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Folds analysis techniques

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

2 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

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

4 2. π-axes and β-axes Define fold axis using measurements of bedding from around the fold Plot on stereonet

5 π-girdle: best-fit girdle to poles to bedding π-axis: pole to π-girdle

6 Finding π-axes 1. Plot poles to bedding 2. Find best-fit great circle


8 β-axis: defined by intersections of bedding planes plotted on net

9 Find a fold axis using these bedding measurements
350°, 44°-NE 088°, 72°-NW Use both π-axis and β-axis methods

10 Interpreting π-axis diagrams


12 Fold with narrow hinge Concentric fold


14 Asymmetric fold Use caution!!
Chevron fold

15 3. Refolded folds

16 Refolded folds recognition
Can often be found in outcrop Regional patterns in facing direction Interference patterns


18 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

19 Interpret these maps: Draw in F1 and F2 axes and indicate plunge directions

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

21 Sketch where these fold axes would plot on a stereo net

22 Type-2 Type-3

23 Type-0 Type-1 Type-2 Type-3

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