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 bedding 2. 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 foldFold with narrow
hinge
Chevron foldAsymmetric 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 F1
and F2 axes and
indicate plunge
directions
Steps to interpret maps1. 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
Type-2 Type-3
Type-0 Type-1
Type-2 Type-3
