Ductile Structures: Foliations

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Ductile Structures Foliations Lineations
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Foliations

• Fabric-any geometric arrangement of component
  features in a rock (e.g., mineral grains,
  clasts) can be either tectonic or primary (e.g.,
  bedding) two types of fabrics (1) foliations
  (2) lineations

lineation
foliation
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Foliations
L-tectonite
LS-tectonite
S-tectonite
Tectonite-rock with a penetrative fabric
Penetrative fabric
Spaced fabric
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Foliations

• Foliation-planar fabric in a rock (e.g., bedding,
  cleavage)
• cleavage-fabric element that causes rock to
  split along planes (generally form at T350C)
  forms normal to direction of maximum shortening

Concentration of insoluble minerals (clays and
organics)
Development of a spaced cleavage (e.g.,
stylolites) Pressure Solution mechanism
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Pressure Solution
Need a film of water bonded to grain dissolved
mineral ions diffuse through water film
Very high stress at grain contacts
precipitated mineral
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Foliations
Pencil structures
Slaty cleavage (phylittic cleavage if T350C)
Types of cleavage
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Foliations(Phyllitic Cleavage and Schistosity)

• Both form at T350C (schistosity generally
  500C)
• Clay in shale (and slate) recrystalize at higher
  temperature (by diffusion) into fine-grained
  chlorite and muscovite (if phyllite) and
  coarse-grained muscovite/biotite if a schist
• Schist often contain porphyroblasts (newly grown
  large crystals), such as garnet

Garnet-mica schist
mica schist
slate
phyllite
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Foliations Gneissic Layering (or, how to form
Gneiss)
Inheritance from original rock types
Transposition
Metamorphic differentiation (diffusion
Lit-par-lit intrusion
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Foliation and Folds
Formation of cleavage in a fold-thrust belt
Layer-parallel shortening
Axial-planar cleavage (in shale)
Flexural folding reorients cleavage (fanning
cleavage) in sandstone
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Foliation and Folds
Slaty cleavage in a synform
Direction of maximum shortening
slaty cleavage (parallels fold axial surface
formed normal to maximum shortening)
bedding
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Lineations

• Linear fabric elements
• a. fold-hinge lineation (orientation of
  imaginary fold hinge)
• b. boudins (sausage-shaped lenses of a rigid
  layer in a less viscous layerforms by extension)
• c. surface lineation (cleavage/bedding
  intersection slip lineations on a fault)
• d. mineral lineation (occur on foliation plane
  and shear surfaces, like in a mylonite)
• stretching lineation-parallel alignment of
  linear minerals (e.g., kyanite, amphibole) due
  to rotation or crystal growth forms parallel to
  direction of extension or transport

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Lineations
Fold-hinge lineation
Boudin neck
Stretched fold limb
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Lineations
Forms parallel to fold hinges
Slip lineations on a fault (e.g., slickenlines)
Stretching lineation on phyllitic cleavage