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
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.
symmetric
asymmetric
symmetricasymmetric
symmetrical: median trace and axial trace are perpendicularasymmetrical: median trace and axial trace intersect at oblique angle
parallel/concentric folds: layer thickness does not change (lower T)
similar folds: layer thickness changes; thickening in hinge and thinning along limbs (higher T)
Cylindrical: well-defined axial surface
non-cylindrical: too complex to have a singlewell-defined axial surface
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
(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