Significance of Foliation
Deformed terranes commonly have several successive generations of foliation.
If these can be distinguished from one another by type and age (cross-cutting
relationships, absolute age dates, and overprinting under microscope),
can help to unravel the tectonic and metamorphic evolution of an area.
Foliations can be used to as reference structures to establish the:
relative growth periods of metamorphic minerals, especially porphyroblasts
deformation phases in an area. Foliation may be related to folds, however, foliation is more
penetrative than related folds and therefore can be seen better.
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Primary Foliation
Structures related to the original rock-forming process.
Originated by sedimentary processes such as transport and deposition:
Bedding (So)
preferred orientation of sedimentary clasts
Originated by primary igneous processes such as flow and crystallization:
magmatic layering in igneous rocks
preferred orientation of bubbles and pumice fragments
Bedding results from discontinuous processes causing considerable variation in thickness,
composition, texture, and structure of individual beds or layers.
Bedding is easily recognized in gently deformed, very low grade metamorphic rocks from
sedimentary features (texture and structure, fossils).
Sedimentary structures can be used for facing (younging direction).
Must be careful for the inversion of graded bedding by the growth of metamorphic minerals
(e.g., large micas may grow in a metamorphosed originally fine pelitic rock. Original
reverse grading is also common.
Bedding is hard to recognize in more intense deformation and higher metamorphic grade.
Is obliterated or disappeared by transposition and recrystallization.
However, it only rarely may be parallel to the axial plane of folds.
Recognition of primary foliation is important for the reconstruction of the structural
evolution after sedimentation crystallization (So, S1, S2, etc).
If bedding is not recognized, only the last part of the evolution can be reconstructed; the
oldest compositional layering has to be labeled Sn, followed by Sn+1 , Sn+2.
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Diagenetic Foliation
Forms by diagenetic processes such as compaction in sediments with detrital mica (i.e.,
pelites).
Are also known as bedding-parallel foliation.
Observed in very low and low-grade pelitic sediments which have undergone little or no
deformation.
Is defined by parallel orientation of thin elongate detrital mica grains with frayed
edges.
The micas are commonly subparallel to bedding.
The preferred orientation of the micas is due to their passive rotation.
Diagenetic foliation is not associated with folds.
It precedes the formation of secondary foliation.
It plays an important role in development of secondary foliation in pelites.
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Secondary Foliation
Forms after lithification and crystallization of rocks.
Forms by some kind of differentiation process in a stress field.
Is commonly (sub)parallel to the fold axial plane.
Is related to strain (parallel to the XY plane) and deformed features.
Foliation forms perpendicular to the maximum shortening direction (Z).
Forms as a result of:
ductile deformation (by crystal plasticity or cataclastic flow)
metamorphism.
Includes:
cleavage
schistosity
differentiated compositional layering
mylonitic foliation (S and C).
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Morphological Classification of Secondary Foliation
(Powell, 1979; Borradaile, 1982; Passchier and Trouw, 1996)
Secondary foliation shows a large variation of morphological features.
The following descriptive classification scheme is independent of origin (non-genetic).
It is based on the fabric elements that define the foliation such as:
elongate or platy grains
compositional layers or lenses
planar discontinuities.
Two general types of foliation:
Spaced foliation
Continuous foliation
NOTES:
Infinitely many transitional forms between foliation types may occur in nature.
A foliation may change its morphology or even disappear in a single thin section
Foliation development is strongly dependent on:
lithotype
strain
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Spaced foliation:
Fabric elements are not homogeneously distributed.
The rock is divided into lenses or layers of different composition.
Rock consists of two types of domains:
Cleavage domain:
Planar, and have fabric elements subparallel to the trend of the domain.
In metapelites, it is rich in mica and other minerals such as ilmenite, graphite, rutile, apatite,
and zircon.
Microlithons
lie between cleavage domains
contain fabric elements with weak or no preferred orientation
may contain fabric elements oblique to the cleavage domains.
Spaced foliations are subdivided based on the structure in the microlithons.
Crenulation cleavage:
Microlithons contain microfolds of an earlier foliation.
Disjunctive foliation:
Microlithons have no microfolds.
Called disjunctive cleavage is rock is fine-grained.
Compositional layering: A special hype of spaced foliation where microlithons and
cleavage domains are wide and continuous enough to form layers visible to the unaided eye
in hand specimen.
Morphological features used in the description of spaced foliation (Fig. 4.6).
Spacing of the cleavage domains
Shape of the cleavage domains
rough, smooth, wriggly, stylolitic
The % of cleavage domains in the rock
The spatial relation between cleavage domains
parallel, anastomosing,
conjugate (two intersecting directions without any sign of overprinting)
The transition from cleavage domains to microlithons
gradational, discrete
The shape of microfolds in crenulation cleavage
symmetric, asymmetric
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Continuous Foliation
Fabric elements are homogeneously distributed, to the scale of grain individual minerals.
Consists of a non-layered homogeneous distribution of platy mineral grains with a preferred
orientation.
minerals are commonly mica and amphibole; sometimes quartz, etc.
The terminology is based on observation under the microscope.
cleavage in slate under the microscope is continuous; it is spaced under SEM.
Fabric elements such as grain shape and size are used to classify continuous foliations.
Continuous Schistosity: grains defining the foliation are visible by the unaided eye.
Continuous Cleavage or slaty cleavage: grains are finer and need microscope.
Just like the distinction between mineral lineation and stretching lineation
(linear shape fabric), continuous foliations are subdivided into:
Mineral foliation: defined by the preferred orientation of platy but undeformed mineral
grains such as micas or amphiboles.
Planar shape fabrics: defined by flattened crystals such as quartz or calcite.
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Likely Mechanisms of Secondary Foliation Development
Factors controlling the development of foliation during deformation are:
Rock composition
Orientation and magnitude of stress
Metamorphic conditions
T, Plithostatic, Pfluid
Fluid composition
Mechanical rotation of Tabular or elongate grains
Solution transfer during pressure solution
Crystal plastic deformation
Dynamic recrystallization
Mimetic growth
Oriented growth defined by a stress field
Microfolding
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Foliation Formed by Mechanical Rotation of Tabular or Elongate Grains
During homogenous ductile deformation
A set of randomly oriented planes such as
tabular or elongate grains with high aspect ratios (e.g., mica and amphiboles)
will tend to rotate such that their mean orientation will trace the direction of the XY plane
of the finite strain.
If an earlier preferred orientation was present, the foliation will not trace the XY plane.
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Foliation formed by Solution Transfer During Pressure Solution
Pressure solution:
Dissolution of grains at grain boundaries in a grain boundary fluid phase under high normal
stress. Effective under presence of abundant fluid phase, and is therefore most active
under diagenetic and low-grade metamorphic conditions.
Solution transfer:
Diffusion of dissolved material away from the sites of high solubility down a stress induced
chemical potential gradient to nearby sites of low solubility.
Pressure solution may lead to the formation of inequant grains defining a foliation.
Pressure solution plays an important role in development of secondary foliation by
microfolding (Fig. 4.17).
Microfolding of an earlier foliation produces a difference in orientation of planar elements,
such as mica and quartz contacts, with respect to the instantaneous 3, enhancing preferred
dissolution in fold limbs, producing a differentiated crenulation cleavage and later a
compositional layering.
Stress-induced solution transfer may also aid development of foliation either by increased
rotation of elongate minerals due to selective solution and redeposition of material or by
truncation and preferential dissolution of micas which lie with (001) planes in the
shortening direction, coupled with preferential growth of micas with (001) planes in the
extension direction.
The intrinsic growth rate of mica is anisotropic and fastest with (001) planes in the
extension direction.
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Foliation formed by Crystal Plastic Deformation
Dislocation creep or solid state diffusion may flatten and/or elongate mineral shape
with maximum extension along the XY plane of finite strain.
Produces a preferred orientation often accompanied with undulose extinction.
Foliation formed by dynamic recrystallization
Dynamic recrystallization and oriented new growth of e.g., mica are important
mechanisms of foliation development.
Foliation formed by mimetic growth
In some rocks, elongate crystals that define secondary foliation may actually have
grown in the direction of the foliation after the deformation phase responsible for the
foliation ceased.
The elongate crystals may have replaced existing minerals inheriting their shape.
They may have nucleated and grown within a fabric with strong preferred orientation,
following to some extent this orientation.
They may have grown along layers rich in components necessary for their growth.,
mimicking the layered structure in their shape fabric.
Foliation formed by oriented growth defined by a stress field
Nucleation and growth of metamorphic minerals in a differential stress field is
thermodynamically possible.
It may lead to both shape preferred orientation (SPO) and lattice preferred orientation
(LPO) without necessarily a high strain.
Foliation formed by microfolding
If an older planar fabric is present, the mechanical anisotropy may lead to a harmonic,
regularly spaced folding producing crenulation cleavage.
The alignment of the fold limbs defines the foliation.
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Relationship of Foliation to Folds
Foliation is commonly associated with folds.
Foliation in the hinge zone of a fold is parallel to the axial plane of the fold.
Foliation on the fold limbs may fan around the axial plane
Foliation may be refracted at boundaries between layers of different lithology.
Convergent fan - foliation converges from the convex toward the concave side of the folded
layer, e.g., in competent rocks such as sandstone.
Divergent fan - foliation diverges from the convex toward the concave side of the folded
layer, e.g., in the less competent rocks such as shale or schist.
Foliation formed by folding should be more steeply inclined than bedding on the fold limbs
unless the fold has been overturned.
Rules for areas with a single episode of folding (i.e., no refolding):
If So and S1 dip in opposite direction, then So is upright.
If So and S1 dip in the same direction, then So is upright if the dip of the foliation is steeper
than that of the bedding (i.e., S1 > So).
If So and S1 dip in the same direction, then So is overturned if the dip of the bedding is steeper
than that of the foliation (So > S1).
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