• Folds form from curving, buckling, and bending of originally planar rock layers (e.g., beds, foliation) through ductile deformation
• Practically, folds are defined by the attitude of their
• axis and/or hingeline, and axial plane
• Folds occur in any geologic layer such as bedding, lava flow layers, foliation
• Folds range in size from mm to km
• Folds are a manifestation of ductile deformation
– i.e., form at depth where T, P are high, and fracturing does not occur
1

2

• Hingeline (HL)
– Lines joining points of maximum (tightest) curvature
• Inflection lines
– Lines connecting points of zero curvature
– Half-wavelength (w/2) : distance between two inflection points ( i )
• Fold domains : are separated by the inflection lines
• The following terminologies are based solely on geometry:
– Antiform : Domains with upward closure (- curvature)
– Synform : Domains with downward closure (+ curvature)
– Neutral fold : Fold that closes sideways
3

• Anticline: A fold with the oldest rocks at the core
(i.e., at the concave side)
• Syncline: A fold with the youngest rocks at the core
• Note: In simply-folded areas, anticlines are generally antiformal, and synclines are synformal
• However, in refolded areas this is not generally the case, and the following structures are common:
– Antiformal syncline
– Synformal anticline
4

• The direction of younging along the axial plane of a fold
• Antiformal anticlines or synformal synclines are upward facing folds
• Antiformal synclines or synformal anticlines are downward facing folds .
– This is because of overturning due to refolding
5

• A special fold axis, connecting points of maximum curvature on the folded surface
• Hingeline is a physical line that can be marked on the folded layer with a pen and be measured directly with a geologic compass
• Folds may have one or more hingelines
– Example of a multi hingeline fold: box fold
• Non-cylindrical folds have many hingelines but no axis!
• Hingeline and axis are parallel in cylindrical folds
6

7

• Hinge zone : The region around the hingeline
• Limb : The region between two adjacent hingelines
• Axial trend :
Refers to the attitude of the hingeline or axis
• A hingeline generally undulates; i.e., changes attitude for a given fold
– This produces structures such as: culmination, depression, saddle, dome, basin
8

• In a multi-layered folded rock, every folded layer has its own hingeline that may or may not parallel
– They parallel if the fold is simple, cylindrical
• A plane that contains all these hingelines is called the hinge plane (or hinge surface if curved)
• Hinge plane may or may not parallel to the axial plane
– Hinge plane parallels the axial plane in cylindrical folds
• Not so for non-cylindrical folds!
9

• Everywhere on a cylindrical fold, there is a line ( axis ) which parallels the hingeline
– i.e., hingeline and axis of cylindrical folds are parallel
• Fold axis ( not a physical line ) cannot be marked on the folded surface. Hingeline can be marked with a pen or chalk
• There are infinitely many axes on the cylindrically folded layer, and these are not restricted to the hinge area of the fold
• The fold axis, therefore cannot be directly measured in the field; it has to be determined indirectly
• A Brunton compass is used in the field to measure the attitude of the limbs, which are then plotted, using the stereonet , to find the axis of the cylindrical fold at their intersection!
10

Terminology of a cylindrical fold in a multi-layer 11

• An imaginary line that generates the fold if it is moved parallel to itself in space
• Fold axis makes sense only for cylindrical folds
• Most folds are non-cylindrical at a large scale
• Areas with non-cylindrical folds can be broken into smaller areas (domains) in which folds are cylindrical
• Axis ( l i
) is defined by the intersection of the:
– Axial plane (e.g., S the folded layer (S
1 o or axial plane cleavage if it exists) with
), for example, for 1 st generation folds, l i =
S
1 xS o
– Limbs ( S n
) of the fold l n
= S n xS n
, where n denotes the generation of the fold (0, 1, 2, …)
12

13

From: Analysis of Geological Structures: Neville J. Price, John W. Cosgrove
14

• Tightness : Is defined by the interlimb angle
• Interlimb angle : The angle between the tangents to the fold surface drawn through the inflection lines
Shape Interlimb angle gentle 180 o -120 o open 120 o -70 o close 70 o -30 o tight 30 o - 0 o isoclinal 0 o elastica Negative values
15

16

• The attitude of a fold is given by the a ttitude of its: Axial plane (strike, dip) & Hingeline (trend, plunge)
• Vertical fold: vertical axial plane and vertical axis
• Upright plunging: vertical axial plane, plunging axis
• Upright horizontal: vertical axial plane, horizontal axis
• Inclined plunging: inclined axial plane, plunging axis
• Inclined horizontal: inclined axial plane, horizontal axis
• Reclined: plunging axis trends along the dip of the inclined axial plane
• Recumbent fold: horizontal axis and axial plane

• Folds are classified based on the relative values of the dip of the axial plane, and the plunge of the hingeline
Fleuty, 1964 – Fold Orientations
18

19

• Axial plane of a fold is an imaginary plane that contains the axes of several layers in a multi-layer fold
• The axial plane and hinge plane are the same only in cylindrical folds (for which axis is parallel to the hingeline)
– We can measure the hinge plane in the field
– We can only determine the axial plane indirectly from two lines (e.g., fold axis and axial trace; two axial traces)
20

If we designate the bedding with S o
, and an axial planar foliation as S
1
, then the trace of the foliation on the folded bedding is the axial trace (AT) which is the fold axis, and which is defined by the intersection of the bedding and foliation
(fold axis = S o x S
1
)
S
1
S o
21

• Axial trace of a fold is the line of intersection of the axial plane of a fold (AP) with any other plane, e.g., with:
– a folded layer (in this special case, the axial trace is the same
as hingeline or axis), i.e.,
– For a first generation fold :
– For a second generation fold :
AT = L
1
= AP
1 xS
0
= S
1 xS
0
AT = L
2
= AP
2 xAP
1
= S
2 xS
1
– a scarp, joint, or any other inclined plane
– horizontal plane of a map (horizontal AT)
• Having two axial traces we can find the axial plane!
• There are an infinite number of axial traces for a fold
(we only need one or two of them for finding the axial plane)!
22

23

• Axis is a special axial trace
– They are generally not the same thing
• The axial trace of a non-plunging fold is parallel to the strike of the fold limbs, which are parallel to the fold axis
• The axial trace of an inclined, plunging fold on the horizontal plane (map), is not parallel to the fold axis!
• The dip of the folded layer at the hinge zone of a symmetric, plunging fold is equal to the plunge of the fold axis!
24

• Polyclinal Fold –
Folds with more than two axial plane
• Conjugate fold : Has converging paired axial surfaces
– Axial planes intersect along the axis (if cylindrical)
– Axial plane may displace another axial plane
• Box Fold : Conjugate folds with round hinge zones
• Kink Fold : Conjugate fold with sharp hinge zones
25

26

• Monocline:
– A local steepening in otherwise uniformly dipping strata.
• Isoclinal fold:
– Limbs are parallel to the axial plane.
• Recumbent fold:
– Fold with horizontal axial plane. Commonly isoclinal
• Symmetric vs. asymmetric folds
27

• The median plane (plane through inflection points) and the axial plane are perpendicular, and the axial plane divides the fold into mirror quarter waves
• Polyharmonic fold : Fold waves with two or more orders of wavelengths and amplitude. Large polyharmonic folds have parasitic (smaller) fold
Disharmonic fold in quartzite/shale
Disharmonic fold in gneiss
28 http://pages.uoregon.edu/millerm/disharm.html

• The asymmetry of smaller folds is:
– Z-, S-, W-, or M-shaped
• These asymmetries are defined looking downplunge, or projected onto a horizontal plane
• The asymmetries of the smaller folds can be used to determine the closure of the larger fold
29

http://sanuja.com/blog/geometry-of-folds
30

31

32