Foliation and lineation

Goal: To interpret some common types of foliation and lineation in deformed rocks

• Typically forms in metamorphic rocks• Combines mineral “Shape-preferred orientation”

and compositional banding

Foliation: mesoscopically penetrative subparallel alignment of planar fabric elements in a rock

A. Foliation nomenclature

• S0 = bedding

• S1 = first foliation

• S2 = second foliation....

Continuous vs.

discontinuous

foliations

B. Continuous foliations1. Slaty cleavage and phyllitic

cleavage

2. Schistose foliation

3. Gneissic banding

1. Slaty Cleavage

Pressure solution vs. rotation

• M-domains

• Q-F-domains

Can have volume loss > 50%

Sample with discrete M-domains and Q-F-domains

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

4. Gneissic foliation• Shape-preferred alignment of minerals

and compositional banding

• Requires compositional segregation... not fully understood– Pre-existing sedimentary– Metamorphic/igneous segregation

C. Discontinuous foliations1. Spaced cleavage

2. Crenulation cleavage

1. Spaced cleavage• Array of anastomosing, fracture-like

partings

• Generally weak deformation at low temps

2. Crenulation cleavage• mm- to cm-scale folding of a pre-existing

foliation

• Marks a second phase of deformation

Part-II Lineations — OutlineA. Lineation nomenclature

B. Elongation lineations

C. Intersection lineations

D. Other linear elements

A. Nomenclature

• L1 = first lineation

• L2 = second lineation

B. Elongation lineations1. Mineral lineations

2. Elongated clasts

• Preferred alignment of tabular or acicular mineral grains

• Not always a true elongation lineation!!!

1. Mineral lineations

2. Elongated clasts• Cobbles

• Pebbles

• Sand grains

• Breccia clasts

C. 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....

D. Other linear elements1. Boudinage

2. Mullion structure

3. Fold hinge lines

4. Slickenlines and ridge-in-groove lineations

1. BoudinageForms due to competency contrast between

layers

Examples of boudinage

2. Mullion structureAlso forms due to competency contrast

between layers... different layer orientations

Combined boudinage and mullion structure