Transcript Structural Geology (Geol 305) Semester (071)

Structural Geology
(Geol 305)
Semester (071)
Dr. Mustafa M. Hariri
CLEAVAGE AND FOLIATIONS
CLEAVAGE
A tendency to split along planes
other than bedding. Cleavage
is directly linked to other
deformation processesespecially 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
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.
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.
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.
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.
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)
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.
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.
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.
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.
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. 1716)
 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)

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.
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.
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
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 threedimensional blocks.
Importance of
Boudins
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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.
LINEATION AS SHEAR-SENSE INDICATORS
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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 shearsense indicators-the direction of
movement is perpendicular to
the lineation (rotation axis)
FOLDS AND LINEATIONS
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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.