Cleavage, foliation and lineation (chapter 8 in davis and reynolds)

Crenulation cle avageThe vertical foliation in this rock is a crenulation cleavage, and developed after the horizonal foliation.. Relationships between deformation and metamorphism•Conne

Cleavage, foli ation and l ineation (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 f eatures

map view

Example; an Ordovician carbonate

An important term: fabric ,

is the total sumof grain

… Cleavage is often seen developed at microscopic scale.

Distinct “domains” of quartz and mica These domains are

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 f irst 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.

This is a sample of the Ira Phyllite, Vermont.

Note the wavy foliation and the overall fine-grain size of this rock.

Crenulation cle avage

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

Rock type

Muscovite-biotite -garnet schist

Locality

New Mexico

Bedding-cleavage relationships in Otago Schist, Lake Hawea, South Island, New Zealand

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 f olding-cleavage-tranposition-”pseudostratigraphy”

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,

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 graine d, phylonite

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

Rock type

mylonite

Locality

Ragged Ridge, NC

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

geometric, not genetic):

•S

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….

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

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.

EXA MPLE-Mi neral King pendant, Si erra Nevada, CA

Foliation-near vertical Lineation-near-vertical

EXA MPLE-Mi neral King pendant, Si erra Nevada, CA

Foliation-near vertical

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

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 f ormed under

brittle conditions.

The shear zones to be considered here are formed either under

intermediate, brittle-ductile or strictly ductile conditions.

1 Overall geometry

2 Tectonic setting

3 Transitions f rom 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 (f aults) 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?

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 - 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

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

7 Pressure shadows and fibers.

HOMEWORK FOR NEXT TIME:

Other shear sense indicators:

1 Veins

2 Shear bands

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