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Geology 3120 - Folding Mechanisms

Learn about the various folding mechanisms in geology, including decollement buckle folds, passive and kink folds, shear and flexural slip folds. Understand multi-layer folds and shear zones with examples and practical demonstrations.

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Geology 3120 - Folding Mechanisms

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  1. Geology 3120 - Folding Mechanisms

  2. Outline • Decollement Buckle Folds • Passive (a.k.a. Free) Folds • Kink Folds • Shear Folds • Flexural Slip • Sheath Folds

  3. Decollement Buckle Fold Zagros salt folds Zagros salt folds Zagros salt folds

  4. Passive-Free-Buckle Folds m2 m2 m1 If m1 > m2

  5. Passive, or Free Folds

  6. Mullions

  7. Mullions

  8. Multi-Layer Folds

  9. Layer thickness and rheology define fold wavelength Layer thickness and rheology both define fold wavelength What color strata are weakest? Light or Dark?

  10. Multi-Layer Folds

  11. Parasitic Folds

  12. Parasitic Folds

  13. Shear Folds

  14. Shear Folds

  15. Kink Folds (photograph Taiwan kink-bands)

  16. Kink Folds (common in shales, slates)

  17. Flexural Slip

  18. Vergence

  19. Flexural Flow

  20. Flexural Flow

  21. Flexural Flow (aka Karl’s favorite rock!) Fractures open outward Extensional fractures end at sliding surface Offset fracture shows vergence

  22. Tangential Longitudinal Strain

  23. Tangential Longitudinal Strain

  24. Tangential Longitudinal Strain

  25. Sheath Folds High Shear Strain Folds (i.e. shear zones) Sheath, or Tongue Shaped Stretching Lineations parallel to fold axis’

  26. Sheath Folds High Shear Strain Folds (i.e. shear zones) Sheath, or Tongue Shaped Stretching Lineations parallel to fold axis’ Sheath Fold in Calcite Marble

  27. Examples

  28. Examples

  29. Examples

  30. In class stunts Use Fig Newtons as an analgue to illustrate extension on outer arc of fold; Wear two old shirts, wrap with rubber band and cut off sheath fold

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