Term 1 - Fold mechanisms + Analysis of fold geometries

Question 1

3 basic configurations by which external forces can act on layers (such as bedding) to cause folding

Answer 1

Not mutually exclusive – can get “mixed” boundary conditions, or boundary conditions that change during progressive strain
Active vs passive mechanisms

Layer-parallel compression: Force applied parallel to bedding
Bending: Force applied across bedding
Shearing: Force applied oblique to bedding
- Called “passive folding” if the strength of the layer does not influence fold development

Question 2

Layer-parallel compression and competence

Answer 2

Force applied to each side of bedding resulting in either
o Homogeneous shortening
o Folding

• “Competence” is a qualitative description of rock strength
• In this case, “competence” is equivalent to viscosity [μ, a measure of the dependence of stress on strain rate: ‘ease of flow’]
• Competence of the layer relative to that of the matrix is a key factor

Low competence = shortening
High competence = folding

Question 3

Role of layer thickness during buckling

Answer 3

• Folds in thick layers have greater wavelengths (λ) than folds in thin layers
• Where the competent layers are far apart, the layers will fold independently of each other

Question 4

Shear (flow) folding:

Answer 4

• Commonly observed in shear zones
• Form by progressive simple shear of initial irregularity
• Shear folds cannot be used to estimate shortening

Question 5

Strain within buckle folds:

Answer 5

• Ideal buckle folding produces folds with constant layer thickness
• Tangential longitudinal strain: outer arc extension; inner arc compression separated by a neutral surface
• Bent eraser analogy

Question 6

Flexural slip folding in multilayers:

Answer 6

• Ideal flexural slip folding produces folds with constant layer thickness
• Slip is maximum on the fold limbs; zero at the hinge
• Phone book analogy
• Favoured by thin beds separated by weak layers (e.g. shale)
• Slickenlines preserved on bedding planes in fold limbs

Question 7

Flexural flow folding

Answer 7

• Ideal flexural flow folding produces folds with constant layer thickness
• Occurs during deformation at high temperatures, or when incompetent layer is “sandwiched” between competent beds
• Layer-parallel simple shear decreases towards the hinges (last slide

Question 8

Contours on stereonets

Answer 8

• Data can be contoured to highlight preferred orientations

* Contours are typically labelled as % of data per 1% area of the stereonet