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Fabric A fabric is built of minerals and mineral aggregates with a preferred orientation

that penetrates the rock at the microscopic to centimeter scale.

Fabric is used to describe the spatial and geometric relationships that make up the rock. It includes planar and linear structures-bedding, cleavage, and the

orientation of minerals and their relationship to texture.41

Types:

Linear Fabric – elongate elements with a preferred orientation.

Planar Fabric – tabular or platy minerals or other “flat” objects with a preferred orientation e.g. mica is a platy mineral.

Random fabric – elements show no preferred orientation e.g. Hornfels.

Primary fabric – characteristic of the original rock e.g granite show primary fabric. It

is shown by undeform rock.

Tectonic fabric – result of deformation

Fabric is a configuration of objects penetrating the rock. Linear objects form L-fabrics

(top) while planar objects constitute S-fabrics (bottom). The rocks are known as L and

S-tectonites respectively.

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Foliation (planar textural elements):

Folios = page or leaf-like.

Fabric-forming planar structure is called foliation.

Foliation is broadly defined as any planar arrangement of mineral grains /structural features in a rock.

Structural geologist's definition:

Planar fabric is secondary and due to mineral recrystallization and/or plastic behavior during deformation at elevated temperatures

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Foliation is easily recognized if there is an alternate of quartz and feldspars with

mica and amphibole.

Types of foliation

Primary foliation:

Primary – forms during deposition of sediments and formation of magmatic rocks. In granite there is primary foliation.

Flow banding in rhyolite or granite Flow foliation in dikes or other intrusives

OR

Structures related to the original rock-forming process.

Originated by sedimentary processes such as transport and deposition:

Bedding (So)

Preferred orientation of sedimentary clasts

Originated by primary igneous processes such as lava flow and crystallization:

Magmatic layering in igneous rocks (cumulates)

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Preferred orientation of bubbles and pumice fragments

Recognition of primary foliation

Recognition of primary foliation is important for the reconstruction of the structural

evolution after sedimentation crystallization (So, S1, S2, etc.)

Secondary Foliation:

Forms after lithification and/or crystallization of rocks

Secondary foliation is the product of stress and strain (tectonic foliation).

Examples are cleavage, schistosity, and mylonitic foliations.

OR

Secondary foliation—the result of microscopically penetrative deformation and

distortion of sedimentary, volcanic, or intrusive igneous rocks, usually under

metamorphic condition.

Cleavage:

Cleavage refers to the ability of a rock to split or cleave into more or less parallel

surfaces

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Cleavage Development

Compaction cleavage develops due to reorientation of mineral grains and collapse of pore space. Some dissolution.

Pencil cleavage develops due to tectonic stress and development of a secondary

cleavage. Pressure solution important in it. In case of pencil cleavage clay minerals

oriented in two planes.

Slaty cleavage develops due to tectonic shortening, reorientation of clay grains,

and solution of quartz. QF- and M-domains.

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Slaty cleavage is used to describe rocks that split into thin, planar slabs when hit

with a hammer.

Slaty cleavage commonly occurs under low-grade metamorphic conditions

Phyllitic Cleavage : (thin wavy foliation)

In phyllitic cleavage QF- and M-domains become more pronounced because of

increasing of grain size as temperature increase with depth.

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Schistosity: Higher metamorphic grades lead to schistosity.

Schistosity describes rocks with foliated mineral grains that are large enough to see without magnification

Schistosity tends to affect all the different mineral components.

Rocks with schistosity are generally referred to as schist.

Gneissic foliation:

Gneissic textures occur when the silicate minerals in the rock separate and recrystallized into alternating bands of light (quartz and feldspar) and dark (biotite, amphibole, or hornblende) grains of silicate minerals.

Crenulation cleavage develops when the principal stress direction changes.

Symmetric versus asymmetric depends on the orientation of the original cleavage

compared to that of the secondary cleavage. Cleavage develops perpendicular to the

direction of shortening.

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Axial plane cleavage parallels the axial surface. Cleavage refraction occurs

when cleavage crosses layers of varying viscosity or competency.

Styolitic Cleavage:

Styolite is dissolution feature. This feature is found mostly in carbonate rocks e.g.

limestone

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Mylonite foliation formed by shear-related plastic grain size reduction of a

coarse-grained granitic rock. This fabric forms because of high non-coaxial strain and

it is found in shear zones or thrust zones that involve large (kilometer scale or more)

displacements.

OR

Mylonitic foliation: Forms due to grain-size reduction by a mix of brittle and plastic

deformation in shear zones.

Migmatite: Compositional banding due to in-situ partial melting. Swirly

appearance.

Flattening Fabric:

In a conglomerate, flattened pebbles may define a foliation.

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Tectonites

• Rocks that are PERVADED by foliation, lineation and/or cleavage.

S: Schistosity (foliation) only due to flattening- no lineation L: Lineation only, due to unidirectional stretching/ constriction LS: Foliation and Lineation, related to non coaxial strain- shearing

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OR

L -tectonites

Rocks that show a marked linear fabric (INDICATE CONSTRICTIONAL STRAIN)

S tectonites

Rocks showing pronounced planar fabric (FLATTENING STRAIN)

LS tectonites

Deformed rocks that contain both linear and planar fabric (PLANE STRAIN)

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S-Surfaces

All planar and some curved structures in deformed rocks

i.e. Cleavage, foliations, and bedding

S0 – Oldest surface, S1 – next oldest, etc. Fold Sets – F1, F2, etc. Linear Structures – L1, L2, etc. Deformation Structures – D1, D2, etc

RELATIVE AGE TERMINOLOGY

Lineation (linear textural elements)

Fabric forming linear structure is called lineation.

Parallelism or alignment of linear elements in the rock

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Types of lineations:

We will focus on those that are related to deformation

1) Intersection

2) Crenulation

3) Mineral

4) Stretching

Intersection lineation (the most common) :

Intersection of two planar features- an "apparent" lineation in that there is no fabric that is linear

OR

Intersection lineations—formed by the intersection of any two foliations Bedding and cleavage (parallel to the fold axis) Cleavage and a second cleavage

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e.g., intersection between cleavage and planar surface

Crenulation lineation (best seen in hand specimen): Intersection between fold

hinges and foliation

Mineral lineation: preferred alignment of minerals due to deformation and/or

recrystallization during deformation

Aligned needle shaped grains (hornblende) Streaked or smeared mineral fibers Long axes of platy minerals like micas

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Stretching lineation: elongation of minerals due to "stretching" deformation

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