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==Basic concepts==
Folds are bends or flexures of layered rock that form in response to motion along [[fault]]s, [[diapirism]], [[compaction]], and regional subsidence or [[uplift]]. Folds are expressed in [[seismic reflection profile]]s as one or more regions of dipping reflections ([[dip]] domains) that correspond to inclined [[stratigraphic]] contacts.
Folds are bends or flexures of layered rock that form in response to motion along [[fault]]s, [[diapirism]], [[compaction]], and regional subsidence or [[uplift]]. Folds are expressed in [[seismic reflection profile]]s as one or more regions of dipping reflections ([[dip]] domains) that correspond to inclined [[stratigraphic]] contacts.
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File:ST53_Part01_Pg03A.jpg
File:ST53_Part01_Pg03B.jpg
File:ST53_Part01_Pg04.jpg|Folds in seismic sections.
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Dip domains are separated by [[axial]] surfaces; imaginary planes which, when viewed in two dimensions, form axial traces. [[Anticlinal]] axial surfaces occupy concave-downward [[fold hinge]]s; [[synclinal]] axial surfaces occupy concave-upward fold hinges.
Dip domains are separated by [[axial]] surfaces; imaginary planes which, when viewed in two dimensions, form axial traces. [[Anticlinal]] axial surfaces occupy concave-downward [[fold hinge]]s; [[synclinal]] axial surfaces occupy concave-upward fold hinges.
==Folds and bedding thickness==
Folds are classified based on whether or not the thickness of stratigraphic layers changes in dip domains or across axial surfaces.
Parallel folds preserve layer thickness, and are common in strata that deformed predominantly by flexural slip (see inset at right). Axial surfaces bisect inter-limb angles in parallel folds.
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File:ST53_Part01_Pg05A.jpg
File:ST53_Part01_Pg05B.jpg
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Parallel folds commonly form by a deformation mechanism called flexural slip, where folding is accommodated by motions on minor faults that occur along some mechanical layering — usually bedding. Flexural-slip surfaces, which can be observed in core or outcrop, may vary in spacing from a few millimeters to several tens of meters in spacing.