# Folds analysis techniques

## Presentation on theme: "Folds analysis techniques"— 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: Bedding upright, dipping more shallowly than foliation Bedding overturned dipping more steeply than foliation 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
350°, 44°-NE 088°, 72°-NW Use both π-axis and β-axis methods

Interpreting π-axis diagrams

Fold with narrow hinge Concentric fold

Asymmetric fold Use caution!!
Chevron fold

3. Refolded folds

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

Sketch where these fold axes would plot on a stereo net

Type-2 Type-3

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