Cleavage, foliation and lineation (Chapter 8 in Davis and Reynolds) Closely spaced planar to linear features that tend to be associated with folds, especially.

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

Cleavage-mostly axial plane featuresmap view

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) andDiscontinuous (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

Note the fine grain size and the unimpressive foliation in this weakly-metamorphosed rock.

Rock type slateLocalityVermont

Phyllite

This is a sample of the Ira Phyllite, Vermont. Note the wavy foliation and the overall fine-grain size of this rock.

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.

Schistosity  

Rock type quartz-mica schistLocalityunknown

Crenulation cleavage  

The vertical foliation in this rock is a crenulation cleavage, and developed after the horizonal foliation.

Rock type Muscovite-biotite -garnet schist

LocalityNew Mexico

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.

Spaced cleavage

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”

original

final

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 developed in the Creston argillite (lower Purcell Group) near crest of anticline, west of the Rocky Mountain Trench, British Columbia.

Boudin

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

Note the extremely fine grain size and strong foliation in this mylonite. These features were probably caused by intense shearing.

Rock type myloniteLocalityRagged Ridge, NC

Lineations

Intersection lineation (the most common)MineralCrenulation 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….

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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

T

P

prograde

retrograde

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

Lineation-near-verticalAnswer:-syn-kinematic

Tectonites and Plate Tectonics

-tectonites, most commonly associated with plate margins;

Can you think of any example of a plate tectonicsSetting that will produce tectonites?

Keys: rocks had to be hot enough and located in an area ofhigh 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 boulderwe 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 geometry2. Tectonic setting3. Transitions from brittle to ductile and viceversa in the ral world4. 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 andnon-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. Folds4. S-C fabrics5. 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.

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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.

6. Porprphyroblasts, porphyroclasts and their rotation as shear-sense indicator

7. Pressure shadows and fibers.

Coaxial ? Non-coaxial? Can you tell?

HOMEWORK FOR NEXT TIME:

Other shear sense indicators:1. Veins2. Shear bands

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