Structural Geology (Geol 305) Semester (071). Dr. Mustafa M. Hariri. FOLDS. 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
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Dr. Mustafa M. Hariri
By the end of this unit you will be able to:
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
Folds can present in all scales
Pumpelly’s rule: small-scale structures are generally mimic larger-scale.
Slip lines:lines of fibers or slicken-sides on a layer surface that indicate the direction of motion of one layer past another
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.
To distinguish between the different type
of folds Fig. 14.13 (after Fleuty 1964) is used.
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.
based on interlimb angle and hinge area (See Fig. 14-21)
Gentle, Open, Closed, Tight, Isoclinal and Elastica
based on ductility and ductility contrast (Fig. 14-27)
Quasi-Flexural, Passive-slip, Passive-flow, Flexural flow, and Flexural slip
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
Fold mechanism is influenced by factors affecting deformation:
Anisotropy is affected by changes in temperature and pressure.
Fold mechanisms include:
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 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.
For Bending and Buckling see Figures 15-8 and 15-9
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.