Fold mechanics

Fold mechanics

1. Fold definition

   • Structure produced when planar beds are contorted by deformation
   • The result of gradual and extended deformation
   • Made up of
     • Hinge point
       • Hinge line
         • Line that runs across the entire hinge point
         • The hinge line of multiple layers of a fold makes up the axial surface
           • Continuous axial surfaces
             • Harmonic folds
           • Discontinuous axial surface
             • Disharmonic folds
         • Straight line
           • Cylindrical fold
         • Curved line
           • Curvilinear fold
     • Limb
       • Inter limb angle
         • 180- 120 degree angle
           • Gentle
         • 120-70
           • Open
         • 70-30
           • Tight
         • 30 - 0
           • Isoclinal

2. Folds with known stratigraphy

   • Syncline
     • • Beds young upwards
   • Anticline
     • • Beds young upwards
   • Synformal anticline
     • Beds young downwards
       • Upside down anticline
   • Antiformal syncline
     • Beds young downwards
       • (upside down syncline)

3. Plunge

   • angle of hinge line compared to the axial surface

4. Classification of folds based on orientation of hinge line and axial surface

   • Upright
   • Horizontally inclined
   • Recumbent
   • Plunging upright
   • Verticle

5. Vergence

   • Direction of apparent shear on the upper side of a layer
   • Recorded towards it geographic direction
     • e.g NE

6. Z folds

   • Left side of hinge

7. S folds

   • Right side of hinge

8. M folds

   • On the hinge

9. Mechanisms of folding

   • Buckling
     • Flexural slip
     • Flexural shear
       • Whole bed shears to accommodate shear
   • Bending
   • Passive folding
   • Viscosity
     • Competent layers
     • Incompetent layers
   • Tangential longitudinal strain
     • Extended outer arc
       • Tension gashes form
     • Compressed inner arc
       • Solution cleavage forms
       • Minor thrusts
   • Fault bend folds
   • Kink bands
     • Formed adjacent to max stress

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