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Fold development

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This document discusses different types of folds that can form in the Earth's crust. It describes how folds are caused by plastic deformation from compressive forces acting under conditions of stress, pressure, temperature gradients. There are three main mechanisms of folding: flexural folding where both competent and incompetent beds are bent, shear folding which involves slip along fractures, and flow folding where rocks fold like a fluid at high temperatures and depths. Common fold types include anticlines, synclines, and monoclines. Folds can form from tectonic processes like horizontal compression or mantle convection, or from non-tectonic processes like hillside creep or collapse structures.

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FOLD DEVELOPMENT Introduction Causes Mechanics Folding in primary & secondary rocks Common types of folds Origin of folds Chingshom N Konyak Roll no. 172031
FOLDS Undulations or waves in the stratified rocks of the Earth’s crust formed by bending of strata by permanent deformation. A fold is measured by its strike, dip and direction of fold axis. Axial Plane: Imaginary plane that divides the fold as symmetrical as possible Fold axis: Line of intersection of axial plane and ground surface Plunge: Angle fold axis makes with the horizontal
SIZE OF FOLDS Few centimetres or less To Several kilometres across Mt. Head, Canada
CAUSES OF FOLDING Plastic deformation by compressive forces under varied conditions of:  Stress  Hydrostatic Pressure  Pore Pressure  Temperature Gradient Synsedimentary fold: Generated by the liquefaction of the soft sediment prior to lithification.
PLASTIC DEFORMATION Plastic deformation causes permanent deformation. CONDITIONS  Ductility of material (e.g., clay, gypsum)  Deform under heat and pressure  Confining pressure does not exceed internal strength  Deformation must be slow. A: Tough(elastic) C: Ductile(plastic) B: Brittle X: Plastic limit
MECHANICS OF FOLDING 1. Flexure folding 2. Shear folding 3. Flow folding
1. Flexure folding -True folding Rocks are sufficiently plastic Both competent and incompetent beds Beds are bent Concave side thickens & convex side thins Also called flexural-slip fold Can interpret drag folds
Rainbow Basin syncline in the Barstow Formation near Barstow, California Cape Fold Belt, South Africa
2. SHEAR FOLDING -Slip folding Minute displacement along closely spaced fractures Fractures across bedding planes Realignment of the fractures due to friction These small fractures give rise to a major fold May be temporary in case the rock is recrystallised
3. FLOW FOLDING -Passive folding At high temperature and/or great depth Fluid nature of rocks. e.g., rock salts, deep buried rocks Little resistance is offered Strata appear shifted undistorted but thin with displacement Take the shape off surrounding rocks
Folded schist Sedimentary rock layers at Ardalanish Beach Park, Scotland Folded schist and quartzite layers on the side of Mt. Monadnock, New Hampshire. Folding in primary & secondary rocks
TYPES OF FOLD
ORIGIN OF FOLDS TECTONIC Result more or less directly from forces operating within the earth. NON-TECTONIC Result of superficial processes
TECTONIC PROCESSES 1. Horizontal compression (as in Appalachians & Jura Mountains)  Compressive force acting parallel to the surface of the earth  Greatest stress axis is horizontal
2.Mantle convection Sub crustal convection When the lower parts are sufficiently heated This currents drag along the base of the crust Overlying sediments are folded
3. Vertical uplift & Gravity sliding Large open folds that are ten to hundreds of miles across Dipping only a few degrees Took millions of years to form Cincinnati & Nashville domes Michigan basins
1. Hillside creep Slow downward progression of rock and soil down a low grade slope Differential dip in a stratum In incompetent rocks like shale NON-TECTONIC PROCESSES
2. Collapse structure A type of gravity sliding
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Fold development

  • 1. FOLD DEVELOPMENT Introduction Causes Mechanics Folding in primary & secondary rocks Common types of folds Origin of folds Chingshom N Konyak Roll no. 172031
  • 2. FOLDS Undulations or waves in the stratified rocks of the Earth’s crust formed by bending of strata by permanent deformation. A fold is measured by its strike, dip and direction of fold axis. Axial Plane: Imaginary plane that divides the fold as symmetrical as possible Fold axis: Line of intersection of axial plane and ground surface Plunge: Angle fold axis makes with the horizontal
  • 3. SIZE OF FOLDS Few centimetres or less To Several kilometres across Mt. Head, Canada
  • 4. CAUSES OF FOLDING Plastic deformation by compressive forces under varied conditions of:  Stress  Hydrostatic Pressure  Pore Pressure  Temperature Gradient Synsedimentary fold: Generated by the liquefaction of the soft sediment prior to lithification.
  • 5. PLASTIC DEFORMATION Plastic deformation causes permanent deformation. CONDITIONS  Ductility of material (e.g., clay, gypsum)  Deform under heat and pressure  Confining pressure does not exceed internal strength  Deformation must be slow. A: Tough(elastic) C: Ductile(plastic) B: Brittle X: Plastic limit
  • 6. MECHANICS OF FOLDING 1. Flexure folding 2. Shear folding 3. Flow folding
  • 7. 1. Flexure folding -True folding Rocks are sufficiently plastic Both competent and incompetent beds Beds are bent Concave side thickens & convex side thins Also called flexural-slip fold Can interpret drag folds
  • 8. Rainbow Basin syncline in the Barstow Formation near Barstow, California Cape Fold Belt, South Africa
  • 9. 2. SHEAR FOLDING -Slip folding Minute displacement along closely spaced fractures Fractures across bedding planes Realignment of the fractures due to friction These small fractures give rise to a major fold May be temporary in case the rock is recrystallised
  • 10. 3. FLOW FOLDING -Passive folding At high temperature and/or great depth Fluid nature of rocks. e.g., rock salts, deep buried rocks Little resistance is offered Strata appear shifted undistorted but thin with displacement Take the shape off surrounding rocks
  • 11. Folded schist Sedimentary rock layers at Ardalanish Beach Park, Scotland Folded schist and quartzite layers on the side of Mt. Monadnock, New Hampshire. Folding in primary & secondary rocks
  • 13. ORIGIN OF FOLDS TECTONIC Result more or less directly from forces operating within the earth. NON-TECTONIC Result of superficial processes
  • 14. TECTONIC PROCESSES 1. Horizontal compression (as in Appalachians & Jura Mountains)  Compressive force acting parallel to the surface of the earth  Greatest stress axis is horizontal
  • 15. 2.Mantle convection Sub crustal convection When the lower parts are sufficiently heated This currents drag along the base of the crust Overlying sediments are folded
  • 16. 3. Vertical uplift & Gravity sliding Large open folds that are ten to hundreds of miles across Dipping only a few degrees Took millions of years to form Cincinnati & Nashville domes Michigan basins
  • 17. 1. Hillside creep Slow downward progression of rock and soil down a low grade slope Differential dip in a stratum In incompetent rocks like shale NON-TECTONIC PROCESSES
  • 18. 2. Collapse structure A type of gravity sliding