Structural GeologyStructural Geology(Geol(Geol 305) 305)

Semester (071)Semester (071)

Dr. Mustafa M. HaririDr. Mustafa M. Hariri

FOLDSFOLDS

ObjectivesObjectivesBy the end of this unit you will be able to:By the end of this unit you will be able to: Where folds occur and their Where folds occur and their

nomenclature nomenclature What are the different fold typesWhat are the different fold types Understating folding processUnderstating folding process Differentiate between different type of Differentiate between different type of

foldsfolds Understand the fold mechanisms and Understand the fold mechanisms and

where different type of folds occurwhere different type of folds occur

FoldsFoldsFolds are wave like structures Folds are wave like structures

that produced by that produced by deformation of bedding, deformation of bedding, foliation or other planar foliation or other planar surfaces in the rocks. They surfaces in the rocks. They occur occur on all scales form on all scales form microscopic to kilometers microscopic to kilometers sizessizes. They form in all . They form in all deformational deformational environments from near environments from near surface brittle to lower-surface brittle to lower-crust ductile and from crust ductile and from simple shear to pure shear. simple shear to pure shear. They occur singly and in They occur singly and in extensive fold trainsextensive fold trains

Importance of foldingImportance of folding

Hydrocarbon traps.Hydrocarbon traps. Concentration of Concentration of

valuable minerals valuable minerals (saddle-reef deposits) (saddle-reef deposits) sulfide minerals localized sulfide minerals localized in the hinges of the foldin the hinges of the fold

Scale types of Folds

Folds can present in all Folds can present in all scales scales

microscopicmicroscopic (require magnification)

mesoscopicmesoscopic (specimen and outcrop size)

macroscopic macroscopic (larger scale)(larger scale)

Pumpelly’s rule:Pumpelly’s rule: small-scale small-scale structures are generally structures are generally

mimic larger-scale.mimic larger-scale.

ANATOMY OF FOLDSANATOMY OF FOLDS

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

VergenceVergence VergenceVergence of a fold applies only to

folds having one limb that dips one limb that dips more steeply and is shorter than more steeply and is shorter than the other-an asymmetric foldthe other-an asymmetric fold. In symmetrical folds vergence is not a property. However, small folds on However, small folds on the limbs of symmetrical fold the limbs of symmetrical fold may exhibit vergencemay exhibit vergence.

Study of vergence may be useful Study of vergence may be useful in working out the overall in working out the overall direction of tectonic transport of direction of tectonic transport of all structures in an area and help all structures in an area and help to fix an observer’s location on to fix an observer’s location on large fold.large fold.

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

Fold ordersFold orders The largest folds in a given area are The largest folds in a given area are

often called first-order folds, often called first-order folds, smaller folds on the limbs (flanks) smaller folds on the limbs (flanks) are second order folds.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 Enveloping surfaces are useful for studying folds at outcrop scale for studying folds at outcrop scale or in cross section where many or in cross section where many small folds occur on limbs of small folds occur on limbs of larger folds, but the geometry of larger folds, but the geometry of the larger folds not clear.the larger folds not clear.

Types of Folds Anticline:Anticline: folds that are concave folds that are concave

towards the older rocks.towards the older rocks. Syncline:Syncline: folds that are concave folds that are concave

towards the younger rocks.towards the younger rocks. Antiform:Antiform: fold is concave fold is concave

downward and rocks may not be downward and rocks may not be older in the middle or age of the older in the middle or age of the rocks is not known.rocks is not known.

Synform:Synform: fold is concave upward fold is concave upward and rocks in the middle may not and rocks in the middle may not be younger or age is not known.be younger or age is not known.

Dome:Dome: layering dips in all layering dips in all directions away from a center directions away from a center point.point.

Basin:Basin: layering dips inward layering dips inward toward a central point.toward a central point.

Antiformal synclineAntiformal syncline:: Downward Downward facing syncline in which layering facing syncline in which layering dips away from axis, but the dips away from axis, but the rocks in the center are younger.rocks in the center are younger.

Synformal anticline:Synformal anticline: upward upward facing anticline, where in layering facing anticline, where in layering dips inward as syncline but the dips inward as syncline but the rocks in the center are older. rocks in the center are older.

DomesDomes

Homocline:Homocline: rocks that dip uniformly in one rocks that dip uniformly in one direction (Fig. 14.8)direction (Fig. 14.8)

MonoclineMonocline:: a local steepening with homocline a local steepening with homocline Structural terrace:Structural terrace: local flattening of a uniform local flattening of a uniform

regional dipregional dip Cylindrical:Cylindrical: The hinges are parallel every where and The hinges are parallel every where and

the fold can be generated by moving the fold axis the fold can be generated by moving the fold axis parallel to itself (Fig. 14.9)parallel to itself (Fig. 14.9)

Non-cylindrical:Non-cylindrical: The hinges are not parallel and The hinges are not parallel and can converge in one point (Fig.14.9)can converge in one point (Fig.14.9)

Sheath folds:Sheath folds: are non-cylindrical and closed at one are non-cylindrical and closed at one end the fold hinges curve within axial surface (Fig. end the fold hinges curve within axial surface (Fig. 14.10)14.10)

Upright folds:Upright folds: have vertical axial surface (Fig. have vertical axial surface (Fig. 14.11)14.11)

Overturned foldsOverturned folds:: have one inverted limb (Fig. have one inverted limb (Fig. 14.11) 14.11)

Reclined folds:Reclined folds: axes plunge at nearly same angle axes plunge at nearly same angle as the dip of the axial surface, plunge of the axis as the dip of the axial surface, plunge of the axis normal or at high angle to the strike of the axial plane normal or at high angle to the strike of the axial plane (Fig. 14.11)(Fig. 14.11)

Recumbent folds:Recumbent folds: Have horizontal axes and axial Have horizontal axes and axial surfaces.surfaces.

Isoclinal folds:Isoclinal folds: are tight folds wherein axial are tight folds wherein axial surfaces and limbs are parallel surfaces and limbs are parallel

Types of FoldsTypes of Folds

To distinguish between the different type of folds Fig. 14.13 (after Fleuty 1964) is used.

Classification of folds based on the Classification of folds based on the bedding thickness, and hinge bedding thickness, and hinge curvaturecurvature

(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 foldsFault-bend and fault-propagation folds: : ((Fig. 11.11) these type of folds associated with thrust fault

Parasitic folds are used to Parasitic folds are used to determine the position in a folddetermine the position in a fold

parasitic or small size fold on the parasitic or small size fold on the limb of big size fold can be used limb of big size fold can be used to determine the position as they to determine the position as they have Z sense of rotation clockwise have Z sense of rotation clockwise in one limb and S sense of in one limb and S sense of movement anti-clockwise in the movement anti-clockwise in the opposite limb.opposite limb. W and M sense of W and M sense of movement are found at the hinge movement are found at the hinge of the big size fold.of the big size fold.

Stereonet is also used to determine Stereonet is also used to determine the direction, vergence, and the direction, vergence, and sense of movement of big fold by sense of movement of big fold by plotting the vergence and plotting the vergence and parasitic small folds. parasitic small folds.

FOLDS CLASSIFICATIONFOLDS CLASSIFICATION

Fleuty Classification:

based on interlimb angle and hinge area (See Fig. 14-21)based on interlimb angle and hinge area (See Fig. 14-21)

Gentle, Open, Closed, Tight, Isoclinal and ElasticaGentle, Open, Closed, Tight, Isoclinal and Elastica Donath and Parker Classification :

based on ductility and ductility contrast (Fig. 14-27)based on ductility and ductility contrast (Fig. 14-27)

Quasi-Flexural, Passive-slip, Passive-flow, Flexural flow, and Flexural slip

Donath and Parker ClassificationDonath and Parker Classification

Donath and Parker Classification (1964)Donath and Parker Classification (1964)

Flexural-slip folds:Flexural-slip folds: parallel concentric folds form by buckling parallel concentric folds form by buckling or bending. Slip in these folds is parallel to the layering and or bending. Slip in these folds is parallel to the layering and characterized by slickensides, fibers. They have constant layer characterized by slickensides, fibers. They have constant layer thickness. thickness.

Flexural-slip foldsFlexural-slip folds

Passive-slip foldsPassive-slip folds Passive-slip folds:Passive-slip folds: type of similar folds, form by type of similar folds, form by

shearing along planes inclined by layering, shearing along planes inclined by layering, form by simple shear and not pure shear. form by simple shear and not pure shear.

Flexural-flow folds:Flexural-flow folds: form in rocks from low and form in rocks from low and moderate metamorphic grade. They are similar like moderate metamorphic grade. They are similar like folds. Some layers maintain constant thickness but folds. Some layers maintain constant thickness but others thickened into axial plane and thinned into others thickened into axial plane and thinned into limbs, indicating higher contrast in internal ductility. limbs, indicating higher contrast in internal ductility. Example shale (change thickness) and quartzite (fixed Example shale (change thickness) and quartzite (fixed thicknessthickness) )

Flexural-flow foldsFlexural-flow folds

Passive-flow foldsPassive-flow foldsPassive-flow folds:Passive-flow folds: are similar folds that involve plastic deformation. are similar folds that involve plastic deformation. The layering acts only as a displacement marker. Passive flow folds The layering acts only as a displacement marker. Passive flow folds form in metamorphic rocks with low mean ductility and ductility form in metamorphic rocks with low mean ductility and ductility contrast. contrast. Example salt, glacial ice and water saturated Example salt, glacial ice and water saturated unconsolidated sedimentsunconsolidated sediments

Quasi-flexural foldsQuasi-flexural foldsQuasi-flexural folds:Quasi-flexural folds: are similar to the passive- are similar to the passive-

flow folds but they are dis-harmonic foldsflow folds but they are dis-harmonic folds

FOLD MECHANICSFOLD MECHANICSFold mechanism is influenced by factors Fold mechanism is influenced by factors

affecting deformation:affecting deformation: temperaturetemperature pressurepressure fluidfluid properties of the rock as determined by properties of the rock as determined by

composition, texture, and anisotropy.composition, texture, and anisotropy.Anisotropy is affected by changes in Anisotropy is affected by changes in

temperature and pressure.temperature and pressure.

Fold mechanismsFold mechanismsFold mechanisms include:Fold mechanisms include: BucklingBuckling BendingBending Passive (ductile) flowPassive (ductile) flow Flexural slipFlexural slip KinkingKinking Flexural flowFlexural flowThe end shape of a fold is may be a produced of one or more fold The end shape of a fold is may be a produced of one or more fold

mechanism. (see Fig. 15-4)mechanism. (see Fig. 15-4)

Buckling may be accompanied by flexural slipBuckling may be accompanied by flexural slip act earlyact early in the fold formation and in the fold formation and buckling buckling accompanied with flexural flow dominatedaccompanied with flexural flow dominated later later as a result of tighten and pressure as a result of tighten and pressure increases during progressive deformation.increases during progressive deformation.

Under high temperature and pressure layers Under high temperature and pressure layers may no longer control the shapes of the folds may no longer control the shapes of the folds but may serve only as strain markers.but may serve only as strain markers.

FLEXURAL SLIPFLEXURAL SLIP Act usually in Act usually in low temperature low temperature

and pressure found at shallow and pressure found at shallow depth within the Earth Crust.depth within the Earth Crust.

Layers maintain their Layers maintain their thickness thickness through slip past one another through slip past one another (book pages)(book pages)

Flexural slip usually Flexural slip usually accompanies the bending and accompanies the bending and buckling mechanisms and is buckling mechanisms and is recognized by slickensides or recognized by slickensides or fibers on bedding surfacefibers on bedding surface. Fibers . Fibers may be oriented perpendicular to may be oriented perpendicular to the fold hinge lines.the fold hinge lines.

BENDINGBENDING Bending involves Bending involves application of force across layers.application of force across layers. Generally produce folds that are Generally produce folds that are very gentle with large interlimb very gentle with large interlimb

angles.angles. They involves They involves flexural flow and are common in continental flexural flow and are common in continental

interiors-cartons- where vertical forces may be directed at high interiors-cartons- where vertical forces may be directed at high angle to the originally horizontal beddingangle to the originally horizontal bedding, producing the broad , producing the broad domes and basins (domes and basins (example arching cover rocks over basement)example arching cover rocks over basement)

Flexural bending of Flexural bending of lithospheric plates also occurs at subduction lithospheric plates also occurs at subduction zones and adjacent to oceanszones and adjacent to oceans..

Layers in bending are bent like an Layers in bending are bent like an elastic beam the has been elastic beam the has been supported at the ends and loaded in the middle.supported at the ends and loaded in the middle.

In this type of fold mechanism layers are also go flexural slip.In this type of fold mechanism layers are also go flexural slip.

BUCKLINGBUCKLING Folds form by buckling where force Folds form by buckling where force is applied parallel to is applied parallel to

layering in rocks.layering in rocks. The product of buckling is buckled fold. The product of buckling is buckled fold. Flexural slip commonly accompanies buckling at low Flexural slip commonly accompanies buckling at low

temperature and pressure.temperature and pressure. The result of this mechanism at low temperature The result of this mechanism at low temperature is parallel is parallel

concentric folds (in low temperature).concentric folds (in low temperature). In high temperature the resulted type of fold may be similar In high temperature the resulted type of fold may be similar

like foldslike folds.. Buckling and thrust fault in-between anticline and syncline Buckling and thrust fault in-between anticline and syncline

may produce fault-propagation folds at low temperature.may produce fault-propagation folds at low temperature. Buckling is usually Buckling is usually produce layers shorteningproduce layers shortening. -. - Folds formed by Folds formed by a combination of buckling and pressure-a combination of buckling and pressure-

solution strain maintain the shapes of buckle folds but may solution strain maintain the shapes of buckle folds but may develop a strong cleavage because of associated flatteningdevelop a strong cleavage because of associated flattening

For Bending and Buckling see Figures 15-8 and 15-9For Bending and Buckling see Figures 15-8 and 15-9

PASSIVE SLIPPASSIVE SLIP See Figure 15-17 Is defined as slip at an angle to layering compared to flexural slip where 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 slip is parallel to layering. Slip in passive slip results in a new cleavage or schistosity to accommodates movement parallel to the new surface.or schistosity to accommodates movement parallel to the new surface.

In this type of slip bedding or compositional layering serve only as strain In this type of slip bedding or compositional layering serve only as strain marker that record the displacement parallel to the cleavagemarker that record the displacement parallel to the cleavage.

KINK FOLDINGKINK FOLDING Kink and chevron folds Kink and chevron folds have straight limbs and narrow angular have straight limbs and narrow angular

hingeshinges. They form in . They form in minerals and rocks and occur on any scale minerals and rocks and occur on any scale from crystal lattices to amp scalefrom crystal lattices to amp scale..

Kink folds requires local slippage (flexural slip) between layers.Kink folds requires local slippage (flexural slip) between layers. If shear strength is exceeded and free slippage can occur in all If shear strength is exceeded and free slippage can occur in all

layers throughout the rock mass sinusoidal buckle folds will layers throughout the rock mass sinusoidal buckle folds will form.form.

FLEXURAL FLOWFLEXURAL FLOW In flexural flow some In flexural flow some layers flow layers flow

ductility while others remain ductility while others remain brittle and buckle.brittle and buckle.

Flexural flow requires Flexural flow requires moderate-moderate-to high ductility contrast to high ductility contrast between layers.between layers.

Strong layers may not undergo Strong layers may not undergo thickness changes but weak thickness changes but weak layers may go extreme thickness layers may go extreme thickness changes.changes.

The products of flexural The products of flexural flow are flow are similar fold.similar fold.

In flexural fold In flexural fold amplitude and amplitude and wavelength may be controlled by wavelength may be controlled by the original thickness, spacing the original thickness, spacing and strength of the strong layers.and strength of the strong layers.

PASSIVE FLOW Involves uniform ductile Involves uniform ductile

flow of the entire rock flow of the entire rock mass.mass.

Layering, foliation, Layering, foliation, gneissic banding gneissic banding serving serving only as a strain marker.only as a strain marker.

In passive flow their must In passive flow their must be little or no ductility be little or no ductility contrast between layers.contrast between layers.