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Published byJuniper Sutton Modified about 1 year ago

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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: a)Bedding upright, dipping more shallowly than foliation b)Bedding overturned dipping more steeply than foliation c)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 bedding2. 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 050°, 80°-SE 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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Concentric fold Fold with narrow hinge

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Chevron fold Asymmetric fold Use caution!!

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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 F 1 and F 2 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-2Type-3

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

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