Transcript Foliation (Passchier and Trouw, 1996)

Foliation
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)
• 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)
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
Foliation Includes:
•
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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
Significance of Foliation
Deformed terrains commonly have several successive
generations of foliation
1. If these can be distinguished from one another by type and
age, by cross-cutting relationships, absolute age dates, and
overprinting under microscope, then they can help to
• unravel the tectonic and metamorphic evolution
of the area
Significance …
2. Foliation can be used to as reference structure 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 over a larger area
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 lava
flow and crystallization:
• magmatic layering in igneous rocks (cumulates)
• 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 intensely deformed
rocks of higher metamorphic grade
• Bedding is obliterated or disappeared by transposition and
recrystallization
– However, bedding only rarely may parallel 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
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 the parallel orientation of thin elongate
detrital mica grains with frayed edges
• The micas are commonly subparallel to bedding
• Mica preferred orientation 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 the development of
secondary foliation in pelites
Secondary Foliation
• Forms after lithification and/or 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 (|| the XY plane) and deformed features
• Foliation forms  the maximum shortening direction (Z)
• Forms as a result of:
– ductile deformation (crystal plasticity or cataclastic flow)
– metamorphism
• Includes:
– Cleavage; schistosity; differentiated compositional layering
– mylonitic foliation (S and C)
Morphological Classification (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
Spaced foliation:
• Fabric elements are not homogeneously distributed
• 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.
Spaced foliation …
2. 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 if 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
Morphological features used in the description of
spaced foliation
• 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
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
• Schistosity: grains defining the foliation are
visible by the unaided eye
• 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
Likely mechanisms of sec. foliation formation
•
1.
2.
3.
4.
5.
6.
7.
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 (parallel to preexisting minerals after deformation)
Oriented growth defined by a stress field
Microfolding
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 ellipsoid
– If an earlier preferred orientation was present, the
foliation will not trace the XY plane
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
• 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
– Microfolding of an earlier foliation produces a
difference in the 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 the
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
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
4. Foliation formed by dynamic
recrystallization
• Dynamic recrystallization and oriented new
growth of e.g., mica are important mechanisms of
foliation development
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
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
7. Foliation formed by microfolding
• If an older planar fabric (S1) is present, the
mechanical anisotropy may lead to a harmonic,
regularly spaced folding producing crenulation
cleavage (S2)
• The alignment of the fold limbs defines the
foliation, with the fold hingelines || (S1xS2)
Relationship of foliation to folds
• Foliation is commonly associated with folds
– In the hinge zone it is commonly || the axial plane
– 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).