True Dip and Apparent Dip

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True Dip and Apparent Dip

• Dip is always between 0 and 90
• True dip is measured in vertical plane that is
  perpendicular to strike line
• Apparent dip is measured in vertical plane that
  is at arbitrary angle to strike line
• Apparent dip is always less than True Dip

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Relationship between True Dip and Apparent Dip
or
Where
apparent dip
true dip
angle between line of section and strike line
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Primary and Secondary Structures

• Primary - structures due to formation of the rock
  (e.g., before diagenisis, before magma
  solidifies)
• Secondary - structures due to diagenesis and
  deformation
• Diagenesis process of sediment becoming a rock
  compaction, cementation, recrystallization

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Causes for Secondary Structures

• Pressure
• Non-hydrostatic stress
• Temperature
• Fluid activity

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How Do Rocks Respond to Stress?

• Stress (hydrostatic and non-hydrostatic)
• Orientation of stress compared to bedding fabrics
• Stress and strain history (rate of strain)
• Temperature
• Material Properities

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Responses to Stress

• Brittle Fracture (breaking into pieces)
• Frictional (slip on pre-existing fracture)
• Ductile (macroscopic flow of rocks)
• - cataclastic - microscopically brittle
• - plastic (actual creep of the crystals)

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Review of Sequence and superpostion

• Principal of superposition
• Original horizontality
• Cross cutting relationships

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Folds

• Axial Line line on a fold separating dip in one
  direction from dip in another direction
• Fold axis direction contained within the beds
• Cylindrical Fold
• Hinge Line line of maximum curvature in a
  single folded surface
• Hinge Surface axial surface surface made of
  many hinge lines

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Symmetry of Folds

• Asymmetric Fold has one limb dipping steeper
  than the other
• Overturned fold has at least one limb tilted by
  more than 90

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• Monocline place where homogeneously dipping
  beds are locally steepened
• Anticline fold with older rocks in concave
  center
• Syncline fold with younger rocks in concave
  center

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• Synform fold that closes (bends) downward,
  regardless of which way is younger
• Antiform fold that closes (bends) upward,
  regardless of which way is younger
• Antiformal syncline syncline (younger rocks in
  concave center) shaped like anticline
• Synformal anticline an anticline (older rocks
  in concave center) shaped like a syncline

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Plunging Folds

• Folds with inclined hinge lines
• In map view you get chevron shapes for plunging
  folds

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The scale of structures

• 10-10m atom 10-10m 1 A (angstrom)
• 10-9m - 10-8m dislocations
• 10-7m - 10-1m fabrics
• 10-8m - 104m folds
• 10-8m - 106m fractures

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