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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.
A plunging anticline has a hinge that is not parallel to the earth's surface. All anticlines and synclines have some degree of plunge. Periclinal folds are a type of anticlines that have a well-defined, but curved hinge line and are doubly plunging and thus elongate domes. [5] Model of anticline. Oldest beds are in the center and youngest on ...
Fold axis plunge is measured in dip and dip direction (strictly, plunge and azimuth of plunge). The orientation of a fold axial plane is measured in strike and dip or dip and dip direction. Lineations are measured in terms of dip and dip direction, if possible. Often lineations occur expressed on a planar surface and can be difficult to measure ...
A horizontal line would have a plunge of 0°, and a vertical line would have a plunge of 90°. [ 3 ] [ 7 ] A linear feature which lies within a plane can also be measured by its rake (or pitch). Unlike plunge, which is the feature's azimuth, the rake is the angle measured within the plane from the strike line.
The vergence of a fold lies parallel to the surrounding surfaces of a fold, so if these surrounding surfaces are not horizontal, the vergence of the fold will be inclined. For a fold, the direction and the extent to which vergence occurs can be calculated from the strike and dip of the axial surfaces, along with that of the enveloping surfaces ...
This photograph illustrates a pronounced and prominent stretching lineation plunging steeply to the north, as a rake upon the main shear foliation parallel with the protractor. Stretching lineations may form in any faulting regime when conditions are such that rocks deform ductiley, including extensional, compressional, transpressional, and ...
By differential compaction over an underlying structure, particularly a large fault at the edge of a basin due to the greater compactibility of the basin fill, the amplitude of the fold will die out gradually upwards. [1] By mild reactivation of an earlier extensional fault during a phase of inversion causing folding in the overlying sequence. [2]
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).