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{{publication
{{publication
| image =
| image = ST53Cover.jpg
| width = 120px
| width = 120px
| series = Studies in Geology
| series = Studies in Geology
| isbn = 0891810609
| isbn = 0891810609
}}
}}
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.
<gallery mode=packed heights=200px widths=200px>
File:ST53_Part01_Pg03A.jpg|{{figure number|1}}
File:ST53_Part01_Pg03B.jpg|{{figure number|2}}
File:ST53_Part01_Pg04.jpg|{{figure number|3}}Folds in seismic sections.
</gallery>
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 on [[:file:ST53_Part01_Pg05A.jpg|Figure 4]]). Axial surfaces bisect inter-limb angles in parallel folds.
<gallery mode=packed heights=200px widths=200px>
File:ST53_Part01_Pg05A.jpg|{{figure number|4}}
File:ST53_Part01_Pg05B.jpg|{{figure number|5}}
</gallery>
Parallel folds commonly form by a [[deformation]] mechanism called flexural slip, where folding is accommodated by motions on minor [[fault]]s 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.
==Shortcomings in seismic images of folds==
Folds can be distorted or only partially imaged in seismic sections.
<gallery mode=packed heights=200px widths=200px>
File:ST53_Part01_Pg06A.jpg|{{figure number|6}}
</gallery>
==Locating axial surfaces in seismic sections==
Migration moves dipping reflections upward and laterally to properly image the fold geometry, but reflections on non-migrated or under-migrated sections do not accurately represent fold shape. However, axial surfaces can be inferred on these sections by mapping the truncations of horizontal reflections.
<gallery mode=packed heights=200px widths=200px>
File:ST53_Part01_Pg06B.jpg|{{figure number|7}}
</gallery>
==Interpreting folds in poorly imaged zones==
Poorly imaged zones on folds are commonly caused by, and interpreted as, faults or steep limbs. Both solutions are often permissible and should be evaluated. In [[:file:ST53_Part01_Pg07.jpg|Figure 8]], a method of interpreting parallel folds in poorly imaged zones is described.
<gallery mode=packed heights=200px widths=200px>
File:ST53_Part01_Pg07.jpg|{{figure number|8}}
</gallery>