Folds and folding
Outline

Terms For Describing Folds

Fold systems

Fold geometries

Mechanics of folding

Kimematic models of folding
Initial Layer Cake
Convexity and Age of Beds
• Anticline
•
- a fold that is convex in the direction of youngest beds
Syncline - a fold that is convex in the direction of oldest beds
Direction of Fold Closing
inflection point
inflection point - change in curvature (i.e., concave to convex)
Folds - Definitions
• Antiforms are anticline-shaped folds (convex-down)
whose stratigraphic order has not been determined.
• Synforms are syncline-shaped folds (convex-up)
whose stratigraphic order has not been determined.
• We apply these terms to any fold in which facing
direction and/or stratigraphic order is unknown or
uncertain.
•Determining stratigraphic succession - which way
is up!
Folds - Definitions
• Overturned folds are
those who have a limb that
us technically upside down,
it has rotated beyond
vertical - dipping past 90°.
Folds - Geometric Properties
• The most basic element of a fold is the folded surface
• We usually describe folds in normal profile view as seen
by looking down the fold axis or down plunge.
Folds - Geometric Properties
• In normal profile view, folded
surfaces can be divided up into
limbs and hinges.
• If the hinge is sharp, that point is
called the hinge point otherwise it
is called a hinge zone.
• Fold limbs commonly curve, and
the location where segments of
opposite convexity join is called
the inflection point.
It is the place where the fold is setting
up for the next hinge.
Folds - Geometric Properties
• The hinge line of a fold is defined by successively
connecting the hinge points along the strike length of the
fold.
• The orientation of the hinge line is recorded as a lineation
(plunge & trend). Hinge lines are typically not straight and
their orientations can vary considerably.
Take hinge points along a
single folded surface, taken
together define a hinge
line
The orientation of a folded
surface can be defined by
the orientation of a hinge
line, using plunge & trend.
Hinge line
inflection point
inflection point - change in curvature (i.e., concave to convex)
Folds - Geometric Properties
• To establish the orientation or attitude
of a fold, it is necessary to know its
hinge orientation and the orientation
of the axial plane or axial surface.
trend and plunge of hingeline of a fold
is not
uniquely
defineof the
overall
The
trend
and plunge
a hinge
line of
oreintations
of thedefine
fold the
a fold does
not uniquely
orientation of the fold
Determining the Fold Axial Surface
Determining the Fold Axial Surface
Profile Plane of a Fold
Folds - Geometric Properties
• The axial surface of
a fold connects all the
hinge points in all
successive layers.
•It may be planar - an
axial plane, or a
curvi-planar surface an axial surface.
Folds - Hinge lines & Axial Surfaces
Hinge lines are lines described by a lineation that lies on the axial
surface, which is itself described by strike and dip..
Axial surface - Surface created by
the hinge lines of consecutive layers
within the fold area - it may be planar
or curved. Described by strike and
dip
How can we measure the axial surface?
We can measure its
dip direction and the
angle of dip
 Strike can always be
determined by
remembering that
strike is perpendicular
to dip

How
is the AP shown on a stereonet?
Interlimb Angle
Four Categories:
Gentle
Open
Tight
Isoclinal
Interlimb angle: classifying fold shape
Angularity of Interlimb Angle
Attitude of Axial Surface
Cylindrical or Non-Cylindrical Folds
non-cylindrical fold
Fold Types: Cylindrical Folds
Cylindrical folds: Folds
where the hinge line is
straight.
If traced far enough, few
hinge lines are ever
straight, but segments of
the hinge lines are straight,
so this is a useful concept.
Think of plotting poles to
bedding for the Mt. Baldy Lab
Stereographic Determination of Fold Orientations
Cylindrical and non-cylindrical folds
Poles to bedding planes are co-planar if the fold has a
cylindrical geometry.
Stereographic Determination of Fold Orientations
 It is usually impossible to directly
measure the axis and axial surface
of large folds.
 The trend and plunge of the hinge
line (fold axis) and strike and dip of
the axial surface can be calculated
using a stereonet.
An axial surface, by definition, passes
through the hinge line of successive
folded surfaces within a fold.
The point representing the trend and
plunge of the hinge line lies on a great
circle that describes the orientation of
the axial surface (great circle).
By definition, the fold
axis (hinge line) lies
upon the axial plane,
which bisects the foldlimbs.
Stereographic Determination of Fold Orientations
How to determine a fold axis
and axial surface of a large
fold in the field
Two methods are:
1) Beta diagrams: -diagrams
2) Pi diagrams: -diagrams
Stereographic
Determination of Fold
Orientations
 Beta diagrams: -diagrams
 Intersection shows the trend
and plunge of fold axis.
The intersection of two
bedding planes (e.g., great
circles) represents a close
approximation to trend and
plunge of the hinge line.
The intersection of the great
circles is labeled beta ().
This is called a beta ()
diagram.
Stereographic Determination
of Fold Orientations
 Pi diagrams: -diagrams
Another way to calculate the
orientation of a fold.
 plots uses at least 2 poles to
bedding, results in the
orientation of the fold axis.
 uses multiple poles to
bedding, fits a best-fit great
circle to those poles, and also
results in the orientation of the
fold axis.
Stereographic Determination
of Fold Orientations
 plots uses at least 2 poles to
bedding, results in the orientation
of the fold axis.
 Pi diagrams: -diagrams
 Pole to pi great circle shows the
orientation of the fold axis
 uses multiple poles to bedding,
fits a best-fit great circle to those
poles, and also results in the
orientation of the fold axis.
The angle between
limb 1 and limb 2 and
the axial plane are
the same - a bisector!
Bisecting surface:
Simple view in stereographic
method that the bisecting surface
approximates the axial surface.
 The bisecting surface and the
axial surface do not always
coincide.
 The axial surface connects
individual hinge lines
Determining the
orientation of the
bisecting surface of
a fold.
1)
Construct beta
diagram
2)
Plot poles to the fold
limbs
3)
Measure angle
between the poles.
4)
Fit a great circle to the
bisector and 
Determining the
orientation of the
bisecting surface of a
fold
Stereographic view of
bisecting surface in
proper orientation.
Stratigraphic Facing
Fold Symmetry and fold vergence
Fold Harmonics
Parasitic Folds
Parasitic folds
always verge
towards anticlines
and away from
synclines
Parasitic Folds
Parasitic folds verge towards anticlines and away from synclines
Parasitic folds verge towards anticlines and away from synclines
Vergence
The direction in which the
next antiform can be
found.
Vergence occurs in the
direction in which thrusting
took place.
Vergence
Vergence
Parasitic folds
Gives us information about
sense of shear on the fold
limbs as well as the location
of larger-scale fold hinges..
Think of S and Z folds, their
asymmetry will give a sense
of rotation, when viewed
down plunge.
Vergence
Small scale folds define fold shape
Vergence
Which cross-section is correct?
Identify major isoclinal fold:
antiform or synform?
Use asymmetry of the folds suggests
flexural slip on the limbs of an
overturned synform.
Expected layer parallel slip (flexural
slip) indicates sense of shear.
Flexural slip folding (buckling)
transforms symmetrical folds into
asymmetrical folds
Vergence
Which cross-section is correct?
Identify major isoclinal fold:
antiform or synform?
References
Most figures from:
http://earth.leeds.ac.uk/folds/describing/folddesc.htm