Foliation

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Foliation
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Foliation(Passchier and Trouw, 1996)

• Any closely-spaced, systematically oriented
  planar feature that occurs penetratively in a
  body of rock, and commonly associated with folds.
• Penetrative means that
• the foliation occurs throughout the volume of the
  rock.
• spacing or the scale of the structure in a rock
  is very small compared to the size of the rock
  volume under consideration (foliation must be on
  the order of tens of centimeter. If spacing of a
  planar structure is on the order of meters, then
  it is not a foliation (e.g., fracture).

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• A homogeneously distributed planar structure in a
  rock.
• Foliation is a characteristic of tectonites,
  i.e., rocks formed by deformation which are
  commonly, but not necessarily, metamorphosed.
• Tectonites are rocks with pervasive foliation
  (S-tectonite) and/or lineation (L-tectonite or
  LS-tectonite)

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Foliation may be defined by a

• spatial variation in mineral composition or grain
  size.
• preferred orientation of platy grains in a matrix
  without fabric.
• e.g., mica in micaceous quartzite or gneiss.
• preferred orientation of grain boundaries of
  deformed elongate grains.
• e.g., elongate quartz or calcite.
• preferred orientation of lenticular mineral/grain
  aggregates.
• planar discontinuities such as microfractures and
  microfaults
• in low grade quartzite and foliated cataclasite.
• combination of the above.

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Foliation Includes

• Rhythmic bedding in sedimentary rocks
• Compositional layering in igneous rocks
• planar alignment of sedimentary clasts.
• parallel alignment of conglomerate pebbles.
• planar alignment of fused clasts in ignimbrite
• S-C foliation in metamorphic rocks
• Excludes joints because they are not sufficiently
  penetrative.

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Significance of Foliation

• Deformed terrains 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

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• 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.

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• 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 Sn1 , Sn2.

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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.

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Two general types of foliation

• Spaced foliation
• Continuous foliation

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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
• 1. 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.
• 2. Microlithons
• lie between cleavage domains
• contain fabric elements with weak or no preferred
  orientation
• may contain fabric elements oblique to the
  cleavage domains.

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• Spaced foliations are subdivided based on the
  structure in the microlithons.Crenulation
  cleavage Microlithons contain microfolds of an
  earlier foliation.Disjunctive
  foliationMicrolithons have no
  microfolds.Called disjunctive cleavage is rock
  is fine-grained.Compositional layering A
  special type of spaced foliation where
  microlithons and cleavage domains are wide and
  continuous enough to form layers visible to the
  unaided eye in hand specimen.

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• Morphological features used in the descri
• ption 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.

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• 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.

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• 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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1. 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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2. 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.

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• 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.

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• 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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3. 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.

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4. Foliation formed by dynamic recrystallization

• Dynamic recrystallization and oriented new growth
  of e.g., mica are important mechanisms of
  foliation development.

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5. 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.

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6. 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.

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7. 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.

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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).