Joints and veins

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Joints and veins
Goal To understand the formation and interpret
the morphology of joints and vein sets.
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Part-I Joints

• Why do we care? Fractures can be dominate
  pathways for fluid migration... oil and gas,
  mineralizing fluids, ground water, and
  contaminate plumes.

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Joints are tensile fractures with no offset

• Cracks that open in the direction of the local
  minimum stress direction
• Tensile strength much lower than compressive
  strength

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Side trip into stress land

• Sress Force/Area
• 3 principal vectors s1, s2, and s3 at right
  angles to each other
• s1 s2 s3
• s1 is the maximum principal stress direction, s2
  is the intermediate principal stress direction,
  and s3 is the minimum principal stress direction

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Systematic joint set Where was s3?
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Weathering of jointed rocks
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Causes for opening

• Homogeneous contraction (shrinkage) or expansion
  of rock
• Bending or warping of rock layers
• Minor changes in the regional stress field

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Contraction
Bending
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Pattern of joints in neutral-surface fold
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Columnar joints in cooling igneous rocks
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Part-II Veins

• Vein Fracture in-filled by minerals precipitated
  from fluids

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Vein arrays

• Planar-systematic
• Stock work
• En echelon

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Planar systematic veins

• Form by in-filling of systematic fracture network
• Outline homogeneous stress field

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Stockwork veins

• Form where rock has been shattered
• Randomly oriented

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En echelon veins

• Form in en echelon joints or shear fractures

En echelon systematic network of close,
over-stepping features oblique to overall
structural trend
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Vein fill

• Blocky crystals
• Fibrous veins

Grow randomly, Can also form by replacement of
rock along fracture
Single crystals grow from walls or center as vein
opens
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Fibrous veins
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