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.
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
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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: type of
similar folds, form by shearing
along planes inclined by
layering, form by simple shear
and not pure shear.
Donath and Parker
Classification
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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: 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: are similar to the
passive-flow folds but they are disharmonic 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.
BENDING
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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.
BUCKLING
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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.