Fold

What is a Fold?

A fold is formed by one or more systematically curved layers, surfaces, or lines in a rock body. A fold denotes a structure formed by the deformation of a geologic structure, such as a contact which the original undeformed geometry is presumed, to form a structure that may be described by the translation of an abstract line (the fold axis) parallel to itself along some curvilinear path (the fold profile). Folds have a hinge zone (zone of maximum curvature along the surface) and limbs (parts of the deformed surface not in the hinge zone). Folds are described by an axial surface, hinge line, profile geometry, the solid angle between the limbs, and the relationships between adjacent folded surfaces if the folded structure is a Layering fabric.

Realizations

Data model	Concept name	Definition
OGC GeoSciML	Fold	A fold is formed by one or more systematically curved layers, surfaces, or lines in a rock body. A fold denotes a structure formed by the deformation of a geologic structure, such as a contact which the original undeformed geometry is presumed, to form a structure that may be described by the translation of an abstract line (the fold axis) parallel to itself along some curvilinear path (the fold profile). Folds have a hinge zone (zone of maximum curvature along the surface) and limbs (parts of the deformed surface not in the hinge zone). Folds are described by an axial surface, hinge line, profile geometry, the solid angle between the limbs, and the relationships between adjacent folded surfaces if the folded structure is a Layering fabric.
IFC	Fold	Same as OGC GeoSciML

Properties

PropertyName	Definition
profileType	FoldProfileType contains a term from a controlled vocabulary specifying the concave/convex geometry of fold relative to earth surface, and relationship to younging direction in folded strata if known. (e.g., antiform, synform, neutral, anticline, syncline, monocline, ptygmatic).
amplitude	The amplitude property reports the length from line segment connecting inflection points on adjacent fold limbs to the intervening fold hinge.
axialSurfaceOrientation	The property axialSurfaceOrientation is used to characterize the geometry of a fold. The axial surface of a particular fold may be located based on observations of the folded geologic structure, but in general it has no direct physical manifestations. As a geologic surface, it has geometric properties, including orientation, which may be specified by observations at one or more locations, or generalized using terminology (upright, inclined, reclined, recumbent, overturned). Dip and Dip Direction are one approach to specifying the value.
geneticModel	The property geneticModel contains a term from a controlled vocabulary describing the specification of genetic model for fold, e.g. flexural slip, parallel.
hingeLineCurvature	The hingeLineCurvature property contains a term from a controlled vocabulary that describes the variation in orientation of fold hinge along trend of fold, distinguishing sheath from cylindrical folds (e.g. sheath, dome, basin, cylindrical.).
hingeLineOrientation	The property hingeLineOrientation reports the specification of the hinge line orientation for fold. GSML_LinearOrientation allows for a term value specification or a numeric specification of either or both the trend and plunge of hinge line. Hinge plunge term examples: sub-vertical, steeply plunging, sub-horizontal, reclined and vertical for special cases in which hinge plunge is close to axial surface dip. 0..* cardinality allows for both a numeric specification and a term specification.
hingeShape	The property hingeShape reports a term from a controlled vocabulary describing the hinge shape, e.g. Rounded vs. angular hinge zones. This property has to do with the proportion of the wavelength that is considered part of hinge.
interLimbAngle	The property interLimbAngle contains a term from a controlled vocabulary describing the interlimb angle using a tightness term (e.g. gentle (120-180°), open (70-120°), close (30-70°), tight (10-30°), isoclinal (0-10°)).
limbShape	The limbShape property contains a term from a controlled vocabulary describing the shape of the limb (e.g. straight vs curved limbs, kink, chevron, sinusoidal, box).
span	The span property reports a value describing the linear distance between inflection points in a single fold.
symmetry	The symmetry property contains a term from a controlled vocabulary describing the concordance or discordance of bisecting surface and axial surface, or the ratio of length of limbs. The folded surface may have asymmetry defined by limb length ratio if inflection points are defined. The definition based on bisecting surface/axial surface angle depends on having multiple surfaces defined such that the axial surface may be identified (symmetric, asymmetric).
DipDirection	The geographic direction of a line created by the intersection of a plane and the horizontal plane. If non specific convention is used, this angular value is in the range 0 to 18 degrees. In this case, there is an ambiguity on the dip orientation. For example, a plane with an orientation of 90 degrees from the nord could either have a dip direction to the North or to the South. Thus, to avoid this ambiguity, a strike value is generally completed with an indication of the dip orientation (Quadrant).

FAQ

GitHub issue

https://github.com/opengeospatial/Geotech/issues/9