Week 11

Folds I. Read pages 372-388 and 391-397 in Chapter 7: Folds

• readings
• terms
• questions
• classnotes

You are expected to read all the sections listed below. Information from the sections in italics will be discussed in class. You are expected to read the other sections and you may be called on in class to answer questions based on that material.

Incentives for Study p.372-379

• Visual Impact
• Mechanical Contradiction of Folding
• Geometric Pleasures
• Informative Minor Structures
• Tectonic Considerations
• Guides to Exploration and Mining

Anticlines and Synclines p.380-382

• Basic Definitions
• Overturned Folds
• Antiforms and Synforms
• Anticlinoria and Synclinoria

Geometric Analysis of Folds p.382-388 & 391-397

• Geometric Properties of Individual Folded Surfaces
• Describing the Shape and Size of a Folded Surface
• Classifying Folds on the Basis of Layer Thickness

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You should become familiar with the following terms during this weeks lectures and readings:

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You should be able to answer the questions below following this week:

1. What geometric elements must be defined to uniquely constrain the orientation of a fold?
2. What does a steeply plunging, gently inclined fold look like?
3. Use Fleuty's Classification Scheme to classify folds illustrated in Figures 7.1B and 7.32.
4. Classify the fold pictured on the right using Ramsay's classification scheme based on the relative thickness of the folded layer and the relative curvature of the bounding fold surfaces.
5. Enlarge Figure 7.44 and draw the dip isogons for 0, 10,20,30 and 40 degree dips for the middle light colored layer.
6. Define the following terms: dip isogon, Class 1B fold, synformal anticline, open fold, fold amplitude, inflection point, vergence.

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Fold geometry and classification

Basic elements of a folded surface

All definitions assume a symmetrical, upright fold pair

• Wavelength - distance between adjacent fold crests (or troughs)
• Hinge - point of maximum curvature in a fold profile
• Hinge line - line of maximum curvature on a folded surface
• Inflection point - point on fold limb where direction of curvature (convex, concave) changes
• Amplitude - distance from inflection point to fold crest
• Axial surface - surface that bisects the interlimb angle of a fold
• Symmetrical folds - limbs have similar inclinations but dip in different directions
• Asymmetrical folds - one limb is steeper than the other, limbs dip in different directions
• Overturned folds - one limb is steeper than the other, both limbs dip in the same direction, one limb is overturned (term in disfavor as it suggests complete overturning of the fold)
• Vergence - direction of asymmetry (viewed from gentle limb toward steep limb)
• Homocline - rocks dip uniformly in one direction
• Monocline - local increase in regional dip, dip direction remains constant (image to right shows view of Shell Canyon monocline, Wyoming)
• Structural terrace - local decrease in regional dip, dip direction remains constant

Fold shape vs. Relative age of beds

• Antiform - convex upward folded surface
• Synform - convex downward folded surface
• Anticline - folded layers where oldest rocks are surrounded by progressively younger rocks
• Syncline - folded layers where youngest rocks are surrounded by progressively older rocks
• Antiformal anticline - convex upward folded layers surrounded by progressively younger rocks
• Synformal syncline - convex downward folded layers surrounded by progressively older rocks
• Antiformal syncline - convex upward folded layers surrounded by progressively older rocks
• Synformal anticline - convex downward folded layers surrounded by progressively younger rocks
• Anticlinorium/Synclinorium - regional scale anticlines/synclines that contain smaller folds

Angular folds, Pinto, Maryland

Fold classifications

 Interlimb angle - easy to use, little information on fold shape, plunge etc.

• Gentle - interlimb angle 120-180^o
• Open - interlimb angle 70-120^o
• Closed - interlimb angle 30-70^o
• Tight - interlimb angle 0-30^o
• Isoclinal - interlimb angle 0^o

 Plunge and orientation of axial surface - provides accurate orientations but little information on fold shapes

 Relative thickness of folded layer vs. relative curvature of inner and outer fold arcs (Ramsay's classification)

There are three Classes of folds based on relative curvature

• Class 1 - inner arc curvature > outer arc curvature
• Class 2 - inner arc curvature = outer arc curvature [Similar fold]
• Class 3 - inner arc curvature < outer arc curvature

How to use dip isogons to tell the difference between fold classes.

Dip isogon - line connecting points of equal inclination on the inner and outer arc

• dip isogons converge downward in Class 1 folds
• dip isogons are parallel in Class 2 folds
• dip isogons converge upward in Class 3 folds

 Class 1 folds can be further subdivided based on relative thicknesses of the fold hinge and limbs

• Class 1A - t_o < t_a
• Class 1B - t_o = t_a
• Class 1C - t_o > t_a

_{Detached fold in Cambrian Arbuckle limestone, southern Oklahoma}

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