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Types of Foliations and lineations and their characteristics

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sruthy sajeev

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

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FOLIATIONS AND LINEATIONS -Sruthy Sajeev Department of Geology Kerala University, Kariavattom Campus STRUCTURAL GEOLOGY
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.
 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
 Foliations have a structure characterised by alternating domains.  Domains show marked difference in preferred orientation of mineral grains, in structure, or in composition.
SPACED FOLIATION  defined by domains having a spacing of 10 micrometer or more. Categorised into; 1. Compositional foliation 2. Disjunctive foliation 3. Crenulation foliation
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
Fig:Compositional foliation in dunites
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.
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
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;
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.
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.
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.
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.
Types of Foliations and lineations and their characteristics
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;
Fig: Crenulation cleavage affecting the phyllitic cleavage
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.
Fig:Symmetric crenulation cleavage formed by layer-parallel shortening
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.
Fig:Discrete cleavage in phyllite, axial planar to mesoscopic folds. Joma area, Central Norwegian Caledonides
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.
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.
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.
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.
Fig: Cleavage refraction caused by localized shear in incompetent layers on fold limb
Types of Foliations and lineations and their characteristics
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.
2.C-foliation  Defined by very thin seam of very fine-grained mica connected to the ends of the micafish.
3.TRANSPORTATION FOLIATION  Results from a superimposition of tectonite foliation on earlier compositional layering.
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.
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.
Fig: Phyllitic cleavage in lower greenschist facies phyllite.
3.GNEISSIC FOLIATION  Develop in gneisses.  Provide best weak cleavage.
Fig:Gneissic banding, formed during shearing of heterogeneous intrusive complex
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.
5.FRACTURE CLEAVAGE  A variety of disjunctive foliations or discrete crenulation foliation.  Term has applied to disjunctive foliations in which microlithon has no fabric.
Types of Foliations and lineations and their characteristics
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.
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.
REFERENCE  Twiss R.J. and Moores E.M.,structural Geology, W.H.Freeman&company,New York,298-306.

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Types of Foliations and lineations and their characteristics

  • 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
  • 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.