Structural Geology(Geol 305)Semester (071) Dr. Mustafa M. Hariri
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
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
Importance of folding • 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 • Crest, trough, Limbs, hinge zones, fold axis, axial plane, axial surface, plunge, wavelength, inflection point and vergence.
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 observer’s location on large fold.
Slip lines:lines of fibers or slicken-sides 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 • 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 • 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) • 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 • Donath and Parker Classification : based on ductility and ductility contrast (Fig. 14-27) Quasi-Flexural, Passive-slip, Passive-flow, Flexural flow, and Flexural slip
Flexural-slip folds • 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 • 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 • 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 passive-flow 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 slipact early in the fold formation and buckling accompanied with flexural flow dominatedlater 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 • 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 • 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 • 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
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 • 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 • 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.
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.