Parasitic folds and Boudin In Tie Shan

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Parasitic folds and Boudin In Tie Shan

• Lecturer Whang Shu qing
• 01002142
• Instructor Mr. ZengZuoxun

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Introduction

• 1.parasitic folds
• A set of small folds found occurring on the
  limbs or hinge zone of another larger fold are
  termed parasitic fold.

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• 2.Boudin
• When relatively strong layers are stretched
  during deformation ,they may separate into blocks
  or form lensoid or pillow-shaped separated by
  narrow necks. Such structures are called
  boudins .

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Field phenomenon

• We have seen this on a rock broken in JianShan

The upper is parasitic fold while the structure
under it is boudin
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Analysis

• Characteristics for this structure ,the most
  interesting feature is that the strong layers
  (boudin layers) are bended along the larger fold
  in the parasitic folds while other boudins layers
  are almost straight.
• So, what is the reason ?

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Analysis for boudin

• As we know ,the boudin can indicate the
  compression perpendicular to the layers.

s1
s3
s2
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• If the bended layers are applied with
  compression, the competent layers will be
  compressed and will become boudins in the limb
  and similar folds in the hinge zone,
• but the structure in JianShan is special that
  the competent layer is all broken without folding
  .So the layer was applied with compression
  perpendicular to the layer plane ,and after this
  is bending ,to

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• form bended boudin zone like we saw in
• the field, if the compression occurred before
  the bending ,the competent layer form boudin in
  the limb and folds in the hinge zone .So I get
  the conclusion that the boudinage is before the
  bending.

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Analysis for parasitic folds

• In this field, we watched parasitic folds.
• I think there can be two mechanisms of
  parasitic folds in tie shan.
• 1.Influence of the competent layer
• In the rock there are layers with different
  competencies ,if the distance between competent
  layers is a special one ,the thiner layer will be
  influenced by other

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• layers ,and it not only bend with small
  wavelength ,but also bend along other layers,
  just like the following pictures

Soft layer
Competent layer
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• 2.Influence of the shear of the other layers
• Another mechanism of parasitic folds is the
  shear of the other layers shear. In buckling ,if
  it is easy for the layers to slide ,when the
  layers are applied with stress along the layers
  ,they will bend and slide ,so the soft layer
  between the competent layers will form parasitic
  folds .
• Just like the follow pictures showing

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shear
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Conclusion

• In the fieldwork ,I noticed this structures
  and analyzed it ,so I got some concluion
• 1.The special boudin can indicate two periodical
  strain first compression ,then bending
• 2.The parasitic folds mechanism---the influence
  of the other layers or the shear of the competent
  layers.

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welcome for questions