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# Structural Geology: Parts of Folds

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Structural Geology: Folds

PARTS OF A FOLD

Limbs

Ø    These are the sides or flanks of a fold. An individual fold will have a minimum of two limbs but when the folds occur in groups, as they very often do, a middle limb will be common to two adjacent folds.

Hinge

Ø     In a folded layer, a p oint can be found where curvature is maximum and one limb ends and the other li mb starts from that point. This is the hinge poin t.

Ø     When rocks occur in a sequence and their all hinge points are joined together, they make a line, called the hinge line.

Axial surface

Ø     When the hinge line is traced throughout the depth of a folded sequence a surface is obtained which may be planar or non-planar. It is referred to as axial surface

Axial plane

Ø     Axial plane is the im aginary plane that passes through all the points of maximum curvature inclined or horizontal in nature.

Ø     A fold surface is plan ar in nature; otherwise it in a folded sequence.

Ø     It may be vertical, is sometimes called a planar fold if the axial is a n on-planar fold.

Axis of a fold

Ø  It is simply defined a s a line drawn parallel to the hinge line of a fold .

Ø    A more precise defin ition of an axis of a fold would be the line repr esenting the intersection of the ax ial plane of a fold with any bed of the fold.

Plunge of a fold

Ø    The angle of inclination of the fold axis with the horizontal as meas ured in a vertical plane is term ed the plunge of the fold.

Crest and Trough.

Ø Most folds are variations of two general forms; uparched and downarched bends. The line running thr ough the highest points in an uparched fold defi nes its crest.

Ø A corresponding lin e running through the lowest point in a down arched fold makes its trough. The crest and trough may or may not coincide wit h the axis of the fold.

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