Folds analysis techniques

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Folds analysis techniques
Goal To apply some of the basic geometric
techniques for analyzing folds and refolded folds.
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1. Foliation bedding intersections

• Three cases for overturned fold
• Bedding upright, dipping more shallowly than
  foliation
• Bedding overturned dipping more steeply than
  foliation
• Bedding highly oblique to foliation

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• Note that for cylindrical folds,
  bedding-foliation intersection lineation lies
  close to fold hinge.

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2. p-axes and ß-axes

• Define fold axis using measurements of bedding
  from around the fold
• Plot on stereonet

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p-girdle best-fit girdle to poles to
beddingp-axis pole to p-girdle
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Finding p-axes
1. Plot poles to bedding
2. Find best-fit great circle
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ß-axis defined by intersections of bedding
planes plotted on net
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Find a fold axis using these bedding measurements

• 050, 80-SE
• 350, 44-NE
• 088, 72-NW

• Use both p-axis and ß-axis methods

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Interpreting p-axis diagrams
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Fold with narrow hinge
Concentric fold
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Asymmetric fold Use caution!!
Chevron fold
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3. Refolded folds
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Refolded folds recognition

1. Can often be found in outcrop
2. Regional patterns in facing direction
3. Interference patterns

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Interference patterns Four end-member classes
Type-0 Tightening of pervious folds Type-1 Dome
and basin (Bullseye shapes) upright folds at a
right angle to each other Type-2
Boomerang Type-3 Hook folds
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Interpret these maps Draw in F1 and F2 axes and
indicate plunge directions
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Steps to interpret maps

1. Draw two lines representing fold axes. Which one
   is refolded?
2. Identify pattern of younger and older rocks. e.g.
   youngest on outside
3. Is the hook/boomerang shape a syncline or
   anticline?
4. What decide what kind of fold refolded the first
   fold.

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Sketch where these fold axes would plot on a
stereo net
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Type-2
Type-3
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Type-0
Type-1
Type-2
Type-3