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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?
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…..
… Cleavage is often seen
developed at microscopic scale.
Distinct “domains” of quartz and mica. These domains are often
called “microlithons”.
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.
Slate
Rock type
slate
Locality
Vermont
Note the fine grain size and the unimpressive foliation in this weakly-metamorphosed rock.
Phyllite
This is a sample of the Ira Phyllite, Vermont.
Note the wavy foliation and the overall fine-grain size of this rock.
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.
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.
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.
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
QuickTime™ and a
Graphics decompressor
are needed to see this picture.
Stratigraphy-bedding- isoclinal folding-cleavage-tranposition-”pseudostratigraphy”
final
original
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).
Boudin
Boudin developed in the Creston argillite (lower Purcell Group) near crest of anticline,
west of the Rocky Mountain Trench, British Columbia.
Foliation- is a “cleavage” typical for metamorphosed rocks.
Slaty cleavage- schistosity.. ...We already know that
In addition gneissic structure and migmatisation
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.
Lineations
Intersection lineation (the most common)
Mineral
Crenulation lineation (best seenin hand specimen)
Rodding, pencil structures
mineral
Intersection lineation
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
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
Strain analysis:
Objective- determine the magnitude and direction of distortion; not easy.
What kind of deformation to expect in tectonites?
S-tect = S1=S2>S3 (coaxial)
L-tect = S1>S2=S3 (coaxial)
LS-tect = S1>S2>S3 (non-coaxial)
Are there any strain markers in these strongly deformed rocks????
1. Deformed object that were originally spherical (the usual way)- good
luck;
2. The ellipticity method- measure strain in deformed conglomerates more of a variation on the same theme;
3. The Fry method;
4. (my favorite) A forward model resembling the Fry method….
QuickTime™ and a
Graphics decompressor
are needed to see this picture.
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
P
T
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.
EXAMPLE-Mineral King pendant, Sierra Nevada, CA
Foliation-near vertical
Lineation-near-vertical
Was deformation :
-pre-kinematic
-syn-kinematic
-post-kinematic?
EXAMPLE-Mineral King pendant, Sierra Nevada, CA
Foliation-near vertical
Answer:
-syn-kinematic
Lineation-near-vertical
Tectonites and Plate Tectonics
-tectonites, most commonly associated with plate margins;
Can you think of any example of a plate tectonics
Setting that will produce tectonites?
Keys: rocks had to be hot enough and located in an area of
high strain.
Good examples:1. 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!!
Oceanic transforms; e.g. Mid-Atlantic ridge
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.
1. Overall geometry
2. Tectonic setting
3. Transitions from brittle to ductile and viceversa in the ral world
4. Strain in shear zones
Sense of shear- similar to fault zones- dextral, sinistral, reverse, normal...
Tectonic setting
Transitions from ductile (shear zones) to brittle (faults) domains.
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?
My favorite shear sense indicators:
1. Fractured and offset grains (can’t 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.
3, Folds
4. S-C fabrics - combination of foliation and shear bands. Among the best
shear sense indicators.
5. Mica-fish fabrics. Typical for sheared rocks with muscovite
and/or biotite. A special form of S-C fabrics.
6. Porprphyroblasts, porphyroclasts and their rotation as shear-sense indicator
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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.
7. Pressure shadows and fibers.
Coaxial ? Non-coaxial? Can you tell?
HOMEWORK FOR NEXT TIME:
Other shear sense indicators:
1. Veins
2. Shear bands
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