Introduction to Folds (p ) 1)Nomenclature 2)Geometric analysis 3)More nomenclature 4)Fold mechanisms - flexural-slip folding - buckling - kink folding

Why are folds important? fundamental to deformation of the Earth’s crust Sensitive markers to tectonic stresses- everywhere!! tectonic transport direction

Structural traps for oil Controls ore geometry in strata-bound deposits

Main types of folds Anticline: fold that is convex in the direction of the youngest beds Syncline: Fold that is convex in the direction of the oldest beds *requires that you know facing direction (direction of youngest beds); know stratigraphy! Antiform: convex up Synform: convex down *simply describes geometry anticline syncline synformal anticline antiformal syncline

Geometric analysis inflection point: point of opposing convexity median surface: imaginary surface connecting inflection points fold width, fold height symmetrical vs. asymmetrical concept of vergence

hinge zone – hinge line: zone of max. curvature fold axis: imaginary line, which when moved parallel to itself can define the form of a fold Geometric analysis cont.

axial surface: surface that passes through successive hinge lines axial trace: line of intersection of axial surface and ground surface symbology: anticline, syncline, antiform, synform Geometric analysis cont.

Chevron: planar limbs meet at discrete hinge point names of common fold shapes

Ptygmatic

Circular fold "lift-off" fold

symmetric asymmetric symmetric asymmetric symmetrical: median trace and axial trace are perpendicular asymmetrical: median trace and axial trace intersect at oblique angle

more terminology

gentle (>90) open (70-120) tight (~10) isoclinal (~0)

Regional structures that have limbs folded by smaller folds

overturned: at least one of the limbs exhibits overturning, or a downward facing direction

recumbent folds- "lie on their side"

recumbent fold with secondary box folds

parallel/concentric folds: layer thickness does not change (lower T) similar folds: layer thickness changes; thickening in hinge and thinning along limbs (higher T)

More descriptive terms?

Cylindrical: well- defined axial surface non-cylindrical: too complex to have a single well-defined axial surface

monocline: step-like fold, only one limb

folded folds!

Fold mechanisms for "free folds", where fold shapes depend on layer properties (1) Flexural-slip folding- accommodates buckling by layer-parallel slip -direction of relative slip is perpendicular to hinge -individual displacement small, but sum is enough to accommodate bending of rock -marked by strong stiff layers with contacts of low cohesive strength -occurs in uppermost levels of crust

minor structures related to flexural- slip folding

minor structures related to flexural-flow folding occur at higher temperature

(2) Kink folding -Occurs when there is strong cohesion between layers -Marked by sharp hinge, straight limbs (one short, two long)

Another mechanism: (3) Buckling -Instability develops when layers of different mechanical properties are subjected to layer-parallel stresses -Wavelength depends on layer thickness and stiffness- thinner layers yield shorter wavelengths ptygmatic folds: high competence contrast cuspate-lobate folds: low competence contrast

anticline syncline antiform synform anticlinorium synclinorium inflection point hinge line fold axis symmetrical vs. asymmetrical axial plane and trace chevron fold Important terminology/concepts ptygmatic fold circular fold fold tightness recumbent fold box fold parallel/concentric vs. similar folds cylindrical vs. non- cylindrical folds monocline flexural-slip folding parasitic folds kink folding buckling instability