Fold Mechanism FOLD MECHANISM

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Fold Mechanism FOLD MECHANISM Powered By Docstoc
					                                                   Class 1A:                                                    Class 1B:
                                                   Thickened                                                    Parallel
                                                   limbs                                                        folds

                                                                                                                Class 1C,
                                                                                                                Class 2,3:
                                                                                                                Thinned
            Fold Mechanism                         Class 2:                                                     limbs
                                                   Similar folds



                                                   Fold classification based on dip isogons Class 1: fold with
                                                   convergent isogons; Class 2: fold with parallel isogons; Class 3: fold
                                                   with divergent isogons. Class1A: strongly convergent isogons.
                                                   Class 1B: parallel folds with isogons perpendicular to fold surface.
                                                   Class 1C: weakly convergent isogons.
                                                                                                      (Ramsay, 1967)




         FOLD MECHANISM
 • Bending
 • Buckling
 • Flexural flow (A) and flexural slip folds




                                                          Bending                  Flattening             Buckling




Buckling:                    Bending:
force parallel to            Force perpendicular
the layer                    to the layer
                                                          Flexure by bending (Twiss &
                    (Park, 1997)                                  Moores, 2007)




                                                                                                                             1
                                  Flexure by
                                   buckling
                                   (Twiss &
                                   Moores,
                                     2007)
                                                          Monocline formed by bending
                                                            (From Twiss & Moores, 2007)
                                               A. Basement faulting B. Rattlesnake mountain monocline,
                                                                      Wyoming




                                                                                       Fold mechanism

                                                                                       A.Buckling
                                                                                       B.Flexural slip
                                                                                         (bedding slip)
                                                                                       C.Flexural shear
                                                                                          (bedding shear)
                                                                                       D.Oblique shear
                                                                                         (Heterogeneous
                                                                                         simple shear)
                                                                                       E. card-deck model of
                                                                                         heterogeneous
                                                                                         simple shear
      Formation of a monocline (modeling)                                              F. Kinking
              (G.H.Davis,1978)                                                                 (Park, 1989)




                                                                                                 Initial element




                                                  Neutral surface




                                                                    Orthogonal Flexure
Orthogonal Flexure (Twiss & Moores, 2007)                    (After J.G.Ramsay et al,1987)
                                               A.strain distribution pattern;B.cleavage;C.tension fractures;D.shear
                                                                               fractures




                                                                                                                      2
  Flexural shear (or flexural flow) folding                            Flexural shear fold (A) and flexural slip folds(B)
                 (Twiss & Moores, 2007)                                              (After J.G.Ramsay et al,1987)




          Passive shear folding (Twiss & Moores, 2007)              Orientation of the fold hinge for a passive shear fold
       Also called passive flow folding or simply flow folding                            (Twiss & Moores, 2007)




                                                                                         The volume of folded
                                                                                         material in the thin lamella
                                                                                         ABCD on the limb of a fold
                                                                                         is the same as that in the thin
                                                                                         lamella EFGH on the hinge
                                                                                         of the fold.

                                                                                          A mechanism for the
                                                                                             Formation of
                                                                                             similar folds
                                                                                         (After Bruce E. Hobbs et al,
                                                                                         1976)

Spaces in the hinge zones formed by flexural slip                                                  Profile of the ideally similar fold
                                                                                                   Formed by the mechanism illustrated
             (Filled with ore bodies)                            Ideally similar fold formed by    in the left figure. The thickness
                                                                 slip oblique to the fold layer    parallel to the shear planes is constant.




                                                                                                                                               3
   Development of chevron folds by kinking
     ( From Peterson and Weiss,1966)
                                                                                      A natural fold




     FOLDING PROCESSES AND LAYER GEOMETRY

• Flexural – Slip: Low temperature, high deformation rate
competent (mechanically strong) layers
                                                                                    Problems for review
                 [ Parallel Folds ]

                                                                   •   Distinguish flexural slip from flexural flow
• Flexural – Flow: Metamorphic temperature, moderate               •   Why do we call some folds passive folds?
deformation rate &/or alternating competent & incompetent layers   •   What is the difference between buckling and bending?
                                                                   •   What is one of the formation mechanism of similar folds?
  [ Mixed layers of roughly parallel and similar geometry ]
                                                                   •   What is a neutral surface?

• Passive – Flow: Higher metamorphic temperatures, low
deformation rate
                 [ Similar Folds ]




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