Foliation and lineation

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Foliation and lineation
Goal To interpret some common types of foliation
and lineation in deformed rocks
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Foliation mesoscopically penetrative subparallel
alignment of planar fabric elements in a rock

• Typically forms in metamorphic rocks
• Combines mineral Shape-preferred orientation
  and compositional banding

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Foliation nomenclature

• S0 bedding
• S1 first foliation
• S2 second foliation....

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Continuous vs. discontinuous foliations
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Continuous foliations

• Slaty cleavage and phyllitic cleavage
• Schistose foliation
• Gneissic banding

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1. Slaty Cleavage
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Pressure solution vs. rotation

• M-domains
• Q-F-domains

Can have volume loss gt 50
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Sample with discrete M-domains and Q-F-domains
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3. Schistose foliation

• Individual mineral grains are visible
• May see M-domains and Q-F-domains with naked eye.
• Metamorphic reactions help grow aligned minerals

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4. Gneissic foliation

• Shape-preferred alignment of minerals and
  compositional banding
• Requires compositional segregation... not fully
  understood
• Pre-existing sedimentary
• Metamorphic/igneous segregation

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Discontinuous foliations

• Spaced cleavage
• Crenulation cleavage

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1. Spaced cleavage

• Array of anastomosing, fracture-like partings
• Generally weak deformation at low temps

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2. Crenulation cleavage

• mm- to cm-scale folding of a pre-existing
  foliation
• Marks a second phase of deformation

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Part-II Lineations
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Lineation Nomenclature

• L1 first lineation
• L2 second lineation

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Mineral lineations

• Preferred alignment of tabular or acicular
  mineral grains
• Not always a true elongation lineation!!!

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Elongated clasts
Pebbles Cobbles Breccia clasts etc.
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Intersection lineations

• Form due to the intersection of two planar fabric
  elements (e.g. S0 and S1, S1 and S2....)
• Nomenclature L0x1 S0 crossed by S1, L1x2 S1
  crossed by S2....

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Evaluating competency

• Boudinage
• Mullion structure

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1. Boudinage

• Forms due to competency contrast between layers

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Examples of boudinage
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2. Mullion structure

• Also forms due to competency contrast between
  layers... different layer orientations

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Combined boudinage and mullion structure