or hinges.
• The combination of compositional banding and parallel orientation
of platy minerals provide planar weakness that can impart a strong
fissility to the rock.

30. • Cranulation cleavage is found in rocks which already posses
a strong cleavage or schistosity as a result of an earlier
deformation.
• Suitable rock lithology- very finely laminated shales.
• If the crenulation cleavage is very small, the resulting
cleavage becomes indistinguishable from other type of slaty
cleavage.

31. Solution cleavage
• A new compositional banding can arise through the
migration of certain constituents of the rock during
deformation, and for example, often accompanies
formation of crenulation cleavage.
• This phenomenon which is common in low grade
metamorphosed rock, is caused by the process of
pressure solution and the resulting cleavage is termed
as the solution cleavage.
• The solution and accompanying deposition appears to
be part of diffusion process which takes place by means
of grain boundary fluid phase.

32. Gneissosity
• A parallel banding produced by alternating
layers of different composition is an important
feature of the metamorphic rock.
• Gneisses are coarse grained metamorphic
rocks typically quatzo-feldspathic in
composition.