Fold analysis techniques
Goal: To apply some of the basic geometric techniques for analyzing folds
and refolded folds.
1. Foliation-bedding intersections and facing directions
a) Combine angle between foliation and bedding with facing direction to
determine location on a fold
 Bedding upright, dipping more shallowly than foliation
 Bedding overturned dipping more steeply than foliation
 Bedding highly oblique to foliation
b) Use basic map example to interpret over-turned folds. Pass out and
have students work through.
c) Note that for cylindrical folds, bedding-foliation intersection lineation
lies close to fold hinge.
2. Finding π- and β-axes
a) π-girdle and π-axis: Great circle and its pole that are defined by a
girdle of poles to bedding from a cylindrical fold. π-axis defines the
regional fold axis.
b) β-axis: Point at which all of the great-circles representing bedding
from a cylindrical fold meet. Defines the regional fold axis.
c) Have students plot the planes 050°, 80°-SE; 350°, 44°-NE; and 088°,
72°-NW and find the π- and β-axes of the hypothetical fold. Should
be 070°, 44° with an π-girdle of 340°, 46°-SW
d) Use examples from Marshak and Mitra to illustrate how π-plots can
be used to determine style of folding.
3. Refolded folds
a) Recognizing refolded folds
 Can sometimes be seen in outcrop — hook folds
 Can be recognized using regional patterns in facing direction —
example from Waterville area
 Can also produce interference patterns
b) Fold interference patterns
 Type-0, tightening of pervious folds: Fold axes and axial
surfaces subparallel. Previous folds are tightened or flattened.
 Type-I, dome and basin: Trend of fold axes and strike of axial
surfaces are at near-right angles to one another. Dips of axial
surfaces remain the same
 Type-II, boomerang: Trend of fold axes and strike of axial
surfaces are at near-right angles to one another. Dips of axial
surfaces also at high-angle to one another
 Type-III, hook folds: Fold axes are subparallel. Strikes of axial
surfaces are at near-right angles to one another. Dips of axial
surfaces are similar.
c) Fold interference patterns in-class interpretation — Use Hatcher’s
maps as in-class exercise.