Fold terminology (Geological Maps and Structures) Flashcards
What is the hinge zone?
The region of the folded surface with the highest curvature
What are limbs?
Two areas of lower curvature along a folded surface, on either side of the hinge zone
What is a hinge / hinge line?
The line of maximum curvature along a single folded layer. Hinge lines may be curved or straight
What is a fold axis?
A straight hinge line
What is the hinge point?
The intersection of a hinge line with another surface (a cross section of the Earth’s surface)
What is the axial surface / plane?
The surface connecting the hinge lines of multiple folded layers
What is the axial trace?
The intersection of a fold’s axial surface with another surface. The axial trace is a line.
What is the crest line?
The line connecting the highest points on an antiform?
What is the trough line?
The line connecting the lowest points on a synform
What is a culmination/depression?
Highest/lowest points on a crest/trough line.
What is a dome/basin?
Area around a culmination on a crest line / depression on a trough line.
What is the inflection line?
Points on a single folded layer where the curvature of a limb is minimised / equal to zero
What is the inflection point?
The intersection of an inflection line with another surface (a cross section or the Earth’s surface)
What is the inflection surface?
The surface connecting the inflection lines of multiple folded layers
What is a cylindrical fold?
A fold with a straight hinge line
What is an antiform?
A fold which closes upwards
What is a synform?
A fold which closes downwards
What is a neutral fold?
A fold which closes sideways (folds without a crest/trough line)
What is an anticline?
Fold with oldest layers in the core
What is a syncline?
Fold with youngest layers in the core
What is an interlimb angle?
The angle between two fold limbs. Used to define the tightness of a fold.
What are the ways in which we describe the tightness of an interlimb angle?
Gentle
Open
Close
Tight
Isoclinal
What is an isoclinal fold?
A tight fold with roughly parallel limbs
What is roundness in terms of folds?
A measure of the curvature of a fold hinge.
What are the ways in which we describe the roundness of a fold?
Round/blunt
Sub-angular
Sharp
What is a horizontal fold?
A fold with a horizontal fold axis
What is a plunging fold?
A fold with a non-horizontal fold axis
What is an upright fold?
Fold with a vertical axial surface?
What is an upright fold?
A fold with a vertical axial surface
What is an inclined fold?
A fold with a non-vertical axial surface
What is an overturned fold?
An inclined fold in which both limbs dip in the same direction
What is a recumbent fold?
A fold in which the axial surface is horizontal
What is a symmetric fold?
Folds where the fold axial plane bisects the limbs. Limbs are equal lengths
What is an asymmetric fold?
Fold in which one limb is shorter than the other
What is the vergence?
The direction of apparent shear of the upper, longer limb of an asymmetric fold.
What is the wavelength?
Distance between two adjacent hinges
What is amplitude?
The distance between hinge point to inflection point, measured in the plane of the axial surface perpendicular to the hinge line.
What is a non-cylindrical fold?
A fold with a hinge line that is not straight
