Description and mechanics of folds

Lecture index: Description of single folded surfaces. / Description of mutliple folded surfaces and fold style. / Kinematic analysis of folds in cross section. / Fold formation mechanisms. /


List of key terms:

The adjacent photo is of folds evident in ice and debris material in Malaspina glacier, St Elias Mountains of Alaska. The folds reflect deformation, flowage in the ice. Another factor in the appearance here is that this is not a orthogonal cross section view, but a very oblique slice provided by a melting surface, and thus layer thicknesses are not true layer thicknesses (measured perpendicular to the layer boundaries). Indeed, given that moraine material is often of a stripe, character, the geometry is not simply that of folded parallel layers. Photo source: .

Description of single folded surfaces

A common assumption is that layers start out as planar and horizontal.. This is a good working assumption for many strata. When might it not be a good assumption? We will start out exploring how to describe the character of just a single folded surface. Such a surface might be represented by a surface contour map.

cylindrical geometry:

conical geometry:

irregular geometry:

composite geometry: e.g. a cylindrical middle portion and conical ends.

More complex patterns occur due to refolding and the variety of interference patterns that can develop. The Excel sheet above can model some simple examples.

Geometric descriptors for cylindrical folds:

Folds in carbonate strata in Montana showing a clear vergence (limb asymmetry). View is northward. What direction is the vergence? Photo source:,%20M.R.%20369ct .

Description according to size (terminology works with other structures):

Description according to tightness or interlimb angle:

Description according to profile curvature:

This photo is of a large conjugate or box fold structure with one of the 5 small fishing villages and popular tourist destinations of the Italian west coast, an area known as Cingue Terre, perched above. The strata are largely turbidites, and the deformation is related to a complex tectonic evolution that includes subduction. Note how the fold form changes as you ascend or descend in the sequence, a trait characteristic of parallel fold geometry (described below). These structures are part of the Northern Appenines, with Mesozoic strata deformed during the Cretaceous. More information on the geology of this area can be found at:

Photo of fold from USGS in Cretaceous strata of Chile. What term might you use for the geometery of this fold? Photo source:,%20K.%20563 .

Description according to orientation and age relationships:

This is a cliff side along an island just south of Porto Venere in Italy that provides a natural cross section of a large fold structure evident in a marble rich sequence (the Porotoro marbles). The red dotted line traces the approximate position of the axial plane as defined by hinges. Naturally, one of the two limbs must be overturned and with a close to horizontal axial plane this fold can also be considered recumbant. Fold nappes are known from this area. .

Image showing map traces of large scale anticlines, synclines and monoclines for the 4-corners area of the U.S.. The monoclines only have an arrow on one side. Note how the defiance Uplift changes along strike. Also note the different directions the fold features are oriented at. Image source: .

Description of mutliple folded surfaces and fold style

axial plane: the limb bisector plane for simple geometries.

axial surface: the plane formed by the sum of all the fold hinge lines. Better descriptor to use, can have distinct strain significance.

The image to the right depicts the axial plance for a simple fold. the dashed heavy line represents the fold hinge line. All the hinge lines together define the axial plane or axial surface (if it is not a planar surface). Image source USGS site:

axial trace on surface: the line formed by the intersection of the axial surface and the earth's surface; i.e. the map trace of the axial surface.

parallel folds :

These are folds in Cretaceous strata exposed at Ernst Tinaja in Big Bend National Park, a very popular stop for geologists. They show the disharmonic geometry and maintenance of bedding-perpendicular thickness that characterize a paralel fold style.

Fold in marbles from cliffside of Porto Venere Regional Nature Park (same locality and geologic context as two photos above). Here one can see a disharmonic fold sandwiched between two slip zones both of which are partially bedding parallel. Disharmonic folds are associated with slip along layering and often with detachments, as they are here, and one possibility is that this fold structure is acting as a transfer zone where slip is 'climbing' from one slip horizon to another. Towards the right one can also see the thicker grey carbonate layer truncated. There is fault herewith the layering on the left side parallel to the fault, and the layering on the right side truncated. If you trace that fault upward you see that it bends so that it is parallel to the layering. This structural pattern is not uncommon. As you will see layer parallel slip is a common component in some types of folds and faults.

similar folds:

Schematic diagrams showing these two end-member fold geometries. As you might suspect, the direction in which layer thickness is maintained in the similar fold geometry to the right has kinematic significance.

Composite fold forms are the most common, with some layers retaining layer thickness, and others not. This is due to differences in rock competency and anistropy, which causes different lithologies to fold by different mechanisms.

Folds of chert layers in the marble matrix of the Bruce 'limestone' from the Precambrian Huronian sequence above Lake Huron in Canada. Note how it is difficult to trace for certain the delicately fold layers across the photo. One might describe the layering as show incipient or mild transposition. Also doe the brittle behavior evident in the thicker cher layer near the base. See below for further explanation.

fold transposition:

If you examine the detailed version of this photograph to the left from the NW coast of Ireland (click on the image to see a larger version) you can see that the axial traces of an earlier fold phase (some shown with red dots) are folded around a later phase fold with its approximate axial trace shown with yellow dots. These are Dalradian marbles that have seen complex polphase deformation including a major Caledonian phase.

polyphase deformation and fold interference patterns.

Kinematic analysis of folds (and faults) in cross section

Basic assumptions for simple balanced cross section analysis:

Shallow crustal level fold and thrust belts often meet these requirements to a good enough approximation.

Sinuous bed method:

Equal area method:

Typical amounts of shortening in fold-and-thrust belts = 10s to 100s of km.

Example of USGS balanced cross section. Found at .

Fold formation mechanisms

Flexural slip folds:

Fault propagation folds:

Diagram from USGS site - - showing development of fault propagation fold. Such folds can be important in seismic risk assessment, because the fold reflects a hidden earthquake generating fault hidden at depth.

These are deformed Cretaceous strata from Ernst Tinaja in Big Bend National Park. Note how the thicker limestone layer is brittlely faulted. However, as the fault is traced down into the underlying thinner bedded strata is disappears in a complex of small folds. This is not accidental. The folding and faulting are linked, and this is an example of a tri-shear zone, where slip concentrated on one surface or zone gets distributed over an area.

Fault-bend folds:

USGS block diagram of fault-bend fold developing above a thrust ramp. Source of image:

Buckle folds:

Differential simple shear folds:

Flow or viscous folds:

Note the soft sediment folds in these tilted turbidite beds exposed in the Cingue Terra cliffs of Italy (see some of the photo captions above for more context). A careful look shows extreme changes in bedding thickness, strongly curved axial planar surfaces and fold axes (e.g. in upper left) and a general chaotic style typical of flow folds. Such soft sediment deformation is common in turbidites and accretionary wedge sedments.

Pollen on top of portion of a small lake in the Norwegian forest. The complex and changing flow pattern of the water deforms linear concentrations of the floating pollen into counter-top patterns. Patterns of gneissic banding in rocks can look much the same.

Accentuation of fold form by pure shear - e.g. by pressure solution, or as we will see with cleavage formation (next lecture).

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