Download Introduction to Folds

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 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!
anticline
syncline
synformal
anticline
Antiform: convex up
Synform: convex down
*simply describes geometry
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
Geometric analysis cont.
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
names of common fold shapes
Chevron: planar limbs meet
at discrete hinge point
Ptygmatic
Circular fold
"lift-off" fold
symmetrical: median trace and axial trace are perpendicular
asymmetrical: median trace and axial trace intersect at oblique angle
symmetric
asymmetric
symmetric
asymmetric
more terminology
gentle (>90)
open (70-120)
isoclinal (~0)
tight (~10)
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: welldefined 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 flexuralslip 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 stiffnessthinner layers yield shorter wavelengths
ptygmatic folds:
high competence
contrast
cuspate-lobate
folds: low
competence contrast
Important terminology/concepts
anticline
syncline
antiform
synform
anticlinorium
synclinorium
inflection point
hinge line
fold axis
symmetrical vs.
asymmetrical
axial plane and trace
chevron fold
ptygmatic fold
circular fold
fold tightness
recumbent fold
box fold
parallel/concentric vs.
similar folds
cylindrical vs. noncylindrical folds
monocline
flexural-slip folding
parasitic folds
kink folding
buckling instability