A foliation is a planar structure in
a rock that is homogeneously
distributed throughout the volume.
A lineation is a homogenously
distributed linear structure that can
be: surficial, if present along discrete
surfaces, and penetrative, if they
occur throughout the volume of a
rock.
Depending on how the foliation
and/or lineation forms in the rock, it
is classified as primary or secondary.
On orders of tens of centimeters.
11.1 Tectonites
Tectonites: rocks whose structure is a
product of deformation and are commonly
metamorphosed.
-characteristic of orogenic belts and fault
zones
-develop as a result of deformation
-contains mostly secondary features, but
primary features may be inherited.
S-tectonite- dominated by foliation
L-tectonite- dominated by lineation
Rock Cleavage- tendency of rock to
break/cleave along surfaces of specific
orientation
Banding- describes planar tabular features
in rocks distinguished by differences in
composition or possibly texture from
adjacent rock
Figure 11.1 provides a foliation
classification diagram
11.2 Compositional Foliations
-marked by laminae of different
mineralogical composition
Diffuse foliations- widely spaced weak
concentrations of a mineral, in a rock
relatively homogenous lithology
Banded foliations- closely spaced
compositional
layers
that
are
mineralogically
distinct
comparable abundance.
and
of
11.3 Disjunctive Foliations
-contain thin domains, called cleavage
domains or seams, characterized by
concentrations of oxides and strongly
aligned platy minerals.
Microlithons – tabular domains where
within rocks showing disjunctive foliation
where platy minerals may be less
abundant or more randomly oriented.
Disjunctive Foliations are divided into:
•stylolitic- long continuous, irregular
cleavage
domains
with
tooth-like
geometry in cross-section. Common in
limestones.
•anastomosing- long continuous, wavy
cleavage domains that form irregular
network outlining lenticular microlithons.
Common in limestones, phyllites, and
schists.
• rough-Common in rocks containing
abundant sand-size material.
cleavage
domains
are
formed
in
short
discontinuous bursts with spacing
typically less than 1mm.•smoothcleavage domains are long, continuous,
and smooth with concentrations of highly
oriented platy minerals. Characteristic of
some slates.
11.4 Crenulation Foliations
-formed by harmonic wrinkles or chevron
folds that develop in a pre-existing
foliation. New foliations cut across old
foliations, with old foliation preserved in
the microlithon.
-The orientation pattern of platy minerals
in cleavage domain provides further
subdivision of crenulations:
Zonal- platy minerals in new cleavage
domain are oriented at small angles to
domain & form a continuous variation of
orientations
from
minerals
in
microlithons.
There is often a
composition difference between cleavage
domains and microlithons.
Discrete- orientation of platy minerals in
new cleavage domains is parallel to
domains and sharply discordant with the
1
orientations of platy minerals within the
microlithons.
Discrete
crenulations
contain similar compositional differences
as zonal crenulations.
11.5 Continuous Foliations
-defined by domains with spacing less
than 10 micrometers or by nondomainal
structure
-divisible by grain size into fine (ie. Slate)
and coarse (ie. Schist) continuous
foliations.
Fine continuous foliations may be either:
-microcontinuous- parallel alignment of
all platy or inequant grains in a rock, and
lacks any domainal structure
-microdomainal – microscopic versions of
the disjunctive and crenulation foliations
Both of these terms are impractical in
field work due to microscopic size.
Course
continuous
foliations
are
characterized by the complete orientation
of planar mineral grains.
11.6 The Relationship of Foliations to
Other Structures
i. Relationship with Folds
•axial surface foliations and axial plane
cleavages are examples of secondary
foliations
-secondary foliations are commonly
parallel/subparallel to axial surfaces of
folds.
-orientations fan across folds and can be
convergent or divergent
Refracted foliation/cleavage- A feature
that preserves orientation changes at a
lithologic contact; strongly convergent in
less platy materials and is the most
divergent in platy-mineral-rich rocks
•The relationship of foliations subparalllel
to axial plane of folds is so consistent that
it can determine the geometry of folding
while field mapping.
ii. Relationship with Ductile Shear Zones
S-C tectonites- rocks from ductile shear
zones with S/C foliations form from a
single deformation. Distinguishable from
crenulations by the single period of
deformation.
EPS116 Chapter 11
Anisa “Anastomosing” Ahmadzai 2011
Anisa Ahmadzai 2011
Jessica “Boudinage”
Banaszak2013
2013
Jessica Banaszak
& Melissa Robinson
& Melissa “Mullion” Robinson 2013
S-foliation- continuous coarse foliation
defined by preferred orientation of mica
grains, and commonly by elongate quartz
grains.
C-foliation- set of shear bands in rock that
develop subparallel to boundaries of the
shear zone, and may have fibrous crystals
(slickenfibers) lying on and subparallel to
foliation surfaces.
-transposition foliation- superposition of
tectonite
foliation
on
earlier
compositional layering. Can be dated
isotopically.
11.7 Special types of Foliation and
Nomenclature
Slaty Cleavage - refers fine continuous
foliations characteristic of slate. “Rocks
with slaty cleavage traditionally have
been a valuable source of materials for
roofing slates and blackboards.” Foliation
causes a strong cleavage in the rock, so
the rock breaks easily and weathers
preferentially.
Phyllitic Cleavage - resembles slaty
cleavage but the grain size of rock slightly
coarser. Characterizes phyllites. In hand
samples, the surface of samples has a
sheen. Also has preferential weathering
patterns.
Schistosity – refers to foliation found in
coarse-grained, mica-rich, medium- to
high grade metamorphic rocks with
chlorite, biotite, or muscovite defining the
foliation
Mineral grains are coarse
enough to be visible.
Gneissic Foliation- develop in gneisses –
coarse grained, metamorphic rocks - in
which platy minerals are sparse or absent.
This foliation provides weak cleavage at
best.
11.8 Structural Lineations
-defined by the preferred orientation of
linear structure contained within a rock.
discrete
lineationsformed
by
deformation of discrete objects. For
example: ooids, pebbles, fossils, and
alteration spots
constructed lineations- formed from
planar features constructed or deformed
during deformation and include the
intersection of two foliations, crenulation
hinge lines, boudin lines, structural
slickenlines, and mullions.
boudins- linear segments of a layer
formed when the layer has been pulled
apart along periodically spaced lines of
separation called boudin lines.
pinch-and-swell structures are periodic
oscillations in the thickness of a bed with
pinches becoming thinner as the amount
of lengthening of the bed increases.
mullionslinear
fluted
structures
developed within a rock or at lithologic
interfaces.
map geometry of fold hinges through
these structures
-Lineations may be found parallel or
perpendicular to fold axes and also as
arbitrary angles to fold axes
References & Resources
Robert J. Twiss, Eldridge M. Moores,
Structural Geology 2nd edition, (W. H.
Freeman), p. 297-317, 2006
11.9 Mineral Lineations
-consist of preferred orientation of either
single elongate mineral grains or elongate
polycrystalline aggregates
-Mineral grain lineations are formed by
the parallel alignment of 1) individual
acicular mineral grains such as
amphibole, 2) by grains of minerals that
have been stretched into an elongate
shape, or 3) by mineral fibers that have
grown in a preferred orientation.
Polycrystalline mineral lineations formed by the preferred orientation of
elongate clusters of grains of a particular
mineral measuring at least a few grains in
diameter. Examples of these structures
are rods, mineral cluster lineations, pencil
lineations, and nonfibrous overgrowths.
11.10 Associations of Lineations With
Other Structures
i. Lineations and Foliations
-Some lineations are defined at least in
part by foliations, and must be parallel
-Lineations may develop on surfaces other
than foliations like fault surfaces
ii. Lineations and Folds
-This relationship can decipher structural
geometry of an area and is useful when
interpreting conditions under which the
structures formed.
-Because lineations are usually smaller
structures than folds it may be easier to
2
EPS116 Chapter 11
Anisa “Anastomosing” Ahmadzai 2011
Anisa Ahmadzai 2011
Jessica “Boudinage”
Banaszak2013
2013
Jessica Banaszak
& Melissa Robinson
& Melissa “Mullion” Robinson 2013