Fold features:

 Limbs-  portions of fold separated by hinge


Inflection Point-

   limb point where sense of curvature changes

 Picture (624x457, 15.6Kb)



Axial Surface (Axial Plane)-

connects series of hinge lines within a fold


Fold Axis-

Imaginary straight line that when moved parallel to itself reproduces the form of a fold

Occurs only with Cylindrical folds

Hinge Line-

      connects points of maximum fold curvature


Cylindrical vs. Non-Cylindrical Folds

        Cylindrical Folds-

                Straight hinge lines

                Contains fold axes


Non-Cylindrical Folds-

                Curved hinge lines

                Does not contain fold axes

 Picture (602x371, 9.3Kb)

   Cylindrical        Non-Cylindrical




   Non-Plunging Folds

 Picture (398x441, 10.1Kb)



Plunging Folds

   Picture (397x444, 23Kb)

Picture (419x559, 54.8Kb)

Landsat TM image of nortwest trending Mariscal Mountain Anticline, Big Bend National Park


Picture (700x399, 113.5Kb)

                Limestone beds of Torcer Formation in syncline of Malone Mountains, south of Torcer station. Hudspeth County, Texas. Syncline plunging away from observer. USGS photo.


Fold Symmetry

Symmetrical Folds- limbs have equal dips

Non-Symmetrical - unequal limb dip angles

Picture (560x228, 4.5Kb) 

Symmetrical Fold    Asymmetrical Fold



Form vs Lithologic age relations:

  Antiform- convex upward fold

  Synform- concave upward fold

These terms used when stratigraphic age     relations are unclear between folded layers.

Picture (433x314, 7.1Kb) 


Anticline-convex upward fold  wherein the oldest stratigraphic units occupy core of fold                                              

Syncline- concave upward fold  wherein the youngest stratigraphic units occupy fold core

Picture (640x480, 35.7Kb)

Sideling Hill Syncline (photo by N. Heywood)


Picture (673x359, 12.9Kb)

  Picture (710x389, 72.7Kb)

Green Pond Inlier folds, Newfoundland, NJ



Fold Classification:

     Dip of Axial Surface                      

          Upright:     700-900   

          Inclined:    100-700.                                  

          Recumbent 00-100.          

Picture (582x433, 12.3Kb)   



      Interlimb Angle

Picture (481x386, 8.3Kb)

A. Gentle:          1200-1800

B. Open:             600-1200

C. Tight:              100-600

D. Isoclinal:          0-100.

Picture (473x400, 8.4Kb)



Dip Isogon Analysis-

Dip isogons connect points of equal dip on the upper and lower boundary of a folded layer

Measure bed thickness change throughout fold



Class 1- convergent dip isogons


 Class 1A- limb thickening

Picture (644x459, 11.8Kb)



Class 1B (Parallel)-

          equal bed thickness throughout the fold

Picture (587x455, 12.2Kb)




Class 1C- slight hinge thickening

Picture (599x522, 14.9Kb)

  Picture (423x688, 67.2Kb)



Class 2 (Similar)-

        parallel dip isogon patterns-

        limb thinning; hinge thickening

Picture (322x478, 10.1Kb)




Class 3:

        divergent dip isogon pattern-

        limb thinning, hinge thickening

 Picture (361x451, 10.6Kb)


Distinctive Fold types:


Picture (619x436, 10.4Kb)



Picture (599x482, 16.1Kb)




Picture (603x482, 35Kb)

Chevron Folds

 Picture (652x489, 53.3Kb)

Mechanics of Folding:

      Passive Folding  (Shear Folding) 

            slip on surfaces not parallel to rock layering

            rock layering (black layer) does not influence fold development

           black bedding layer merely acts as a "passive" recorder of strain

                card deck analogy in which slip occurs on individual cards;

           the surface of individual card records no strain

           produces similar (Class 2) folds 










Picture (542x406, 50.9Kb)

Passive folding in layered leucogabbro, Siroua, Morocco






Flexural Slip Folding

    Folding is controlled by rock layering

    Slip (bending) occurs between rock layers

   No strain on the bedding plan surface

   Circles become ellipses in the profile plane

    Amount of slip increases from the hinge line to the inflection point

    produces parallel (Class 1B) folds

  Picture (315x486, 5.1Kb)

                Flexural Slip Folding

  Picture (653x475, 17.8Kb)



Neutral Surface Folding - produces concentric folds

    X-axis is normal to hinge line along top of folded layer

   Neutral surface (no strain) occurs in middle of folded layer

    X-axis is parallel to hinge line along bottom of folded layer

   produces parallel (Class 1B) folds


 Picture (653x509, 26.4Kb)



Superposed Folds- refolded folds

Picture (700x466, 169.2Kb)

Specimen of refolded isoclinal folds in schist. Riverside Mountains, Riverside County, California. January 1967. Photo by Warren B. Hamilton, USGS.


Transposed Folds - hinges distended from limbs

Picture (542x406, 51.9Kb)



Extensional Folds: folds may also be generated by tension

        Reverse Drag

  Picture (382x470, 7.8Kb)


Soft Sediment Deformation- Lastly, folds may occur due to soft sediment deformation, commonly in turbidity current environments


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