Structural Geology (Geol 305) Semester (071)

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Structural Geology(Geol 305)Semester (071)

• Dr. Mustafa M. Hariri

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CLEAVAGE AND FOLIATIONS
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CLEAVAGE

• A tendency to split along planes other than
  bedding. Cleavage is directly linked to other
  deformation processes-especially folding- and
  metamorphism. It can help in understanding the
  fold geometry and the physical conditions during
  deformation. It may serve as a conduit for ground
  water

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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.

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Slaty Cleavage

• Is a penetrative structure (occurs in all scale).
  It consists of parallel grains of thin layer
  silicates (clay minerals or micas) or thin
  anastomosing subparallel zones insoluble residues
  produced by pressure solution.

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S-surfaces

• Planar and some curved structures in deformed
  rocks. They include all cleavages and foliations
  commonly though as penetrative structures. They
  also include nontectonic planar structure,
  bedding. In areas of multiple S-surfaces, a
  series of subscripts is assigned bedding being
  oldest is designated S0, S1 is the oldest
  cleavage (or foliation) and any later structures
  are given numerically higher subscripts.

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Cleavage and Foliation can be divided into

• Continuous Cut all the rock mass.
• Spaced can be resolved into regions of uncleaved
  rock separated by cleavage planes spaced from
  less than a millimeters to several centimeters.
  The uncleaved zones between cleavage surfaces are
  called microlithons.

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Spaced cleavage is divided into

• Disjunctive (cross-cutting and not related to
  original layering)
• Disjunctive cleavage may be divided into
• styloitic
• anastomosing
• rough
• smooth
• Crenulation (which crenulates preexisting
  layering)
• Crenulation cleavage may be divided into
• discrete
• zonal
• Spacing in the different types of cleavage
• slaty cleavage (continuous) 0.01 mm to less than
  1.o mm
• crenulations cleavage 0.1 mm to 3cm
• Shale usually display more closely spaced
  cleavage compared to sandstone that shows wide
  space cleavage (Figs. 17-4 and 17-7)

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Cleavage types

• Pressure solution
• produces spaced cleavage by dissolving the most
  soluble parts of a rock mass leaving behind
  discrete insoluble residues in irregular planar
  zones that define cleavage (Fig. 17-8). Spacing
  of pressure solution ranges from less than a
  millimeter to more than a centimeter. They may be
  irregular ( styloitic to anastomsing to rough) to
  smooth, where rock mass is more severely
  deformed.
• Slaty cleavage
• is a planar tectonic structure resulting from
  parallel orientation of clays, muscovite, and or
  chlorite. It is penetrative and develop generally
  in rocks of fine-grained sedimentary and volcanic
  rocks, such as shale, mudstone, siltstone, and
  tuff.
• FORMATION OF SLATY CLEAVAGE
• Folding, Compression, Pressure solution,
  recrystalliztion and pure and simple shears
  concepts.

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Cleavage types

• Crenulation cleavage
• cleavage marked by small-scale crinkling or
  crenulation. Most crinkles are spaced and
  asymmetric, and the short limb becomes usually
  the cleavage plane. They commonly form by
  deformation of an earlier cleavage or bedding.
• Foliation
• foliation is a term used to describe all type of
  cleavage slaty, crenulation and it is used also
  to describe the planar structure in
  coarser-grained metamorphic rocks, such as schist
  and gneiss where planar orientation of at least
  one mineral dominates the fabric (parallel of
  mica, amphibole, and flatten of quartz grains).
• Schistosity refers to foliation in schistose.
• Foliation is easily recognized if there is an
  alternate of quartz and feldspars with mica and
  amphibole.

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Cleavage types

• Metamorphic differentiation
• formation of new layering by recrystallization
  or pressure solution. It is the production of new
  minerals with new orientation.
• Differential layering
• the foliation that is produced during
  metamorphism and recrystallization.
• At high temperature and pressure this process
  will be enhanced with processes and gniessic
  banding may be produced.
• Crenulations and spaced slaty cleavage may
  produce differential layering at low temperature
  and pressure.

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CLEAVAGE BEDDING RELATIONSHIP

• The angular relationship between cleavage and
  bedding can be used to determine whether one is
  observing the upright or the overturned limb of a
  fold.
• If bedding dips less steeply (lower angle but
  same direction as cleavage) the rock will be on
  upright limb.
• Care should be taken regarding the fold axis and
  timing of cleavage.

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CLEAVAGE REFRACTION

• Refraction of cleavage from layer to layer occurs
  where the texture and composition-ductility- vary
  from layer to layer in rocks. The angle between
  cleavage and bedding changes or refracts as the
  cleavage passes from one layer to another (Fig.
  17-16)
• Most slaty cleavage forms parallel to axial
  surfaces in folds but may be displaced or fanned
  with respect to the hinge as folding proceeds
  (Fig. 17-17)

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LINEAR STRUCTURES

• Any structure that can be expressed as a real or
  imaginary line is linear structure or lineation.
• Lineament is a topographic feature consisting of
  straight or aligned surficial features such as
  valleys and ridges.

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Non-penetrative linear structures

• Non-penetrative linear structures
• Slickenlines are the direct result of frictional
  sliding.
• Slinckensides refer to the entire movement
  surface develop on the fault surface, bedding,
  and foliation.
• Slinkenfibers fibrous crystals of calcite,
  quartz, chlorite or iron oxides where their long
  axes are oriented in the direction of movement.

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Penetrative linear structures

• Penetrative linear structures
• Intersection lineation (two cleavage or foliation
  planes)
• Mineral lineation (alignment of grain aggregates
  of mica, amphibole or feldspars)
• Pressure shadow of quartz, muscovite, chlorite,
  magnetite on either side of single crystal of
  pyrite
• Rotated minerals
• Rods rodding or grain aggregates of one or more
  minerals such as quartz, feldspars and mica (it
  is common in ductile shear zones)
• Natural strain ellipsoids long axes of pebbles,
  boulders, vesicles and reduction spots.
• Mullions form at boundaries between differing
  rock types.
• Boudinage consists of lenticular segments of
  layer that has been pulled apart and flattened
  (layer been segmented is less ductile than
  enclosing).
• Shape of boudins is affected by the degree of
  contrast between the two rocks. Large contrast
  produces boudins with sharp edges, and small
  contrast produces rounded boudins.
• Boudins can produce under conditions of either
  ductile or brittle. Under brittle condition most
  boudins are angler and space between them is
  filled with less-competent rock

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Type of Boudins

• 1) Ordinary boudinage
• consists of segmented, sausage shaped of a single
  layer in which the lenticular segments parallel
  one another. It results from extension of the
  layer in a single direction.
• 2) Chocolate-block (chocolate tablet) boudinage
• It is produced if layer-parallel extension has
  occurred in two directions, the resulting
  boudinage consists of a series of
  three-dimensional blocks.

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Importance of Boudins

• They yield information about strain, shear sense
  and difference in competence.
• The neck line of the boudin is the lineation and
  is commonly oriented parallel to the fold axes.
• Boundinage is frequently seen in the limbs of the
  folds, where most flattening and layer-parallel
  extension occurs.

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LINEATION AS SHEAR-SENSE INDICATORS

• Slickensides directly indicate movement sense by
  the direction of their lines and steps.
• Boudins indicate the extension direction.
• Mineral lineations yields sense of shear if the
  linear mineral is segmented in the movement
  direction.
• Rotated minerals are shear-sense indicators-the
  direction of movement is perpendicular to the
  lineation (rotation axis)

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FOLDS AND LINEATIONS

• Intersection lineations tend to parallel fold
  axes.
• Mullions and Boudin necks generally parallel the
  fold axes.
• Mineral-elongation lineations sometimes parallel
  fold axes and are sometimes oriented oblique to
  normal to fold axes.
• Deformed lineations are strain markers that can
  help to reveal the later deformation.