Folds

1
Folds

• Field and Lab Measurements

2
Data Acquired for Folds

• Detailed structural analysis requires sampling
  of
• Bedding in sedimentary rock
• Compositional layering in gneiss
• Planar fabric (cleavage) with lineation on them
• Fold elements (attitude of axial plane and
  hingeline)
• Shear zones
• Faults, slickensides
• Joints

3
Collecting Data

• Measure as many folds and their elements as
  possible.
• A higher number of folds measured will improve
  the accuracy of later structural analysis.
• Measure all the structural fabric in the same
  folded rock.
• Examine their (e.g., cleavage, lineation)
  relationship to the folds.
• Plot all the fabric data on the stereonet, and
  conduct a careful study of the orientation data.

4
Measuring a Fold

• Folds are completely defined by the attitude of
  their
• Axial plane (strike, dip)
• Hingeline (trend, plunge)
• Most folds are non-cylindrical. Break such folds
  into smaller segments where they are cylindrical
  then measure them.
• The next step is to measure the attitude of
  several tangent planes around the hingeline (in
  the hinge zone).

5
Measuring a Fold

• Measure the attitude of at least two folded
  layers (e.g., bedding (b or So), cleavage (c or
  S1) on either side of the hingeline (note them
  down as b1, b2, b3, b4).
• Measure the attitude of the hingeline (HL), by
  measuring a pencil parallel to the line
  connecting points of max. curvature (Note
  measure trend in the down-plunge direction!).
• Measure the axial plane (AP) directly.
• AP contains the HL (a pencil) and the axial
  trace (AT) (a second pencil) on the profile plane
  (plane perpendicular to the hingeline).
• If HL cannot be measured, measure as many axial
  traces (AT) as possible then line them up on a
  same great circle.

6
Plot the Fold Data (?-diagram)

• Plot the fold elements using an equal-area
  stereonet.
• ?-diagram
• Plot the normal or pole (perpendicular) to each
  folded layer as a point. Use consistent symbols,
  e.g.
• Use ? symbols for the pole to folded bedding
• Use ? symbol for the pole to folded cleavage
• Plot the normal to the axial plane (symbol ?)
• The best-fit great circle through the poles
  defines the profile plane (plane normal to the
  hingeline or axis).
• Fold (?) axis (symbol ?) is the pole to the
  profile plane
• Plot the hingeline (?) and compare it with the
  axis (?)
• Check to see if the ? and/or ? lie on the axial
  plane.

7
The Interlimb Angle

• The poles to the two limbs of a fold may not
  spread over the 180o sector of the profile
  plane(great circle).
• In such a case, the interlimb angle is the angle
  between the two dominant clusters (maxima) of the
  normals (of the two limbs) measured on the
  profile plane.
• If the fold has straight limbs (e.g. chevron
  fold), the poles to the two limbs define two
  maxima. In such a case, the fold axis is the
  intersection of the two planes (great circles)
  drawn perpendicular to these two maxima.

8
Cylindrical Folds (?-diagram)

• ?-diagram
• Plot the folded layers as cyclographs (i.e.,
  great circles)
• Determine the fold (?) axis at the intersection
  of all the cyclographs (they do not intersect
  exactly at a point!).
• The ? axis and the ? axis are generally the same.
• The ? diagram is more cluttered than the ?
  diagram.
• We commonly use both of these diagrams in our
  fold analysis.

9
Construction of the Axial Plane

• Apply any of the following techniques using a
  net
• The axial plane is the great circle that includes
  the axis and any of the measured axial traces (in
  the field or on a map) -(measured on any random
  section of the fold)
• The axial plane is the great circle that contains
  at least two axial traces on two random sections
  (none has to be the hingeline).
• In a symmetric fold, the axial plane may be
  assumed to be the bisector plane of the fold that
  contains the axis of the fold.
• In a similar fold with foliation, the foliation
  may parallel the axial plane of the fold.

10
Construction of Fold Axis from Intersections

• The fold axis can also be determined from
• (So x S1) i.e., intersection of the hinge plane
  or axial plane (e.g. S1) and folded surface (So),
  where So is the original surface such as bedding
  or lava flow, and S1 is the first generation
  surface such as the axial plane of first
  generation fold or an axial-plane cleavage.
• The (So x S1) intersection of an axial-plane
  foliation, S1 and folded layer, So
• The (So x So) intersection of several tangent
  planes to the folded layer in the hinge area of
  the fold (? axis).

11
Fold Classification - Parallel Folds

• Thickness, t, perpendicular to the layer, is
  constant throughout the fold.
• Some parallel fold are concentric i.e., have
  constant, circular curvature
• Parallel folds are typical of competent layers.
• Layer thickness measured parallel to the axial
  plane (T) is greater on the limbs than around the
  hinge.

12
Similar Folds

• Have variation in their layer thickness, t.
• The layer thickness reduces on the limb.
• The curvature of the bounding surfaces are
  identical (hence the word similar).
• The layer thickness measured parallel to the
  axial plane (T) is constant.

13
Intermediate Fold Styles Ramsay 1967

• The similar and parallel folds are not the end
  members of fold style. Other styles fall outside
  of these two.
• Progressive deformation (e.g., flattening) may
  change one geometry to another.
• Ramsay proposed analyzing the variation of the t
  layer thickness with the angle of dip within a
  quarter fold wave sector.

14
Fold Classification Ramsay 67

• Start with a profile plane view of a fold
  (constructed by rotation, or photographed in the
  field looking downplunge).
• Mark the hinge points and inflection points on
  the two bounding surfaces of the folded layer.
• Draw the tangents to the folded layer at the
  hinge points. This is the zero dip (? 0)
  reference.
• At ? 0, measure the orthogonal hinge thickness
  to.
• Construct other tangents at other ? angles.
• Measure the orthogonal thickness (t?) between
  these tangents for these ? angles.
• Determine the ratio t? t? /to
• Plot t? as ordinate against ? as abscissa.
• Repeat for all values of ?.

15
Fold Classification Ramsay 67

• For parallel folds that have a constant
  orthogonal thickness
• t? t? /to 1
• For similar folds thickness measured parallel to
  axial plane
• T? To to
• For similar folds, the orthogonal thickness
  varies as
• t? cos ?

16
Fold Classification

• On the t? against ? diagram
• All other folds fall on either side of the two
  lines
• t? 1 (parallel fold class 1B)
• t? cos ? (similar fold class 2)
• Class 1A folds lie above class 1B parallel folds.
• Class 1A folds are thicker on the limb than near
  the hinge
• Class 1C, class 2, and class 3 folds all show
  thinning on the limb.

17
Dip Isogons

• Lines joining points of equal dip (normal to
  tangents).
• These are drawn at different ? angles (at 10o
  intervals).
• Isogons can be parallel, converging, or
  diverging.
• The sense is from the outer arc to the inner arc
  of the fold.
• Isogons provide clue as to the nature of the
  curvatures
• Parallel isogons
• The average inner and outer curvatures are the
  equal.
• Converging isogons
• Inner arc curvature exceeds that of outer arc.
• Divergent isogons
• Outer arc curvature exceeds that of the inner arc.

18
Fold Classes