AGT 157: Anticline at Calico Ghost Town EarthCache

Fold Classification

Folds are classified on the basis of several geometric factors:

Tightness of folding
The tighness of folds can be described as open (limbs dip gently), tight (limbs dip steeply) or isoclinal (limbs are parallel).

Orientation of axial plane
The orientation of the axial plane relative to the horizontal together with the orientation of fold limbs allow subdivision into upright (axial plane vertical, limbs symmetric), overturned (axial plane moderately inclined, one limb overturned), or recumbent (axial plane near horizontal, one limb inverted).

Thickness of folded beds
Thickly-bedded, brittle units tend to form concentric folds with the bed thickness preserved normal to bedding surfaces. Thinly-bedded, clay-rich units have a tendency to develop a foliation parallel to the axial plane and form similar folds with the vertical distance between top and bottom of the unit preserved through the deformation.

Types of Folds

► Anticline: linear, strata normally dip away from axial center, oldest strata in center.

► Syncline: linear, strata normally dip toward axial center, youngest strata in center.

► Antiform: linear, strata dip away from axial center, age unknown, or inverted.

► Synform: linear, strata dip toward axial centre, age unknown, or inverted.

► Dome: nonlinear, strata dip away from center in all directions, oldest strata in center.

► Basin: nonlinear, strata dip toward center in all directions, youngest strata in center.

► Monocline: linear, strata dip in one direction between horizontal layers on each side.

► Chevron: angular fold with straight limbs and small hinges

► Recumbent: linear, fold axial plane oriented at low angle resulting in overturned strata in one limb of the fold.

► Slump: typically monoclinal, result of differential compaction or dissolution during sedimentation and lithification.

► Ptygmatic: Folds are chaotic, random and disconnected. Typical of sedimentary slump folding, migmatites and decollement detachment zones.

► Parasitic: short wavelength folds formed within a larger wavelength fold structure – normally associated with differences in bed thickness

► Disharmonic: Folds in adjacent layers with different wavelengths and shapes


 

Anticline

In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core. A typical anticline is convex up in which the hinge or crest is the location where the curvature is greatest, and the limbs are the sides of the fold that dip away from the hinge. Anticlines can be recognized and differentiated from antiforms by a sequence of rock layers that become progressively older toward the center of the fold. Therefore, if age relationships between various rock strata are unknown, the term antiform should be used.

The progressing age of the rock strata towards the core and uplifted center, are the trademark indications for evidence of Anticlines on a geologic map. These formations occur because Anticlinal ridges typically develop above thrust faults during crustal deformations. The uplifted core of the fold causes compression of strata that preferentially erodes to a deeper stratigraphic level relative to the topographically lower flanks. Motion along the fault including both shortening and extension of tectonic plates, usually also deforms strata near the fault. This can result in an asymmetrical or overturned fold.


Terminology of anticlines

An Antiform can be used to describe any fold that is convex up. It is the relative ages of the rock strata that separate anticlines from antiforms. The hinge of an anticline refers to the location where the curvature is greatest, also called the crest. The hinge is also the highest point on a stratum along the top of the fold. The culmination also refers to the highest point along any geologic structure. The limbs are the sides of the fold that display less curvature.

The inflection point is the area on the limbs where the curvature changes direction. The axial surface is an imaginary plane connecting the hinge of each layer of rock stratum through the cross sectional anticline. If the axial surface is vertical and the angles on each side of the fold are equivalent, then the anticline is symmetrical. If the axial plane is tilted or offset then the anticline is asymmetrical. An anticline that is cylindrical has a well-defined axial surface, whereas non-cylindrical anticlines are too complex to have a single axial plane.


Formation processes

Anticlines are usually developed above thrust faults, so any small compression and motion within the inner crust can have large effects on the upper rock stratum. Stresses developed during mountain building or during other tectonic processes can similarly warp or bend bedding and foliation (or other planar features). The more the underlying fault is tectonically uplifted, the more the strata will be deformed and must adapt to new shapes. The shape formed will also be very dependent on the properties and cohesion of the different types of rock within each layer.

During the formation of flexural-slip folds, the different rock layers form parallel-slip folds to accommodate for buckling. A good way to visualize how the multiple layers are manipulated, is to bend a deck of cards and to imagine each card as a layer of rock stratum. The amount of slip on each side of the anticline increases from the hinge to the inflection point.

Passive-flow folds form when the rock is so soft that it behaves like weak plastic and slowly flows. In this process different parts of the rock body move at different rates causing shear stress to gradually shift from layer to layer.iii There is no mechanical contrast between layers in this type of fold. Passive-flow folds are extremely dependent on the rock stratums makeup and can typically occur in areas with high temperatures.