Download Foliation and lineation

Foliation and lineation
Goal: To interpret some common types of foliation and lineation in
deformed rocks.
Part-I foliation
Foliation: “Any mesoscopically penetrative subparallel alignment of planar
fabric elements in a rock”. — Expand on “fabric elements”, “mesoscopic”,
and “penetrative”
 Typically forms in metamorphic rocks
 Combines mineral “Shape-preferred orientation” and compositional
banding
A. Nomenclature — S0 = bedding, S1 = first foliation, S2 = second
foliation....
B. Continuous foliations: Will work from “low-grade” to “high-grade” —
Expand on “high-grade” and “low-grade”
1. Slaty cleavage: Low-grade (lower- to middle-greenschist facies),
microscopically penetrative foliation that causes rocks to split into
slabs along “cleavage” planes. — Expand on why I don’t like
“cleavage”. Hard to define, confuses rocks with minerals. Entrenched
in literature.
a) Hand-specimen description (not shiney) — pass around
example
b) Micro-scale description
 M-domains and Q-F-domains at micro-scale
 Roles of pressure-solution/reprecipitation and rotation in
forming slaty cleavage — Enormous volume loss in slates
2. Phyllitic cleavage/foliation (in book): Higher-grade version of slate
(middle- to upper-greenschist-facies conditions).
Hand-specimen description: Shiney, still cannot make out individual
mineral grains with naked eye. Pass around example
3. Schistose foliation: Penetrative foliation in which individual mineral
grains are visible. (upper-greenschist facies to lower-amphibolite
facies)
 May also be able to see M-domains and Q-F-domains with
naked eye.
 May contain large metamorphic minerals (garnet, staurolite, Alsilicates, ect.)
 Metamorphic reactions play an important role in growing
aligned minerals
4. Gneissic foliation: Shape-preferred alignment of medium- to coarsegrained minerals accompanied by compositional banding. (middleamphibolite facies to granulite facies)
Requires compositional segregation... not fully understood
 Pre-existing sedimentary
 Metamorphic/igneous segregation
C. Discontinuous foliations (Disjunctive foliations): Mesoscopically visible,
unfoliated domains between individual foliation surfaces.
1. Spaced cleavage: Array of anastomosing, fracture-like partings that
are often occupied by clay minerals or carbonaceous material
(graphite). Generally weak deformation at low-grades.
2. Crenulation cleavage: Forms by mm- to cm-scale folding of a preexisting continuous foliation. Significant in that it marks a second
phase of deformation with different stress-axes!!
As pre-existing foliation is rotated into the limbs of micro-folds,
pressure solution preferentially removes Q-F-domains in limbs of
these folds, leaving new, concentrated M-domains.
Part-II lineation
Lineation: Subparallel alignment of elongate, linear fabric elements in a
deformed rock.
A. Nomenclature — L1 = first lineation, L2 = second lineation....
B. Elongation lineations: Always in the plane of foliation. Define one of the
principal axes along which the rock was “stretched”.
1. Mineral lineations: Preferred alignment of tabular or acicular (needleor splinter-shaped) mineral grains. Not always a true elongation
lineation!!!
2. Elongated clasts: Can include cobbles, pebbles, sand grains, breccia
clasts, ect.
3. Rodded quartz: Quartz grains that have been elongated into rod or
blade shapes.
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 elements
1. Boudinage: Forms due to competency contrast between two layers.
Stronger layer is pulled apart into sausage-shaped pods.
2. Mullion structure: Also forms due to competency contrast. Very low
wave-length folds with “mullion” pointing outwards towards less
competent layer.
3. Fold hinge lines: Lines of maximum curvature on folded surface
4. Slickenlines: Grooves or fibers that form on a fault surface. Give
direction of fault slip.