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If the axial surface is planar, it is called an axial plane and can be described in terms of strike and dip. 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).
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 deformation event. For example, D 1, D 2, D 3. Folds and foliations ...
In his original use of the term, however, he did, in fact, use the up-dip direction of the fold. The main reason this creates confusion is a result of the common definition of fold-facing in geology, which is described as the direction (normal to the axis of a fold and corresponding to the axial plane) that points towards younger beds.
In geology, strike and dip is a measurement convention used to describe the plane orientation or attitude of a planar geologic feature. A feature's strike is the azimuth of an imagined horizontal line across the plane, and its dip is the angle of inclination (or depression angle ) measured downward from horizontal. [ 1 ]
The foliations are symmetrically arranged with respect to the axial plane, depending on the composition and competency of a rock. For example, when mixed sandstone and mudstone sequences are folded during very-low to low grade metamorphism, cleavage forms parallel to the fold axial plane, particularly in the clay-rich parts of the sequence. In ...
This is related to the axis of folds, which generally form an axial-planar foliation within their axial regions. Measurement of the intersection between a fold's axial plane and a surface on the fold will provide the fold plunge. If a foliation does not match the observed plunge of a fold, it is likely associated with a different deformation event.
It is an asymmetrical fold which means the axial plane is not in the upright orientation but inclined. [7] The axial plane is a hypothetical plane dividing two sides of the fold. [7] The limbs dip in opposite directions and are formed by tilted sedimentary rock layers. [6] Limbs belong to the straight sections on the two sides of a fold.
Based upon their orientation to the axial planes and axes of folds, the types of systematic joints are: Longitudinal joints – Joints which are roughly parallel to fold axes and often fan around the fold. Cross-joints – Joints which are approximately perpendicular to fold axes.