Folds

1
Folds

• Chapter 7

2
Strike
Strike
Dip direction
Dip direction
Antiform
Antiform
Synform


Mechanical layering
Ductile deformation
Compression

FOLDS
3
Strike
Strike
Dip direction
Dip direction
Antiform
Anticline
Antiform
Anticline
Syncline
Synform
Antiformal synclines and synformal anticlines can
form due to refolding
4
Hinge line Join points of maximum curvature
along the same layer (pages 383-384)
http//earth.leeds.ac.uk/folds/describing/3d/index
.htm
5
To construct the AXIAL SURFACE of a fold (page
384)

• Find the hinge lines of different layers

http//earth.leeds.ac.uk/folds/describing/3d/axsur
f1.htm

• Construct the IMAGINARY surface containing all
  the hinge lines for that fold

http//earth.leeds.ac.uk/folds/describing/3d/axsur
f2.htm
6
The hinge lines could be

• Horizontal
• Plunging
• Vertical

The section perpendicular to the hinge line is
called the PROFILE section or profile plane of
the fold
http//earth.leeds.ac.uk/folds/describing/3d/profi
le.htm
7

• The trace of the axial surface on the profile
  section is called AXIAL TRACE (page 384)

Axial trace
The hinge lines can be straight or curved.
Straight segments of hinge lines are considered
for descriptive purposes
8
Fleuty diagram (page 387, fig. 7.25)

• Classifies folds on the basis of
• The DIP of the axial plane, and,
• The PLUNGE of the hinge line

Dip of axial plane CANNOT be lower than the
plunge of the hinge line (why?)
9
Dip of axial plane
Recumbent
Steeply inclined
Upright
Gently inclined
Moderately inclined
10
30
60
80
90
0
0
Sub-horizontal
10
Gently plunging
30
Plunge of hinge line
Moderately plunging
60
Steeply plunging
80
Sub-vertical
90
10
Fold axis vs. hinge line (page 387)

• Fold axis is a mathematical concept (IMAGINARY)
• ALWAYS a straight line
• Generates CYLINDRICAL (or near-cylindrical) folds
  (page 388)
• Useful for detailed structural analyses,
  identifying multiple generations of folds etc.

11
Fold axis vs. hinge line contd.

• Hinge line is a measurable line (REAL) that joins
  points of maximum curvature on a folded surface
• May or may not be a straight line
• All types of folds have hinge lines
• When the hinge line (or a segment of hinge line)
  is straight, it is considered parallel to the
  fold axis

12
Cylindrical folds (page 388)

• Mathematical definition
• Folded surface generated by a straight line (fold
  axis) moving parallel to itself in space
• Real-life characteristic
• Folds with straight hinge lines
• Importance
• Cylindrical folds indicate a SINGLE folding event

13
Cylindrical folds on stereograms (page 389)

• On a stereogram, the great circles representing
  the limbs (or the planar segments of the folded
  surface) intersect along the FOLD AXIS (ß-
  diagram)
• The POLES of the limbs (or the planar segments of
  the folded surface) lie on one great circle
  (p-diagram)
• The pole to this great circle (called p-circle)
  represents the fold axis
• The p-circle represents the PROFILE SECTION of
  the fold

14
Ode to the profile section

• The profile section is the plane perpendicular to
  the hinge line (or fold axis)
• If the limbs of a fold are planar, the angle
  between them (INTERLIMB ANGLE, page 390) is
  measured on the profile section
• The fold axial plane is usually the plane
  bisecting the interlimb angle (the best way to
  find its orientation)