Folds & Folding

Folds and Folding

Earth Structure (2nd Edition), 2004 W.W. Norton & Co, New York Slide show by Ben van der Pluijm

Maryland Appalachians

Swiss Alps

9/18/2010 © EarthStructure (2nd ed) 2 Fold Classification 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

9/18/2010 © EarthStructure (2nd ed) 3 Fold terminology

9/18/2010 © EarthStructure (2nd ed) 4 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

9/18/2010 © EarthStructure (2nd ed) 5 Fold Shape

parallel fold similar fold

ptygmatic folds

9/18/2010 © EarthStructure (2nd ed) 6 Fold shape

a. Parallel fold b. Similar fold t is layer-perpendicular thickness; T is axial trace-parallel thickness

9/18/2010 © EarthStructure (2nd ed) 7 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 (α) 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.

9/18/2010 © EarthStructure (2nd ed) 8 Fold Types – Dip Isogons

DePaor, 2002

9/18/2010 © EarthStructure (2nd ed) 9 Homework – dip isogons

Complete problem 6.3 of the hand-out, which uses dip isogons to classify folds. Due on TBD.

9/18/2010 © EarthStructure (2nd ed) 10 Enveloping surface and fold (a)symmetry

a. Symmetric; orthorhombic; ~90o The fold enveloping surface. b. Asymmetric; monoclinic; < 90o

9/18/2010 © EarthStructure (2nd ed) 11 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.

9/18/2010 © EarthStructure (2nd ed) 12 Fold vergence - Antiform

9/18/2010 © EarthStructure (2nd ed) 13 Fold vergence - Synform

Enveloping Surface Axial Surface

9/18/2010 © EarthStructure (2nd ed) 14 Folds in 3D: cylindrical and non-cylindrical folds

(a) Cylindrical fold (b) Noncylindrical fold; planar axial surface (c) Noncylindrical fold; curved axial surface

9/18/2010 © EarthStructure (2nd ed) 15 Fold orientation

9/18/2010 © EarthStructure (2nd ed) 16 Fold orientation Fold classification based on orientation of hinge line and axial surface

Recumbent fold in the Caledonides of northeast Greenland.

9/18/2010 © EarthStructure (2nd ed) 17 Fold geometries: kink and chevron folds

a. Kink fold (Spain) b. Recumbent chevron (Switzerland) c. Chevron folds (CA)

9/18/2010 © EarthStructure (2nd ed) 18 En-echelon folds

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

9/18/2010 © EarthStructure (2nd ed) 19 Fold Classification

• 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

9/18/2010 © EarthStructure (2nd ed) 20 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

9/18/2010 © EarthStructure (2nd ed) 21 Super(im)posed folding: Fold Interference

Type 1

Type 2 Type 3

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.

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.

DePaor, 2002

fault-propagation fold

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)

(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.

DePaor, 2002

Sheath fold

Asymmetric fold develops at a perturbation (a–d), which in turn gets refolded (e–f).