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Fabric
• The characteristics of the geometry and spacing of the elements that make up a rock. – Linear – Planar – Random
Foliation
• Any fabric-forming planar or curvi-planar structure. • May be primary or tectonic • Many rocks have more than one foliation • Approximate as planes and/or surfaces
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Cleavage
• Describes the tendency of a rock to split along foliation planes • Not the same as cleavage of a single crystal/mineral !
Where well-developed, synonym for foliation.
Lineation • Linear elements in a rock • One axis >> other two • Penetrative, surface or geometric
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Keep thinking about strain ellipse
• Foliations form perpendicular to shortening (e3 axis is pole to foliation plane) • Lineations – Elongation – mineral stretching = e1 – Shear lineations – oblique to strain ellipse axes, gives asymmetry/rotation
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Tectonites
• Fabrics in deformed metamorphic rocks are referred to as tectonites
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S-tectonite
Gneissic banding
• Gneiss – Early layering is folded – Flattened limbs are parallel • Transposed foliation
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Mylonites
• Contain a “mylonitic foliation” formed by crystal plastic deformation in a shear zone (pure or simple) • Transposed layering • Shear zone fault rocks
Describing cleavage
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Metamorphosis of a pelite
• Slaty cleavage – Preferential dissolution of some minerals – Perpendicular to shortening • Minerals line up • Becomes more continuous as it
1mm develops
Crenulation = 2nd foliation folds first
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Crenulation Cleavage
– Early cleavage is folded after a rotation of shortening axis – Symmetric or asymmetric – Hinges of tiny folds = intersection lineation
1mm
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Spaced crenulation – creates “folds”
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Higher grade metamorphism
• Phyllitic cleavage: clays recrystallize to micas • Schist: Mica grains grow, foliation non- planar because of new, stiff phases
Higher grades: recrystallization, grain growth and crystal plastic deformation become important
1mm
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Cleavage and folds • Axial planar cleavage – parallel to axial plane! • Common
Aligns with axial planes of folds!
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Cleavage refraction
• Cleavage refraction – cleavage orientation changes between layers of varying competence
Cleavage and folds
• Sometimes cleavage orientations are complicated
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Cleavage patterns • Cleavage tracks variations in strain around fold hinge
• Neutral point, but no neutral surface because of flexural shear
Cleavage and folds
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Fold information from cleavage
• cleavage dips more steeply than bedding on normal (right way up) limbs • cleavage dips less steeply and in the same direction as bedding on overturned limbs • cleavage almost always is parallel to the fold's axial plane at the fold hinge • the line (lineation) produced by the intersection of cleavage and bedding is parallel to the fold hinge • the cleavage-bedding angle is always smaller in the less competent beds and larger in the more competent beds
Mineral lineations
• Aligned prismatic minerals, or elongate mineral aggregates – Static grain growth – Syn-kinematic recrystallization – Rotation – Cataclasis – Strain shadows • Stretching • Mineral fiber
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Intersection lineations
• Planar foliations intersect bedding (or other plane)
Mullions
• Interface between competent and incompetent layers • Cusp-shapes point to more competent layer
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Lineations and kinematics
• Stretching lineations commonly approximate the transport direction when projected onto shear plane
Are fold axes lineations?
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Lineations in brittle faults
• Fault surface lineations – Slickenlines from wear/abrasion – Mineral fibers from growth
~5cm
Fault lineations and kinematics
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