Week 12

Folds II. Read pages 397-423 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.

Kinematic Analysis p.397-404

• Flexural Folding vs. Passive Folding
• Flexural Slip Kinematics
• Minor Structures Created During Flexural-Slip Folding
• Jointing and Flexural Slip Folding

Mechanics of Buckling p.404-410

• Instability and Dominant Wavelength
• Simple Buckling of a Single Layer, in Theory
• Adding Layer-Parallel Internal Strain, in Theory
• Buckle Folding of a Single Layer, in Practice
• Influence of Competency Contrast on Fold Form
• Buckle Folding of Multilayers

Regional Tectonic Fold Mechanisms p.413-423

• Free Folding
• Forced Folding
• Multiple Mechanisms

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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 these questions after this weeks lectures and readings:

1. Describe the criteria that influence folding mechanisms in single-layer and multi-layer buckle folds.
2. Contrast the development of flexural slip folds and passive folds.
3. Describe how folding mechanisms in multilayer folds are related to variations in the competence of the layers.
4. What is the difference between bending and buckling?
5. What factors control the size of folds in single layer buckle folds?
6. A sequence of rocks includes thick limestone layers above interbedded thin sandstone and shale layers. This sequence undergoes folding in the shallow crust. Describe the potential shape and kinematic evolution of the resultant folds.

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Fold kinematics and mechanics

Folding mechanisms depend on physical conditions and properties of the folded layers

• At low temperatures and pressures - layering is important, has mechanical role (active folding)

Two types of active folding:

• Bending - stress applied across layers (e.g. from above or below) create gentle folds
• Buckling - stress applied parallel to layering

• At high temperatures and pressures - layering is not important, acts as a marker (passive folding)

Kinematic Analysis

Flexural Folding

Orthogonal flexure - lines perpendicular to the layer before folding are perpendicular after folding (sometimes called neutral surface folding)

• Typically generates low curvature folds
• Requires lengthening of outer arc, shortening of inner arc of fold
• Neutral surface within the layer does not change length

Flexural slip folding - deformation accommodated by slip on bedding planes (image on right shows fold pairs, Mt. Kidd, Canada, layers promoted flexural slip)

• Slip toward anticline hinge, away from syncline hinge
• No length change for layers
• Bedding plane slip is zero at fold hinge and maximum at inflection points
• Slip increases with increasing dip of fold limbs

Flexural flow folding - deformation accommodated by shearing of incompetent beds

 Passive folding

• Layer exerts no mechanical influence on folding
• Simple shear on planes that cut across layering
• Creates Class 2 (Similar) folds

The fold shapes formed by these mechanisms may be modified by later superimposed homogeneous strain which may result in the thinning of limbs relative to hinges (moving fold shapes from Class 1B toward 1C).

One mechanism that results in a modification of fold shape is pressure solution that may cause material to be removed preferentially from the inner arc of the fold, increasing the interlimb angle.

Mechanics of Buckling

A dominant fold wavelength develops when layers are subjected to layer-parallel stress.

The dominant wavelength is dependent upon:

• Single layer vs. multilayer
• Strength of folded layer vs. surrounding rocks
• Thickness of the layer.

 Single-layer folds

Single-layer folds have been modeled as with viscous or elastic behavior, generating similar types of equations.

L = fold wavelength; t = layer thickness; E = Young's modulus; h = coefficient of viscosity

Fold wavelength increases with layer thickness and the contrast in stiffness or viscosity between the deformed layer and the surrounding medium.

Multi-layer folds

• If thin layers are interbedded with thick layers, the thin layers will conform to the shape of the thicker layers unless they are sufficiently far away to be unaffected.
• The different mechanical properties of the layers can be evaluated based on their mean competence and competence contrast.

• Ptygmatic folds - mean competence low, competence contrast high
• Flexural slip folds - mean competence high, competence contrast low-high
• Flexural flow folds - mean competence moderate, competence contrast moderate-high
• Passive folds - mean competence low, competence contrast low

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