Cleavage, foliation and lineation (Chapter 8 in Davis and Reynolds)

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Cleavage, foliation and lineation (Chapter 8 in
Davis and Reynolds)
Closely spaced planar to linear features that
tend to be associated with folds, especially in
rocks formed at deeper levels in the crust. How
deep?
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Cleavage and folding
map view
Cleavage-mostly axial plane features
Example an Ordovician carbonate
An important term fabric, is the total sumof
grain shape, grain size, and grain configuration
in the rock. It is relevant to cleavage because..
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Cleavage is often seen developed at microscopic
scale.
Distinct domains of quartz and mica. These
domains are often called microlithons.
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Types of cleavage (based on the
scale) Continuous (domains need to be resolved
with the aid of a microscope) and Discontinuous
(or disjunctive if the domains can be seen with
the naked eye).

• Within the first category, the cleavage is called
  (as scale increases)
• Slaty
• Phylitic
• schistosity

• The discontinuous cleavage is further divided
  into
• Crenulation (a preexisting planar feature is
  crenulated into new microfolds)
• Spaced cleavage (array of fracture-like partings
  often filled with carbonate or other vein-like
  material)- spacing can be 1-10 cm.

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Slate
Rock type slate Locality Vermont
Note the fine grain size and the unimpressive
foliation in this weakly-metamorphosed rock.
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Phyllite
This is a sample of the Ira Phyllite, Vermont.
Note the wavy foliation and the overall
fine-grain size of this rock.
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Schistosity  
Rock type quartz-mica schist Locality unknown
A foliation is any planar fabric in a metamorphic
rock. In this case, the foliation is defined by
aligned sheets of muscovite sandwiched between
quartz grains.
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Crenulation cleavage  
Rock type Muscovite-biotite -garnet schist
Locality New Mexico
The vertical foliation in this rock is a
crenulation cleavage, and developed after the
horizonal foliation.
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Spaced cleavage
Bedding-cleavage relationships in Otago Schist,
Lake Hawea, South Island, New Zealand. Grey /
slaty grey color variation corresponds with
steeply inclined and folded bedding. Axial
planar, spaced cleavage forms prominent partings
at a high angle to bedding. Pressure solution
along cleavage surfaces has disrupted the
continuity of bedding contacts. Minute quartz
veins are evident in the outcrop and may
represent sites of reprecipitation of quartz.
Coin for scale.
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• Strain questions
• Amount of shortening
• Alignment of planar minerals (flattening,
  rotation) Problems why concentrate these
  minerals
• Recrystallization? Take the pressure shadows as
  one of many examples reflecting
  recrystallization
• Pressure solution is it important?
• Grain rotation

Next few slides will contain examples of some key
phenomena in understanding strain - alignment
and concentration of phases -presssure
shadows -stylolites (pressure solution
features) -evidence for grain rotation
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Stratigraphy-bedding- isoclinal
folding-cleavage-tranposition-pseudostratigraphy
final
original
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Flatteningthat accompanies most foliation
formation cause stiff compositional layers
surrounded by softer layers to neck and pull
apart into BOUDINS (sausage-shaped structures
that accentuate gneissic foliation).
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Boudin
Boudin developed in the Creston argillite (lower
Purcell Group) near crest of anticline, west of
the Rocky Mountain Trench, British Columbia.
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Foliation- is a cleavage typical for
metamorphosed rocks.
Slaty cleavage- schistosity.. ...We already know
that In addition gneissic structure and
migmatisation
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Mylonite-proto to ultramylonite, mylonitic
gneiss, mylonitic schist, finally if very fine
grained, phylonite
Rock type mylonite Locality Ragged Ridge, NC
Note the extremely fine grain size and strong
foliation in this mylonite. These features were
probably caused by intense shearing.
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More mylonites
Quartz flows, feldspar does not
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Coding deformation events in foliated rocks S0-
bedding, all other surface forming events are
given a code name- S1, S2, S3. Lineation are
coded with the letter L Folds are given the
letter F Group all structural elements check
if there are synchronous S, F, L, and reconstruct
deformation events coded S
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• Tectonites
• Rocks that are PERVADED by foliation, lineation
  and/or cleavage.
• These rocks flowed in solid state. The
  distribution of foliation and lineation is a
  manifestation of the state of strain.
• Tectonites formed at high T and P (most of them
  anyway).

• Types of tectonites (definition is geometric, not
  genetic)
• S
• L
• LS

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Strain analysis Objective- determine the
magnitude and direction of distortion not
easy. What kind of deformation to expect in
tectonites? S-tect S1S2gtS3 (coaxial) L-tect
S1gtS2S3 (coaxial) LS-tect S1gtS2gtS3
(non-coaxial)
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Relationships between deformation and metamorphism

• Connection between structural processes and
  metamorphism
• Tectonites are subject to grain-size reduction
  but because this process take place at high
  pressures-temperatures, tectonites are also
  subject to grain growth via recrystallization.

time
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prograde
retrograde
P
T
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Relationships between deformation and plutonism

• WHY DO WE CARE?
• Tectonites -commonly associated with plutons
• Igneous rocks- important source of heat
  responsible for metamorphism
• Age can be readily determined on plutons-
  geologic relationships
• between igneous rocks and tectonites can
  constrain the age of deformation

Intrusions can be - pre-kinematic -syn-kinematic
-post-kinematic i.e., before, during or after
deformation.
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EXAMPLE-Mineral King pendant, Sierra Nevada, CA
Foliation-near vertical
Lineation-near-vertical
Was deformation -pre-kinematic -syn-kinematic -p
ost-kinematic?
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EXAMPLE-Mineral King pendant, Sierra Nevada, CA
Foliation-near vertical
Answer -syn-kinematic
Lineation-near-vertical
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Tectonites and Plate Tectonics
Keys rocks had to be hot enough and located in
an area of high strain.
Good examples1. Transform faults in oceanic
settings 2. Gneiss domes in
collisional settings 3. Magmatic arc
terranes
These regional terranes of tectonites are great
illustrations of the transition we need to make
fairly soon in this class from the SMALL SCALE
(i.e. structural geology) to the BIG SCALE (i.e.
tectonics). Instead of calculating strain of a
conglomerate boulder we ought to deal with the
strain of, say the western US!!
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Oceanic transforms e.g. Mid-Atlantic ridge
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Shear zones and progressive deformation Tabular
to sheetlike planar or curviplanar zone of highly
strained rocks, more strained than adjacent
rocks. Clearly STRAIN is the key word, we need to
be able to determine it!! From mm thick to tens
of km !!! You could say that a fault zone is a
shear zone formed under brittle conditions. The
shear zones to be considered here are formed
either under intermediate, brittle-ductile or
strictly ductile conditions.
For the sake of classification shear zones are
continuous and discontinuous.
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1. Overall geometry 2. Tectonic setting 3.
Transitions from brittle to ductile and viceversa
in the real world 4. Strain in shear zones
Sense of shear- similar to fault zones- dextral,
sinistral, reverse, normal...
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Tectonic setting
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Transitions from ductile (shear zones) to brittle
(faults) domains.
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Strain in shear zones is accomodated
by -distorsion of the primarily ductile domains
in the shear zone -rotations of relatively rigid
objects.
Strain- coaxial or noncoaxial (pure or simple)?
Remember coaxial and non-coaxial strain?
Can we distinguish?
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My favorite shear sense indicators 1. Fractured
and offset grains (cant beat that) 2. (similar
to 1) Deflection of markers- dikes etc. 3.
Folds 4. S-C fabrics 5. Mica-fish fabrics 6.
Porphyroclasts and porphyroblasts
1., 2.
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3, Folds
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4. S-C fabrics - combination of foliation and
shear bands. Among the best shear sense
indicators.
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5. Mica-fish fabrics. Typical for sheared rocks
with muscovite and/or biotite. A special form of
S-C fabrics.
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6. Porprphyroblasts, porphyroclasts and their
rotation as shear-sense indicator
Asymmetric recrystallization tails on feldspar
porphyroclasts. Mylonitized Ayer granite from
the Wachusett mylonite zone in eastern
Massachusetts (Goldstein, 1994, Tectonics)
illustrates sigmagrain geometry. Slide is 3.5 mm
in long dimension, cut perpendicular to
foliation and parallel to foliation, viewed in
plane polarized light.
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7. Pressure shadows and fibers.
Coaxial ? Non-coaxial? Can you tell?
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Other shear sense indicators 1. Veins 2. Shear
bands
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Coaxial or non-coaxial?
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Example Catalina Mts
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Interpretative block diagram
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The end