FOLDS

Fold features:

 Limbs-  portions of fold separated by hinge

Inflection Point-

   limb point where sense of curvature changes

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

   Cylindrical        Non-Cylindrical

   Non-Plunging Folds

Plunging Folds

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

                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

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.

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

Sideling Hill Syncline (photo by N. Heywood)

Green Pond Inlier folds, Newfoundland, NJ

Fold Classification:

     Dip of Axial Surface                      

          Upright:     70⁰-90⁰   

          Inclined:    10⁰-70⁰.                                  

          Recumbent 0⁰-10⁰.          

      Interlimb Angle

A. Gentle:          120⁰-180⁰

B. Open:             60⁰-120⁰

C. Tight:              10⁰-60⁰

D. Isoclinal:          0-10⁰.

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

Class 1B (Parallel)-

          equal bed thickness throughout the fold

Class 1C- slight hinge thickening

Class 2 (Similar)-

        parallel dip isogon patterns-

        limb thinning; hinge thickening

Class 3:

        divergent dip isogon pattern-

        limb thinning, hinge thickening

Distinctive Fold types:

Monoclines

Basins

Domes

Chevron Folds

Mechanics of Folding:

      Passive Folding  (Shear Folding)