Presentation on theme: "Folds analysis techniques Goal: To apply some of the basic geometric techniques for analyzing folds and refolded folds."— Presentation transcript:
Folds analysis techniques Goal: To apply some of the basic geometric techniques for analyzing folds and refolded folds.
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
Note that for cylindrical folds, bedding- foliation intersection lineation lies close to fold hinge.
2. π-axes and β-axes Define fold axis using measurements of bedding from around the fold Plot on stereonet
π-girdle: best-fit girdle to poles to bedding π-axis: pole to π-girdle
Finding π-axes 1. Plot poles to bedding2. Find best-fit great circle
β-axis: defined by intersections of bedding planes plotted on net
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
Interpreting π-axis diagrams
Concentric fold Fold with narrow hinge
Chevron fold Asymmetric fold Use caution!!
3. Refolded folds
Refolded folds recognition 1.Can often be found in outcrop 2.Regional patterns in facing direction 3.Interference patterns
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
Interpret these maps: Draw in F 1 and F 2 axes and indicate plunge directions
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.
Sketch where these fold axes would plot on a stereo net