1. Foliation

2. 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)

3. 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)

4. 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

5. 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

6. Significance of Foliation
Deformed terrains commonly have several successive generations of foliation
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

7. Significance …
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

8. 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

9. Sedimentary structures can be used for facing (younging direction)
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.

10. 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

11. 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

12. 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)

13. 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

14. Two general types of foliation:
Spaced foliation
Continuous foliation

15. 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

16. 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.

17. Spaced foliation … lie between cleavage domains
2. Microlithons
lie between cleavage domains
contain fabric elements with weak or no preferred orientation
may contain fabric elements oblique to the cleavage domains

18. 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

19. 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

20. 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

21. 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

22. 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

23. Likely mechanisms of sec. foliation formation
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

24. 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

25. 2. Foliation formed by solution transfer during 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

26. 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

27. 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

28. 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

29. 4. Foliation formed by dynamic recrystallization
Dynamic recrystallization and oriented new growth of e.g., mica are important mechanisms of foliation development

30. 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

31. 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

32. 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)

33. 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

34. 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).