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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. π-axes and β-axes Define fold axis using measurements of bedding from around the fold Plot on stereonet
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π-girdle: best-fit girdle to poles to bedding π-axis: pole to π-girdle
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Finding π-axes 1. Plot poles to bedding 2. Find best-fit great circle
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β-axis: defined by intersections of bedding planes plotted on net
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Find a fold axis using these bedding measurements
350°, 44°-NE 088°, 72°-NW Use both π-axis and β-axis methods
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Interpreting π-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
Can often be found in outcrop Regional patterns in facing direction 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
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.
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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
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