Folds and folding
In Ductile processes, deformation is retained.
Plastic if permanent strain increases at same stress
Some types of folds
Folds and Folding
Folds and folding
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
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!
Direction of Fold Closing
inflection point
inflection point - change in curvature (i.e., concave to convex)
Folds - Definitions
• Overturned folds are those who have a limb that is
upside down, it has rotated beyond vertical, i.e. dipping
past 90o from its original horizontal position°.
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.
•Hinge: Region of Greatest Curvature
• 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.
Folds - Geometric Properties
• The hinge line of a fold is defined by successively
connecting the hinge points of the fold.
• The orientation of the hinge line is recorded as a lineation
(plunge (angle) & trend (compass direction)). Hinge lines are
typically not straight and their orientations can vary
considerably.
Multiple 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.
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
Folds with the same hinge line
Same hinge line direction
Same axial surface
We need more information to distinguish these, maybe orientations of the limbs
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.
Determining the Fold Axial Surface
Determining the Fold 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 local 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
Profile Plane of a Fold
Interlimb angle “fold tightness”, four categories
Angularity of Interlimb Angle
Attitude of Axial Surface
Cylindrical or Non-Cylindrical Folds
Stratigraphic Facing
Graded Bedding
Parasitic Folds
Vergence
Parasitic folds
Give 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.