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Folds can have a fold axis. A fold axis "is the closest approximation to a straight line that when moved parallel to itself, generates the form of the fold". [2] (Ramsay 1967). A fold that can be generated by a fold axis is called a cylindrical fold. This term has been broadened to include near-cylindrical folds.
Topographic map showing detachment folds in the eastern Sichuan Basin, China.. A detachment fold, in geology, occurs as layer parallel thrusting along a decollement (or detachment) develops without upward propagation of a fault; the accommodation of the strain produced by continued displacement along the underlying thrust results in the folding of the overlying rock units.
If there are folds, these are numbered as F 1, F 2, etc. Generally the axial plane foliation or cleavage of a fold is created during folding, and the number convention should match. For example, an F 2 fold should have an S 2 axial foliation. Deformations are numbered according to their order of formation with the letter D denoting a ...
If the angle between the limbs is large (70–120 degrees), then the fold is an "open" fold, but if the angle between the limbs is small (30 degrees or less), then the fold is a "tight" fold. [4] If an anticline plunges (i.e., the anticline crest is inclined to the Earth's surface), it will form V s on a geologic map view that point in the ...
A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimposed smaller folds. [1] Synclines are typically a downward fold ( synform ), termed a synformal syncline (i.e. a trough), but synclines that point upwards can be found when strata have been overturned and folded (an antiformal syncline).
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An anticline is a fold that is concave down, whose limbs are dipping away from its axis, and whose oldest units are in the middle of the fold. [2] These features form in a number of structural settings. In the case of river anticlines, they form due to high erosion rates, usually in orogenic settings.