Week 14
Cleavage II. Read pages 456-492 in Chapter
8:Cleavage, Foliation and Lineation.
You are expected to read all the sections
listed below. Information from the sections in italics
will be discussed in class. You are expected to read the other
sections and you may be called on in class to answer questions
based on that material.
- Definition of Foliation
- Primary vs. Secondary Foliation
- Foliations in Typical Metamorphic Rocks
- Foliations in Mylonitic Rocks
- Definition and Expression
- Telling the Difference Between Lineation and
Foliation
- Types of Lineation
- Types of Linear Structure
Descriptive and Geometric Analyses of Foliation and
Lineation p.472-476
- The Problem
- The Coding System
Tectonites p.476-482
- The Concept
- Types of Tectonite
- Strain Significance of Tectonites
- Flinn Diagrams
- Logarithmic Flinn Diagram
- Deformation Paths
- Identifying Deformation Paths of Natural Tectonites
Relation Between Deformation and Metamorphism p.485-487
Relation Between Deformation and Plutonism p.487-492
- Plate Tectonic Environments of Tectonite Formation
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You should become familiar with the
following terms during this weeks lectures and readings:
| augen |
bedding fissility |
boudin |
constriction |
| crenulation lineation |
eutaxitic structure |
flattened-pebble
foliation |
| flattening |
Flinn diagram |
flow banding |
foliation |
| gneissic structure |
intersection lineation |
L-tectonite |
LS-tectonite |
| lineation |
mineral lineation |
mullion |
mylonite |
| oblate strain ellipsoid |
pencil structure |
primary foliation |
primary lineation |
| prolate strain ellipsoid |
protolith |
S-tectonite |
secondary foliations |
| stretched pebble
conglomerate |
tectonites |
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You should be able to answer the
questions below following this week:
- Describe a deformation history that would allow the
following to form in association with a fold: a)
mullions; b) boudins; c) pencil structure. In each case,
describe the relative orientation of the feature and the
fold.
- Define the following terms: S-tectonite, LS-tectonite,
L-tectonite, mylonite, boudin, mullion, prolate strain
ellipsoid.
- Describe the features of a Flinn diagram.
- Suggest an explanation for the light colored feature
illustrated in the figure to the right. Note the
orientation of foliation. (Image
courtesy of Dr. A. Foos)
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Cleavage,
Foliation and Lineation
Foliation
Foliation - mesoscopically penetrative parallel
alignment of planar fabric elements in a rock
Cleavage is an example of foliation, other secondary
foliations are schistosity, phyllitic structure.
Mylonitic rocks
- Strongly deformed fault rock formed under ductile (or
brittle-ductile) conditions (mylonitization)
- Arrangement of deformed grains gives foliation
Three types - reflect character of protolith from which they
form:
- Protomylonite - 50+% relict grains
- Mylonite - 50-90% matrix with strong foliation
- Ultramylonite - 90+% fine-grained matrix
Lineation
- Lineation - (sub)parallel alignment of elongate linear
fabric elements in a rock, commonly penetrative at
outcrop and/or hand-specimen scale.
- Linear structure - large, parallel aligned elements
observed at the outcrop scale.
Three types of lineation:
- Intersection lineation - created by intersection of
foliations or of foliations with compositional layering.
- Crenulation lineation - alignment of fold hinges in
foliated rocks (e.g. schist, phyllite)
- Mineral lineation - formed by the alignment of minerals
in the foliation plane of metamorphic rocks due to
directional crystallization and comminution of larger
grains.
Linear structure
- Stretched pebble conglomerate - deformed clasts in
original conglomerate
- Mullions - corrugated surfaces between different
rock types, cusps point toward the more competent
material. Form due to buckling instability between stiff
vs. soft layers.
- Pencil structure - formed by the intersection of
bedding fissility and cleavage planes.
- Boudins - sausage-shaped segments of extended
competent layers surrounded by less competent matrix. If
ductility contrast is large, boudins will be subangular
with rectangular forms. As ductility contrasts diminish,
boudins become more lens-shaped in profile.
Tectonites
Tectonites - rocks with pervasive foliations and/or lineations
- All parts of the rock are deformed
- Rocks flow in the solid state due to microscopic
deformation mechanisms
Three types of tectonites:
- S - tectonites - tectonites with foliation but not
lineation
- L - tectonites - tectonites with lineation but not
foliation
- LS - tectonites - tectonites with both foliation
and lineation, lineation lie in the plane of the
foliation
Strain significance of tectonites:
Flattening
- Produces S-tectonites
- Spheres are deformed into oblate strain ellipsoids
(hamburger); S1 = S2 > S3
Constriction
- Produces L-tectonites
- Spheres are deformed into prolate strain ellipsoids
(cigar); S1 > S2 = S3
Plane Strain
- May produce LS-tectonites
- Form in non-coaxial strain conditions; stretching in one
direction compensated by flattening in another; S1
> S2 > S3, assume S2
= 1
Flinn diagrams
- Allow the representation of three dimensional strain on a
two dimensional plot
- If S2 = 1, plane strain
- k = (a-1)/(b-1)
- See overhead
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