Structural Geology (Geol 305) Semester (071)

1
Structural Geology(Geol 305)Semester (071)

• Dr. Mustafa M. Hariri

2
FOLDS
3
Objectives

• By the end of this unit you will be able to
• 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

4
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 lower-crust ductile and from
  simple shear to pure shear. They occur singly and
  in extensive fold trains

5
Importance of folding

• Hydrocarbon traps.
• Concentration of valuable minerals (saddle-reef
  deposits) sulfide minerals localized in the
  hinges of the fold

6
Scale types of Folds

• Folds can present in all scales
• microscopic (require magnification)
• mesoscopic (specimen and outcrop size)
• macroscopic (larger scale)
• Pumpellys rule small-scale structures are
  generally mimic larger-scale.

7
ANATOMY OF FOLDS

• Crest, trough, Limbs, hinge zones, fold axis,
  axial plane, axial surface, plunge, wavelength,
  inflection point and vergence.

8
Vergence

• 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
  observers location on large fold.

9

• Slip lines lines of fibers or slicken-sides on
  a layer surface that indicate the direction of
  motion of one layer past another

10
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.

11
Types of Folds

• 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.

12
Types of Folds

• 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.
13
Classification of folds based on the bedding
thickness, and hinge curvature

• (Fig. 14.14)
• 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

14
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.

15
FOLDS CLASSIFICATION

• Fleuty Classification
• based on interlimb angle and hinge area (See Fig.
  14-21)
• Gentle, Open, Closed, Tight, Isoclinal and
  Elastica
• Donath and Parker Classification
• based on ductility and ductility contrast (Fig.
  14-27)
• Quasi-Flexural, Passive-slip, Passive-flow,
  Flexural flow, and Flexural slip

16
Donath and Parker Classification

• 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.

17
Donath and Parker Classification

• 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 dis-harmonic folds

18
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.

19
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)

20

• 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.

21
FLEXURAL SLIP

• 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.

22
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.

23
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
  pressure-solution 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

24
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.

25
KINK FOLDING

• 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.

26
FLEXURAL FLOW

• In flexural flow some layers flow ductility while
  others remain brittle and buckle.
• Flexural flow requires moderate-to 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.

27
PASSIVE FLOW

• 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.

28
(No Transcript)