Fabric Foliation

2/14/15 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 1 2/14/15 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 Image from your friend Wikipedia 2 2/14/15 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 3 2/14/15 Tectonites • Fabrics in deformed metamorphic rocks are referred to as tectonites L-tectonite 4 2/14/15 S-tectonite Gneissic banding • Gneiss – Early layering is folded – Flattened limbs are parallel • Transposed foliation 5 2/14/15 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 6 2/14/15 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 7 2/14/15 Crenulation Cleavage – Early cleavage is folded after a rotation of shortening axis – Symmetric or asymmetric – Hinges of tiny folds = intersection lineation 1mm virtualexplorer.com.au Spaced crenulation – creates “folds” 8 2/14/15 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 9 2/14/15 Cleavage and folds • Axial planar cleavage – parallel to axial plane! • Common Aligns with axial planes of folds! 10 2/14/15 Cleavage refraction • Cleavage refraction – cleavage orientation changes between layers of varying competence Cleavage and folds • Sometimes cleavage orientations are complicated 11 2/14/15 Cleavage patterns • Cleavage tracks variations in strain around fold hinge • Neutral point, but no neutral surface because of flexural shear Cleavage and folds 12 2/14/15 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 13 2/14/15 Intersection lineations • Planar foliations intersect bedding (or other plane) Mullions • Interface between competent and incompetent layers • Cusp-shapes point to more competent layer 14 2/14/15 Lineations and kinematics • Stretching lineations commonly approximate the transport direction when projected onto shear plane Are fold axes lineations? 15 2/14/15 Lineations in brittle faults • Fault surface lineations – Slickenlines from wear/abrasion – Mineral fibers from growth ~5cm Fault lineations and kinematics 16 .

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