Introduction to Folds(p. 372-413)

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 oilControls 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 meetat discrete hinge point

names of common fold shapes

Ptygmatic

Circular fold"lift-off" fold

symmetric

asymmetric

symmetricasymmetric

symmetrical: median trace and axial trace are perpendicularasymmetrical: median trace and axial trace intersect at oblique angle

more terminology

gentle (>90) open (70-120)

tight (~10)

isoclinal (~0)

Regional structures that have limbsfolded 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 singlewell-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/conceptsptygmatic 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