Folds and folding

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Folds and folding
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Outline

• Terms For Describing Folds
• Fold systems
• Fold geometries
• Mechanics of folding
• Kimematic models of folding

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Initial Layer Cake
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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

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Direction of Fold Closing
inflection point
inflection point - change in curvature (i.e.,
concave to convex)
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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!

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Folds - Definitions

• Overturned folds are those who have a limb that
  us technically upside down, it has rotated beyond
  vertical - dipping past 90.

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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.

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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.

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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.
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Hinge line
inflection point
inflection point - change in curvature (i.e.,
concave to convex)
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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.

The trend and plunge of a hinge line of a fold
does not uniquely define the orientation of the
fold
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Determining the Fold Axial Surface
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Determining the Fold Axial Surface
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Profile Plane of a Fold
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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.

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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
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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?

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Interlimb Angle
Four Categories Gentle Open Tight Isoclinal
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Interlimb angle classifying fold shape
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Angularity of Interlimb Angle
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Attitude of Axial Surface
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Cylindrical or Non-Cylindrical Folds
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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.

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Think of plotting poles to bedding for the Mt.
Baldy Lab
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Stereographic Determination of Fold
Orientations Cylindrical and non-cylindrical
folds
Poles to bedding planes are co-planar if the fold
has a cylindrical geometry.
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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).
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By definition, the fold axis (hinge line) lies
upon the axial plane, which bisects the
fold-limbs.
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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

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• Stereographic Determination of Fold Orientations
• Beta diagrams b-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
(b). This is called a beta (b) diagram.
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• Stereographic Determination of Fold Orientations
• Pi diagrams p-diagrams

Another way to calculate the orientation of a
fold. p plots uses at least 2 poles to bedding,
results in the orientation of the fold axis. p
uses multiple poles to bedding, fits a best-fit
great circle to those poles, and also results in
the orientation of the fold axis.
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• Stereographic Determination of Fold Orientations
• Pi diagrams p-diagrams
• Pole to pi great circle shows the orientation of
  the fold axis

p plots uses at least 2 poles to bedding, results
in the orientation of the fold axis. p uses
multiple poles to bedding, fits a best-fit great
circle to those poles, and also results in the
orientation of the fold axis.
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The angle between limb 1 and limb 2 and the axial
plane are the same - a bisector!
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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

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Determining the orientation of the bisecting
surface of a fold.

• Construct beta diagram
• Plot poles to the fold limbs
• Measure angle between the poles.
• Fit a great circle to the bisector and b

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Determining the orientation of the bisecting
surface of a fold Stereographic view of bisecting
surface in proper orientation.
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Stratigraphic Facing
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Fold Symmetry and fold vergence
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Fold Harmonics
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Parasitic Folds
Parasitic folds always verge towards
anticlines and away from synclines
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Parasitic Folds
Parasitic folds verge towards anticlines and away
from synclines
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Parasitic folds verge towards anticlines and away
from synclines
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Vergence
The direction in which the next antiform can be
found. Vergence occurs in the direction in which
thrusting took place.
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Vergence
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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.
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Vergence
Small scale folds define fold shape
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Vergence
S
Which cross-section is correct? Identify major
isoclinal fold antiform or synform?
Z
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
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Vergence
S
Which cross-section is correct? Identify major
isoclinal fold antiform or synform?
Z
S
Z
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References
Most figures from http//earth.leeds.ac.uk/folds/
describing/folddesc.htm