Transcript Structural Geology (Geol 305) Semester (071)

Structural Geology
(Geol 305)
Semester (071)
Dr. Mustafa M. Hariri
FOLDS
Objectives
By the end of this unit you will be able to:
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Where folds occur and their
nomenclature
What are the different fold types
Understating folding process
Differentiate between different type of
folds
Understand the fold mechanisms and
where different type of folds occur
Folds
Folds are wave like structures
that produced by
deformation of bedding,
foliation or other planar
surfaces in the rocks. They
occur on all scales form
microscopic to kilometers
sizes. They form in all
deformational
environments from near
surface brittle to lowercrust ductile and from
simple shear to pure shear.
They occur singly and in
extensive fold trains
Importance of folding
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Hydrocarbon traps.
Concentration of
valuable minerals
(saddle-reef deposits)
sulfide minerals localized
in the hinges of the fold
Scale types of Folds
Folds can present in all
scales
 microscopic (require
magnification)
 mesoscopic (specimen and
outcrop size)
 macroscopic (larger scale)
Pumpelly’s rule: small-scale
structures are generally
mimic larger-scale.
ANATOMY OF FOLDS
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Crest, trough, Limbs, hinge zones,
fold axis, axial plane, axial surface,
plunge, wavelength, inflection
point and vergence.
Vergence
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Vergence of a fold applies only to
folds having one limb that dips
more steeply and is shorter than
the other-an asymmetric fold. In
symmetrical folds vergence is not a
property. However, small folds on
the limbs of symmetrical fold may
exhibit vergence.
Study of vergence may be useful
in working out the overall
direction of tectonic transport of
all structures in an area and help
to fix an observer’s location on
large fold.
Slip lines: lines of fibers or slickensides on a layer surface that indicate
the direction of motion of one layer
past another
Fold orders
The largest folds in a given area are
often called first-order folds,
smaller folds on the limbs (flanks)
are second order folds.
To relate the geometry of small-to
large scale folds enveloping
surface is used. The enveloping
surface can be constructed through
connecting the inflection points.
Enveloping surfaces are useful
for studying folds at outcrop scale
or in cross section where many
small folds occur on limbs of
larger folds, but the geometry of
the larger folds not clear.
Types of Folds
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Anticline: folds that are concave
towards the older rocks.
Syncline: folds that are concave
towards the younger rocks.
Antiform: fold is concave
downward and rocks may not be
older in the middle or age of the
rocks is not known.
Synform: fold is concave upward
and rocks in the middle may not
be younger or age is not known.
Dome: layering dips in all
directions away from a center
point.
Basin: layering dips inward
toward a central point.
Antiformal syncline: Downward
facing syncline in which layering
dips away from axis, but the
rocks in the center are younger.
Synformal anticline: upward
facing anticline, where in layering
dips inward as syncline but the
rocks in the center are older.
Domes
Types of Folds
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Homocline: rocks that dip uniformly in one
direction (Fig. 14.8)
Monocline: a local steepening with homocline
Structural terrace: local flattening of a uniform
regional dip
Cylindrical: The hinges are parallel every where
and the fold can be generated by moving the fold
axis parallel to itself (Fig. 14.9)
Non-cylindrical: The hinges are not parallel and
can converge in one point (Fig.14.9)
Sheath folds: are non-cylindrical and closed at one
end the fold hinges curve within axial surface (Fig.
14.10)
Upright folds: have vertical axial surface (Fig.
14.11)
Overturned folds: have one inverted limb (Fig.
14.11)
Reclined folds: axes plunge at nearly same angle
as the dip of the axial surface, plunge of the axis
normal or at high angle to the strike of the axial plane
(Fig. 14.11)
Recumbent folds: Have horizontal axes and axial
surfaces.
Isoclinal folds: are tight folds wherein axial
surfaces and limbs are parallel
To distinguish between the different type
of folds Fig. 14.13 (after Fleuty 1964) is used.
Classification of folds based on the
bedding thickness, and hinge
curvature
(Fig. 14.14)
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Parallel folds: folds maintain constant
thickness (Fig. 14.14)
Concentric folds: parallel folds in which
folded surfaces define circular arcs and
maintain the same center of curvature.
Ptygmatic folds: nearly concentric shape,
attenuated limbs and intestinal appearance.
Similar folds: maintain the same shape
throughout a section but not necessarily with
the same thickness.
Chevron and kink folds: have sharp
angular hinges and straight limbs.
Disharmonic: shape or wavelength
changes from one layer to another.
Supratenuous folds: synclines are
thickened and anticlines are thinned. These
folds are usually non-tectonic form in
unconsolidated sediments and when uplift is
taking place.
Fault-bend and fault-propagation folds:
(Fig. 11.11) these type of folds associated with
thrust fault
Parasitic folds are used to
determine the position in a fold
parasitic or small size fold on the
limb of big size fold can be used
to determine the position as they
have Z sense of rotation clockwise
in one limb and S sense of
movement anti-clockwise in the
opposite limb. W and M sense of
movement are found at the hinge
of the big size fold.
Stereonet is also used to determine
the direction, vergence, and
sense of movement of big fold by
plotting the vergence and
parasitic small folds.
FOLDS CLASSIFICATION
Fleuty Classification:
based on interlimb angle and hinge area (See Fig. 14-21)
Gentle, Open, Closed, Tight, Isoclinal and Elastica
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Donath and Parker Classification :
based on ductility and ductility contrast (Fig. 14-27)
Quasi-Flexural, Passive-slip, Passive-flow, Flexural flow,
and Flexural slip
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Donath and Parker
Classification
Donath and Parker Classification (1964)
Flexural-slip folds
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Flexural-slip folds: parallel concentric folds form by buckling
or bending. Slip in these folds is parallel to the layering and
characterized by slickensides, fibers. They have constant layer
thickness.
Passive-slip folds
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Passive-slip folds: type of similar folds, form by
shearing along planes inclined by layering,
form by simple shear and not pure shear.
Flexural-flow folds
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Flexural-flow folds: form in rocks from low and
moderate metamorphic grade. They are similar like
folds. Some layers maintain constant thickness but
others thickened into axial plane and thinned into
limbs, indicating higher contrast in internal ductility.
Example shale (change thickness) and quartzite (fixed
thickness)
Passive-flow folds
Passive-flow folds: are similar folds that involve plastic deformation.
The layering acts only as a displacement marker. Passive flow folds
form in metamorphic rocks with low mean ductility and ductility
contrast. Example salt, glacial ice and water saturated
unconsolidated sediments
Quasi-flexural folds
Quasi-flexural folds: are similar to the passiveflow folds but they are dis-harmonic folds
FOLD MECHANICS
Fold mechanism is influenced by factors
affecting deformation:
 temperature
 pressure
 fluid
 properties of the rock as determined by
composition, texture, and anisotropy.
Anisotropy is affected by changes in
temperature and pressure.
Fold mechanisms
Fold mechanisms include:
 Buckling
 Bending
 Passive (ductile) flow
 Flexural slip
 Kinking
 Flexural flow
The end shape of a fold is may be a produced of one or more fold
mechanism. (see Fig. 15-4)
Buckling may be accompanied by flexural slip
act early in the fold formation and buckling
accompanied with flexural flow dominated
later as a result of tighten and pressure
increases during progressive deformation.
Under high temperature and pressure layers
may no longer control the shapes of the folds
but may serve only as strain markers.
FLEXURAL SLIP
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Act usually in low temperature
and pressure found at shallow
depth within the Earth Crust.
Layers maintain their thickness
through slip past one another
(book pages)
Flexural slip usually
accompanies the bending and
buckling mechanisms and is
recognized by slickensides or
fibers on bedding surface. Fibers
may be oriented perpendicular to
the fold hinge lines.
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BENDING
Bending involves application of force across layers.
Generally produce folds that are very gentle with large interlimb
angles.
They involves flexural flow and are common in continental
interiors-cartons- where vertical forces may be directed at high
angle to the originally horizontal bedding, producing the broad
domes and basins (example arching cover rocks over basement)
Flexural bending of lithospheric plates also occurs at subduction
zones and adjacent to oceans.
Layers in bending are bent like an elastic beam the has been
supported at the ends and loaded in the middle.
In this type of fold mechanism layers are also go flexural slip.
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BUCKLING
Folds form by buckling where force is applied parallel to
layering in rocks. The product of buckling is buckled fold.
Flexural slip commonly accompanies buckling at low
temperature and pressure.
The result of this mechanism at low temperature is parallel
concentric folds (in low temperature).
In high temperature the resulted type of fold may be similar
like folds.
Buckling and thrust fault in-between anticline and syncline
may produce fault-propagation folds at low temperature.
Buckling is usually produce layers shortening. Folds formed by a combination of buckling and pressuresolution strain maintain the shapes of buckle folds but may
develop a strong cleavage because of associated flattening
For Bending and Buckling see Figures 15-8 and 15-9
PASSIVE SLIP
See Figure 15-17
Is defined as slip at an angle to layering compared to flexural slip
where slip is parallel to layering. Slip in passive slip results in
a new cleavage or schistosity to accommodates movement
parallel to the new surface.
In this type of slip bedding or compositional layering serve only
as strain marker that record the displacement parallel to the
cleavage.
KINK FOLDING
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Kink and chevron folds have straight limbs and narrow angular
hinges. They form in minerals and rocks and occur on any scale
from crystal lattices to amp scale.
Kink folds requires local slippage (flexural slip) between layers.
If shear strength is exceeded and free slippage can occur in all
layers throughout the rock mass sinusoidal buckle folds will
form.
FLEXURAL FLOW
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In flexural flow some layers flow
ductility while others remain
brittle and buckle.
Flexural flow requires moderateto high ductility contrast
between layers.
Strong layers may not undergo
thickness changes but weak
layers may go extreme thickness
changes.
The products of flexural flow are
similar fold.
In flexural fold amplitude and
wavelength may be controlled by
the original thickness, spacing
and strength of the strong
layers.
PASSIVE FLOW
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Involves uniform ductile
flow of the entire rock
mass.
Layering, foliation,
gneissic banding serving
only as a strain marker.
In passive flow their must
be little or no ductility
contrast between layers.