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?
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
- alignment and concentration of phases;
-presssure shadows
-stylolites (pressure solution features)
-evidence for grain rotation
Q uickTim e™ and a
Graphics decom pressor
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)
