ES2009_Ch11

Fabrics (): Foliation and Lineations ()

Jyr-Ching Hu, Department of GeosciencesNational Taiwan University

Fabric Terminology

Random fabric (): No preferred orientation (i.e., alignment) of the fabric element.

Nonrandom fabric (preferred fabric, ): Fabric elements are aligned in some

manner and/or repeated at approximately regular spacing.

Planar fabric (foliation,): Fabric element is a planar or tabular feature.

Linear fabric (Lineation, ): Fabric element is a linear feature.

Basic categories of fabricsRandom fabric A 1-D preferred fabric

A foliation A lineation

Distinction between continuous and spaced fabrics

Continuous fabrics ( )

Spaced fabrics ( )

Fabric elements are closer than 1 mm (below the resolution of the eye)

Continuous fabric ()

1 mm

Spaced fabric () (

)

The nature of tectonites ()Tectonites: Rock with a penetrative tectonic fabric ()

L-tectonite: Linear fabric elements dominate;S-tectonite: A rock with dominantly planar fabrics;LS-tectonite: Rock with both types of fabric elements

L-tectoniteS-tectonite LS-tectonite

Tectonites () :

(Rocks with penetrative tectonic fabric).

S-Tectonite L-Tectonite LS-Tectonite

Penetrative()

Foliations () Foliation: Any type of planar fabric in a rock. Bedding, cleavage (), schistosity () and

gneissosity () all qualify as foliations. Fractures are not considered to be foliations, because

fractures are breaks through a rock and are not a part of rock itself.

Convention: Track of different foliations (s0 , s1 , s2 ,)S0 : bedding; S1 : First foliation formed after bedding;S2 : Foliation forms after S1.

L: Lineation, S: surface in English, schistosit in French or Schieferung in German

(foliation) (primary foliation)

(secondary foliation)

What is cleavage ()? Nongenetic definition: A secondary fabric element,

formed under low-temperature conditions, that imparts on the a rock a tendency to split along planes.

1. Feature of rock that forms subsequent to the origin of rock.

2. Tectonic planar fabrics formed at or below lower greenschist facies conditions (i.e., 300oC).

3. The term cleavage is not used when referring to the fabric in schists or in gneiss.

4. A rock with cleavage, there are planes of weakness across which the rock may break when uplifted and exposed at the surface, even though cleavage itself forms without loss of cohesion. Closely spaced fractures array is not a cleavage.

1.

2. (lower greenschist facies)( 3000C)

3.

Slaty cleavage

Courtesy of Professor Yang Chao-Nan, Dept. Geosciences, NTU

Powell (1979) (domainal structures)

(cleavage domain):

(microlithon domain)

(domainal structures)

Spaced disjunctive cleavage (, ,): solution cleavage in

limestone

Disjunctive cleavage () The disjunctive implies that the cleavage domain cut

across a preexisting foliation in the rock (bedding) without affecting the orientation of the preexisting foliation in the microlithons.

Pressure solution is always involved in the formation of a disjunctive cleavage, pressure-solution cleavage (

, ) and stylolitic cleavage () have been used for this structure.

(disjunctive cleavage)

(marls)

Stylolite ()


Pressure shadows ()


Cleavage description and

classification

Sutured domains Planar domains Wavy domains

Anastomosing array ofwavy domains

Spaced cleavage: Domain spacing

Anastomosing array of wavy domains


Pencil Cleavage

Fine-grained sedimentary rock (shale or mudstone) breaks into elongate pencil-like shards because of its internal fabric.

Pencils are 5-10 cm long and 0,5-1 cm in width. It results from the interaction of two fracture sets. Pencil cleavage forms because of the strong

shape anisotropy of clay flakes creates a preferred orientation parallel to bedding.

Progressive development of slaty cleavage via the formation of pencil structure

Layer-parallel shortening

Cleavage formation process begin to take place: Large detrital phyllosilicates fold and rotate, while fine grain undergo pressure solution along domain perpendicular to the shortening direction, and new clay crystallizes.

Strain ellipsoid:pancake

(a) (b)

diagenetic foliation

(c) diagenetic foliation

(pencil cleavage)

(d)

Slaty cleavage () Slaty cleavage is defined by strong dimensionally

preferred orientation of phyllosilicates ( ) in a very clay-rich rock.

Slaty cleavage forms under temperature condition that mark the onset of metamorphism (250oC-350oC), there is a notable decreasing in the amount of interlayered water in clays; that is, smectite (, water-bearing clay) transforms to illite ().

(smectite) (illite)

Phyllitic cleavage () and schistosity ()

When metamorphic conditions reach lower greenschist (), the clay and illite in a pelitic rock (

) react to form white mica (muscovite, ) and chlorite ().

Phyllite (): Rock is composed of strongly aligned fine-grained white mica and/or chlorite.

Phyllitic cleavage is intermediate between slaty cleavage and schistosity.

Schist (): When metamorphic conditions get into the middle greenschist facies (), , the minerals in a pelitic lithology react to form coarser-grained mica and other minerals.

Phyllitic cleavage () and schistosity ()

Conveniently, a schist is named by the assemblage of metamorphic index minerals that in contains, garnet- biotite schist (-).

Phyllite: Rock is composed of strongly aligned fine- grained white mica and/or chlorite.

In schist contains porphyroclasts (, relict large crystals) or porphyroblasts () newly grown large crystals), the schistosity tends to be wavy, as the mica curve around the large crystals.

(Phyllitic cleavage)

(Schistosity)

Continuous cleavage

Crenulation Cleavage ()

Symmetric crenulation cleavage

asymmetric (sigmoidal) crenulation cleavage

A lithology containing a closely and evenly spaced foliation that is shortened in a direction at a low angle of this foliation.

Crenulation cleavage ()

http://www.globalchange.umich.edu/Ben/differentiated%20cren%20cleav.JPG

Differentiation ()during the formation of crenulation cleavage

Crenulation cleavage forms under conditions that are also amenable to the occurrence of pressure solution ().

Qtz: Removed from limbs of the microfold and precipitates in the hinges as the crenulation cleavage forms.

Gneissic layering and Migmatization ( )

Foliated gneiss (gneissosity): metamorphic rock in which the foliation is defined by compositional banding().

Light-colored layers: feldspar and Qtz Dark-colored layers: amphibole (

)/pyroxene ()(and/or biotite) Gnesis formation (amphibolite to granulite facies, ): muscovite reacts to form feldspar, so the rocks contains no schistosity.

Fold hinges in transposed () gneiss: (transposed)

Felsic mineral (): feldspar, Qtz

Mafic mineral (, ):

amphibole/pyroxene,

biotite

Gneissic layering and Migmatization ( )

Paragneiss (): Sedimentary protolith ( ).

Orthogneiss (): Igneous protolith ( ).

Augen gneiss (): Contains relatively large feldspar clasts floating in a fine-grained matrix.

Augen Gneiss ()

http://www.psibertrip.com/geology/metamorphic.asp

(gneiss)

(compositional banding)

(gneissosity)

( )(amphibolite to granulite facies)

(paragneiss)

(orthogneiss)

(gneissic structure)

(compositional banding)

Mechanism of formation of a

gneiss(a) Inheritance from an

original lithology

(b) Creation of new composition banding via transposition: Hinge of fold detached

(c) Metamorphic differentiation: Diffusion, ions to be excluded

(d) Igneous process: Lit- par-lit intrusion

(compositional layering)(a)

(b) (transposition)

(c)

(metamorphic differentiation)

(d)

(lit-par-lit intrusion)

Migmatite ()

http://epod.usra.edu/archive/epodviewer.php3?oid=204160Eric Cohen, Westhampton Beach High School

The metamorphic rocks exposed in this outcrop near Michigamme, Michigan are over 3 billion years old.

Migmatite ()

()

()( )(refractory

minerals) (partial melting or anatexis)

Cleavage and Strain Does the study of a foliation in the field provide

any constraints on the nature of strain in the region?

A whisy-washy answer: sometimes yes, sometimes no.

Strain marks in a cleaved rock

Strain markers: Reduction spots

Strain markers: Reduction spots

The rock has a light colored reduction spot. When the rock forming sediments were deposited there was a small bit of organic material mixed in. As this was decomposed by bacterial processes the Fe3+ was reduced to Fe2+. Reduced Iron is much more soluble in water therefore subsequent water flow through the rock rinsed away the iron. The redish color in the rest of the rock is the oxidized iron mineral hematite that is present in the cement.

http://www.uoregon.edu/~lemerson/rotd/

The lose or not to lose volume

Volume loss strain Volume constant strain

Assuming no change in the third dimension, the intermediate strain axis Y=1

Graptolite: strain marker for finite strain

http://www.ukfossils.co.uk/LIVE/keswick04.htm

Studies of deformed markers and geochemical studies:

Rock volume may have decreased by up to 50% during cleavage formation

Geochemical studiesBased on relative immobile elements TiO2 , Y, and Zr

~45%

Foliations in Folds and Fault Zones

How cleavage develops during progressive deformation?

Layer-parallel shortening

Disjunctive cleavage

Pencil cleavage

Cleavage refraction:

Change in cleavage attitude that occurs where cleavage domain cross from one lithology into another of different competency.

Region shear

Flexural slip

Transecting CleavageCross-cutting cleavage forms in transpressional environments (: components of both pure and simple shear).

Dextral shear (dextral transpression): Counterclockwise transecting cleavage relative to roughly synchronous fold hinges

Sinistral shear: Clockwise transecting cleavage.

Q: Is it subjected to dextral shear of sinistral shear?

()


Cleavage-bedding relationships and cleavage refraction

Facing: younging

A: On the overturned limb of the upward facing part of the F2 fold, bedding dips more steeply than cleavage.

Assumption: S1 cleavage is axial planar to the F1 fold

B: On the upright limb of the upward facing part of the F2 fold, cleavage dips more steeply than bedding.


C: On the upright limb of the downward facing part of the large fold, bedding dips more steeply than bedding.


D: On the overturned limb of the downward facing part of the large fold, cleavage is steeper.


Examples of Lineations

a: Fold and crenulation hinges

b: mullions ()

c: boudins, boudinage ()

Mullions ()

Cusplike corrugations that form at the contact between units of different competencies in a deformed multilayered sequences, the axes of mullions are a lineation.

More rigid lithology occurs in convex bulge that protrude into the ductile lithology.

Because of their mechanical origin, mullions cannot be used as a facing indicator.

Boudinage ()

Elongate pebbles in a stretched conglomerate ( )

Surface Lineation Intersection lineation

Bedding-cleavage interaction Groove lineationsFiber lineations

Slip lineations ( )

Shear wave splitting ()

(isotropic)

SSS

()

(t)

Shear wave splitting

Courtesy of Professor Rau Ruey-JuinDept. Earth Sciences, NCKU

Magnetic fabrics in pressively cleavaged

mudrock

AMS: Anisotropy of magnetic susceptibility (

)

Maximum susceptibility axis:

Minimum susceptibility axis:

The minimum axes are sensitiveIndicator of cleavage intensity

Assignments

Structural Analysis: An interactive course for Earth Science Student by Declan G. De Paor

Chapter 14: Fabrics(1) Foliation(2) Lineation(3) S-C foliation(4) Low/High T

Homeworks: Geometric Rules of bedding and cleavage

1. Foliations in folds and fault zone (p. 285-288)

2. If cleavage/foliation is axial planar the upright limb of an overturned fold will dip less than the fabric whereas the overturned limb will have a steeper dip. Please explain which is the possible structural interpretation in Figures B and C for the outcrop in Figure A.

Fabrics (): Foliation and Lineations ()Fabric TerminologyBasic categories of fabricsDistinction between continuous and spaced fabricsContinuous fabric () 1 mmSpaced fabric () () The nature of tectonites ()Tectonites () : (Rocks with penetrative tectonic fabric). S-TectoniteL-Tectonite LS-Tectonite Penetrative() Foliations () (foliation)(primary foliation)(secondary foliation) What is cleavage ()?1.2.(lower greenschist facies)( 3000C)3.Slaty cleavage(domainal structures) Spaced disjunctive cleavage (,,): solution cleavage in limestoneDisjunctive cleavage () (disjunctive cleavage)Stylolite ()Pressure shadows ()Cleavage description and classificationAnastomosing array of wavy domainsPencil CleavageProgressive development of slaty cleavage via the formation of pencil structureSlaty cleavage ()Phyllitic cleavage () and schistosity ()Phyllitic cleavage () and schistosity () 29(Phyllitic cleavage) (Schistosity) Continuous cleavageCrenulation Cleavage ()Crenulation cleavage () Differentiation ()during the formation of crenulation cleavageGneissic layering and Migmatization ()Fold hinges in transposed () gneiss:(transposed)Gneissic layering and Migmatization ()Augen Gneiss ()(gneiss)(gneissic structure)Mechanism of formation of a gneiss 45Migmatite ()Migmatite ()Cleavage and StrainStrain markers: Reduction spotsStrain markers: Reduction spotsThe lose or not to lose volumeGraptolite: strain marker for finite strainGeochemical studiesFoliations in Folds and Fault ZonesTransecting Cleavage()Cleavage-bedding relationships and cleavage refraction 59 60 61 62Examples of LineationsMullions ()Boudinage ()Elongate pebbles in a stretched conglomerate ()Surface Lineation Shear wave splitting ()Shear wave splittingMagnetic fabrics in pressively cleavaged mudrockAssignmentsHomeworks: Geometric Rules of bedding and cleavage

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ES2009_Ch11