structural geology- lineations and foliations, their types, characteristics with examples and figures.

1. FOLIATIONS AND LINEATIONS
-Sruthy Sajeev
Department of Geology
Kerala University,
Kariavattom Campus
STRUCTURAL GEOLOGY

2. INTRODUCTION
 Foliation ( Latin word folium, meaning leaf) is any
fabric-forming planar or curvi-planar structure in a
metamorphic rock, but may also include primary
sedimentary bedding or magmatic layering.
 Some geologists reserve the term for planar
structures formed by tectonic strain.
 It is now common to include depositional bedding
and other primary planar structures in the
definition of this term.

3.  Primary foliations: form during the deposition of
sediments and formation of magmatic rocks
 eg: bedding in sedimentary rocks, flow banding in
lavas and magmatic layering in intrusive rocks.
 Secondary foliations: products of stress and strain
and most are tectonic foliations. they form in
response to tectonic stress.
 Eg: axial plane cleavages in metamorphic rocks

4.  Foliations have a structure characterised by
alternating domains.
 Domains show marked difference in preferred
orientation of mineral grains, in structure, or in
composition.

5. SPACED FOLIATION
 defined by domains having a spacing of 10
micrometer or more.
Categorised into;
1. Compositional foliation
2. Disjunctive foliation
3. Crenulation foliation

6. COMPOSITIONAL FOLIATION
 Marked by layers or laminae
 Planar alignment of platy or needle-shaped crystals
may be also present.
 Rock has weak tendency to cleave parallel to the
foliation.
subdivisions;
1. Diffuse foliation
2. Banded foliation

7. Fig:Compositional foliation in dunites

8. DIFFUSE FOLIATION
 Defined by widely spaced weak concentrations of
mineral in a rock of predominantly one lithology.
 Common in ultramafic rocks.
 Eg: dunites in which sparse layer concentration of
pyroxene crystals.
 Common in deformed granites in which concentrtions
of mafic minerals define the foliation.

9. BANDED FOLIATION
 Defined by relatively closely spaced compositional
layers that are mineralogically distinct and are of
comparable adundance.
 Common in high grade metamorphic rocks.
Fig: banded
foliation in
gneiss.s

10. DISJUNCTIVE FOLIATION
 Contain thin cleavage domains or seams marked by
concentrations of oxides and strongly aligned platy
minerals.
 Cleavage domains are seperated by tabular or
lenticular domains called microlithons in which platy
minerals are less abundant or more randomly
oriented.
 Commonly form in previously unfoliated rocks such as
limestone or mudstone
 Also develop in some foliated rocks cross cutting an
earlier foliation.
 Subdivided intofour;

11. 1.STYLOLITIC FOLIATION
 Consist of long continuous,but very irregular cleavage
domains.
 A distinct tooth like geometry in cross section.
 Individual domains are called stylolites.
 Common in limestones.
 Spacing of the cleavage domains is commmonly
from1-5cm or more.

12. 2.ANASTOMOSING FOLIATION
 Distinguished by long , continuous , wavy cleavage
domains.
 Domains form an irregular network outlining lenticular
microlithons.
 Common in phyllites and schists.
 The spacing of the clevage domain tend to be smaller than
for stylolitic foliation,averaging perhaps 0.5-1cm.
 The clevage domain contain concentrations of platy
minerals with strong preferred orientation parallel to
domain boundaries.

13. 3.ROUGH FOLIATIONS
 Typically develops in rocks containing abundant sand-
sized material.
 The cleavage domains are short, discontinuous
concentrations of highly oriented platy minerals that
envelop the coarse grains.
 The spacing of the cleavage domains are generally
less than a millimetre.
 Within the microlithons, preferred orientation of
mineral grains may vary widely from random to
strongly oriented.

14. 4.SMOOTH FOLIATIONS
 Represents the end member of the spectrum from
irregular to planar cleavage domains.
 Characteristic of some slates.
 Cleavage domains are long, continuous and smooth
with concentrations of highly oriented platy minerals.
 Cleavage domain spacing is generally less than a
millimeter.
 Fabric develop in microlithons commonly range from
random to completely oriented.

16. CRENULATION FOLIATION
 Formed by harmonic wrinkles or chevron folds that
develop in pre-existingfoliation.
 The new foliation cut across the old foliation.
Defined by;
 Both limbs of symmetric crenulations
 Long limbs of asymmetric crenulations.
 Microlithon width is comparable with the half-
wavelength or the wavelength of crenulations.
 Subdivided into;

17. Fig: Crenulation cleavage affecting the phyllitic cleavage

18. 1.ZONAL CRENULATION
FOLIATION
 Platy minerals in the new cleavage domains are
oriented at a small angle to the domain.
 Form a continuous variation of orientations from platy
minerals in microlithons.
 The microlithon boundaries are gradational.
 Compositional difference occur between the clevage
domains and microlithons.
 Proportion of platy minerals are relatively high in the
clevage domains and low in microlithons.

19. Fig:Symmetric crenulation cleavage formed by layer-parallel
shortening

20. 2.DISCRETE CRENULATION
FOLIATION
 Orientation of platy minerals in the new clevage
domains is parallel to the domains.
 Sharply discordant with the orientation of platy
minerals in the microlithons.
 Crenulations are preserved in the microlithons.
 Cleavage domains are generally narrow and may do
not neccessarily , correspond to the limbs of
crenulation and microlithons.
 Difference in mineralogy between the two domains
are similar to those of zonal crenulation foliations.

21. Fig:Discrete cleavage in phyllite, axial planar to mesoscopic folds. Joma
area, Central Norwegian Caledonides

22. CONTINUOUS FOLIATION
 Defined either by domains with spacing less than
10micrometer.
 Also by nondomainal struture.
 Divisible by grain size into;
1.Fine continuous
2.Coarse continuous.

23. 1.FINE CONTINUOUS
FOLIATIONS
 Microdomainal or micro continuous
 Micro dominal:
fine foliations, may be micro crenulation or micro
disjunctive, microdomainal spacing is less than 10
micrometer.
 Micro continuous: fine foliation characterised by
parallel allignment of all platy or inequant grains, lack
any domainal struture.
 These two are impractical to use as field
classificationterms.
 In fine grained rock, only electron microscope can
reveal these sstrutures.

24. 2.COARSE CONTINUOUS
FOLIATIONS
 Characterised by complete orientation of
homogeneously distributed platy minerals
 Or by alignment of flattened mineral grains.
 No domainal structure.
 Easily revealed by coarse grain size.
 Defined by preferred orientation of deformed objects
distributed within the rock.

25. RELATIONSHIP WITH FOLDS
 AXIALSURFACEFOLIATIONS: parallel or subparallel
to axial surface off olds.
 Fans across the fold.
 Foliation fans are convergent or divergent.
 Orientation of foliation changes significantly at the
lithologic contact –refracted foliation.
 Help to determine the geometry of folding.

26. Fig: Cleavage refraction caused by localized shear in
incompetent layers on fold limb

28. RELATIONSHIP WITH DUCTILE
SHEARZONE
 Ductile shearzone contains;
1.S-foliation
 Continuous ,coarse foliation.
 Defined by preferred orientation of mica grains and
elongate quartz grains
 Orientation is oblique to dutile shearzone.

29. 2.C-foliation
 Defined by very thin seam of very fine-grained mica
connected to the ends of the micafish.

30. 3.TRANSPORTATION
FOLIATION
 Results from a superimposition of tectonite
foliation on earlier compositional layering.

31. SPECIAL TYPES
1.Slaty cleavage:
 Fine ,continuous foliations.
 Characteristic of slate.
 Continuous or microspaced.
 Microdomain spacing is not recognisable in the field.
 Foliation provides very strong clevage.

32. 2.PHYLLITIC CLEVAGE
 Resembles slaty cleavage.
 Grain size is slightly coarser.
 Characteristic of phyllite.
 In handspecimen foliation surface has a sheen to it.
 Intermediate between fine and coarse continuous
foliation.
 Some may be smmoth disjunctive.

33. Fig: Phyllitic cleavage in lower greenschist facies phyllite.

34. 3.GNEISSIC FOLIATION
 Develop in gneisses.
 Provide best weak cleavage.

35. Fig:Gneissic banding, formed during shearing of heterogeneous intrusive
complex

36. 4.FLOW CLEAVAGE
 Term applied to continuous axial surface foliation.
 Resul to a large amount of ductile deformation in
rocks.
 Represented the orientation of ductile
deformation.

37. 5.FRACTURE CLEAVAGE
 A variety of disjunctive foliations or discrete
crenulation foliation.
 Term has applied to disjunctive foliations in which
microlithon has no fabric.

39. 6.Shear/solution/strainslip cleavage
 Used to describe a variety of spaced foliations.
 Refers to disjunctive foliations.
 None of these term is well defined and none
should be used descriptively.

40. SUMMARY
 Foliations are common in metamorphic rocks and
are often used in the definition of the different
types of metamorphic rocks.
 Without foliations it would be difficult to do proper
structural analysis
 Most foliations form perpendicular to or at a high
angle to the shortening direction.
 give us important strain information where regular
strain markers are absent.
 Different types of foliations reflect variations in
lithology and temperature or depth of burial
during deformation.

41. REFERENCE
 Twiss R.J. and Moores E.M.,structural Geology,
W.H.Freeman&company,New York,298-306.