En echelon vein arrays

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En echelon vein arrays

• Faculty of geosciences
• 01000l-05
• Du Shijun

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INTRODUCTION

• There has been considerable debate over the
  nature of fractures which host en echelon arrays
  of veins. Mechanisms which have been proposed
  include extensional fracturing, shear fracturing
  (faulting) with the opposite sense of shear to
  the array (antithetic shear fractures), such as
  what we have seen in the Ya Kou of Nan Wang
  mountain. shear fracturing with the same sense as
  that of the array (synthetic shear fractures) and
  a hybrid of extensional and shear fracturing.

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• Morphological evidence supporting a shear
  fracture origin for the host fractures of en
  echelon veins include (1)continuation of the
  host fractures outside the vein array with
  evidence of shear displacement on host fractures
• (2) shear fibres in veins.
• Although these features indicate shear
  displacement on a fracture they dont exclude the
  possibility of its nucleation as an extension
  fracture.

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• Brittle ductile shear zones often contain en
  echelon vein arrays which can be used to
  determine the sense of shear on the zone, since
  the extension direction is indicated by the
  dilation of the veins. Earlier-formed veins may
  be folded by the later movement on the shear zone
  to produce a sigmoidal pattern , which is also a
  good indicator of the sense of movement.

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Sigmoidal shape veins

• The veins have a sigmoidal shape below show
  evidence that the host fractures are dextral
  fault.

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• The fractures continue beyond the tips of
  sigmoidal veins and displace bedding along the
  fracture.

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• Pinnate extension veins occur on the host
  fractures, particularly at terminations.

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• Quartz shear fibres fill some of these fractures.

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conclusion

• Pinnate veins have extensional fabrics and
  sigmoidal veins have both shear and extension
  fabrics . The sigmoidal curvature of the veins
  produced contraction jogs along the dextral
  faults. Thus dextral slip could not occurred
  during or after sigmoidal vein opening. As
  deformation proceeded, and movement on dextral
  faults ceased, dextral arrays of extension
  fractures formed.

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The problem on the determination of stress and
strain trajectories

• For the conjugate array of en echelon veins, most
  authors agreed that conjugate zones enclose an
  acute bisector parallel to Z (where Z is the
  maximum incremental shorting strain direction),
  many equating this direction with that of
  sigma1(where sigma1 is the maximums effective
  compressive stress direction) in coaxial
  deformation. However, many en echelon arrays
  dont occur in conjugate sets, and many workers
  have proposed that en echelon array may form
  during propagations of an extension fracture.
  These workers view the above method of inferring
  stress orientations with scepticism.

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That may be superficial
Thank you for questions