Download - Foliation and lineation
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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