Fold

1. Fold • 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

2. Canadian Rockies, Alberta

3. Single Folded Surface • 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

4. Single Folded Surface … • 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

5. Facing • The direction of younging along the axial plane of a fold • Antiformal anticlines or synformal synclines areupward facing folds • Antiformal synclines or synformal anticlines aredownward facing folds. • This is because of overturning due to refolding

6. Hingeline • 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 for cylindrical folds

7. Hingeline, hingeplane

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

9. Folds in Multi-layers • In a multi-layered folded rock, every folded layer has its own hingeline • A plane that contains all these hingelines is called the hinge plane (or hinge surface if curved) • Hinge plane may or may not be parallel to the axial plane

11. Fold in Multi-layers … • Median Surface: Surface passing through the inflection points of a single folded layer • Inflection surface: A surface passing through the inflection lines of all the folded layers

12. Cylindrical Fold • 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 • 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!

13. Fold Axis • 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 (li) is defined by the intersection of the: • Axial plane (e.g., S1 or axial plane cleavage if it exists) with the folded layer (So), for example, for 1st generation folds, li = S1xSo • Limbs (Sn) of the fold ln = SnxSn, where n denotes the generation of the fold (0, 1, 2, …)

14. Fold Axis

15. Fold Shape • Tightness: Is defined by the interlimb angle • Interlimb angle: The angle between the tangents to the fold surface drawn through the inflection lines

16. Fold Tightness

17. Fold Attitude • The attitude of a fold is given by the attitude 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 • See Fleuty, 1964 – Fold Orientations

18. Fold Asymmtry and Overturning

19. Axial Plane (AP) • Axial plane of a fold is an imaginary plane that contains the axes of several folded layers in a fold • The axial plane and hinge plane are the same only in cylindrical folds (for which axis is parallel to the hingeline)

20. Fold axis = So x S1 S1 So

21. Axial Trace - AT • 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: AT = L1 = S1xS0for a second generation fold: AT= L2= S2xS1 • 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. Axial trace: Line of intersection of the axial plane with any other plane.It passes through points of max. curvature Axial trace

23. Axis vs. Axial Trace • 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 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. Box Fold • Polyclinal Fold – Folds with more than two axial plane (rare) • 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. Box fold

26. Types of Folds • 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 • Symmetricvs.asymmetric folds

27. Symmetric Folds • The median plane 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

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

29. Fold Asymmtry and Vergence

30. Chevron fold