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Structural Geology: Parts of Folds

Structural Geology: Parts of Folds
Civil - Engineering Geology - Structural Geology And Geophysical Method - Structural Geology: Folds

Structural Geology: Folds

 

PARTS OF A FOLD

 

Limbs

 

    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

 

     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.

 

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

 

Axial surface

 

     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

 

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

 

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

 

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

 

Axis of a fold

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

 

    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

 

    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.

 

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.

 

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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