Folds and Folding

Earth Structure (2nd Edition), 2004

W.W. Norton & Co, New York

Slide show by Ben van der Pluijm

WW Norton; unless noted otherwise

EarthStructure (2nd ed) 29/18/2010

Folds

Maryland Appalachians

Swiss Alps

EarthStructure (2nd ed) 39/18/2010

fold shape in profile

interlimb angle

similar/parallel

symmetry/vergence

fold size

amplitude

wavelength

fold facing

upward/downward

fold orientation

axis/hinge line

axial surface

fold in 3D

cylindrical/non-cylindrical

presence of secondary features

foliation

lineation

DePaor, 2002

Fold Classification

EarthStructure (2nd ed) 49/18/2010

Fold terminology

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Fold facing

(a) upward facing antiform or anticline

(b) upward facing synform or syncline

(c) downward-facing antiform or antiformal syncline

(d) downward-facing synform or synformal anticline

(e) profile view; (f) map view

EarthStructure (2nd ed) 69/18/2010

Fold Shape

parallel fold similar fold

ptygmatic folds

EarthStructure (2nd ed) 79/18/2010

Fold shape

a. Parallel fold

b. Similar fold

t is layer-perpendicular thickness; T is axial trace-parallel thickness

EarthStructure (2nd ed) 89/18/2010

Dip isogons

In Class 1A (a) the construction of a

single dip isogon is shown, which

connects the tangents to upper and

lower boundary of folded layer with

equal angle () relative to a reference

frame; dip isogons at 10 intervals are

shown for each class. Class 1 folds (a

c) have convergent dip isogon patterns;

dip isogons in Class 2 folds (d) are

parallel; Class 3 folds (e) have divergent

dip isogon patterns. In this

classification, parallel (b) and similar (d)

folds are labeled as Class 1B and Class

2, respectively.

EarthStructure (2nd ed) 99/18/2010

Fold Types Dip Isogons

DePaor, 2002

EarthStructure (2nd ed) 109/18/2010

Homework dip isogons

Complete problem 6.3 of the hand-out, which

uses dip isogons to classify folds.

Due on TBD.

EarthStructure (2nd ed) 119/18/2010

Enveloping surface and fold (a)symmetry

a. Symmetric; orthorhombic; ~90o

b. Asymmetric; monoclinic; < 90oThe fold enveloping surface.

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Fold vergence

Clockwise (a) and counterclockwise (b),

defined by (apparent) rotation of axial

surface from a hypothetical symmetric fold

into observed asymmetric fold, without

changing orientation of enveloping surface.

EarthStructure (2nd ed) 139/18/2010

Fold vergence - Antiform

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Fold vergence - Synform

Enveloping Surface

Axial Surface

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Folds in 3D: cylindrical and non-cylindrical folds

(a) Cylindrical fold

(b) Noncylindrical fold; planar axial surface

(c) Noncylindrical fold; curved axial surface

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Fold orientation

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Fold orientation

Recumbent fold in the Caledonides

of northeast Greenland.

Fold classification based on orientation of hinge

line and axial surface

EarthStructure (2nd ed) 189/18/2010

Fold geometries: kink and chevron folds

a. Kink fold (Spain)

b. Recumbent chevron (Switzerland)

c. Chevron folds (CA)

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En-echelon folds

Hand specimen (a). Satellite view of central Appalachians (b).

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fold facing

upward/downward

fold orientation

axis/hinge line

axial surface

fold size

amplitude

wavelength

fold shape in profile

interlimb angle

similar/parallel

symmetry/vergence

fold in 3D

cylindrical/non-cylindrical

presence of secondary features

axial plane foliation

lineation

DePaor, 2002

Fold Classification

EarthStructure (2nd ed) 219/18/2010

Elements of fold style

Characteristic elements:

What is the interlimb angle in profile?

Is the fold classified as parallel or similar (or further refinement)?

In three-dimensions, is the fold cylindrical or non-cylindrical?

Is there an associated axial plane foliation and/or lineation present, and of what type are they (these will be discussed later)?

Note: orientation and symmetry are not style criteria

EarthStructure (2nd ed) 229/18/2010

Super(im)posed folding: Fold Interference

Type 1

Type 3

Type 2

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Fold interference

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Fold interference patterns

Four basic patterns of fold

superposition. F2 shear folds (a2 is

the relative shear direction and b2

is the hinge line) are superimposed

on preexisting F1 folds.

EarthStructure (2nd ed) 259/18/2010

Fold interference scheme

Geometric axes describing

orientation of fold generations F1 and

F2 (a), and corresponding

interference patterns (b). In all

patterns, layering was initially parallel

to front face of cube. F1 resembles

case D; F2 is similar to the folding in

case D, but with different

orientations. Axial surface S1 is

shown with dotted lines and axial

surface S2 with dashed lines.

EarthStructure (2nd ed) 269/18/2010

Superimposed Folding

DePaor, 2002

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Fault-related Folds

fault-propagation fold

fault-bend fold

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Fault-bend fold

(a) Progressive stages during development of

fault-bend fold. Dashed lines are traces of axial

surfaces. (b) Photo of fault-bend fold above

McConnell Thrust, Alberta. Paleozoic strata

moved 5 km vertically and 40 km horizontally, and

now lie above Cretaceous foreland basin deposits.

(mirror image)

EarthStructure (2nd ed) 299/18/2010

Fault-propagation fold

(a) Progressive development of a fault-

propagation fold.

(b) Exposure of a fold in the Lost River

Range, Idaho, showing an asymmetric

fold dying out updip in the core of a fold.

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Fault-related Folds

DePaor, 2002

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Transposition

Sheath fold

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Fold Transposition

Asymmetric fold develops at a

perturbation (ad), which in turn gets

refolded (ef).