Transcript Perspectives of Japan
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
by looking
down the fold axis or down plunge
.
as seen
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 define the overall oreintations of the fold of 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 fold limbs.
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 Pi diagrams: diagrams Pole to pi great circle shows the orientation of the fold axis 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.
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) 2) 3) 4) Construct beta diagram Plot poles to the fold limbs Measure angle between the poles. 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