Flow, deformation, and transport - Physical Processes in Earth and Environmental Sciences - page 173

Hinge point

(a) (b)

(f )

Hinge point

Inflection
point

Inflection
point

Hinge point

Hinge point

(c) Hinge zone

(d)

limb

+

(e)

Closure

Hinge
point

Hinge
point

Inflection
point

Fig. 4.115 Definition of fold geometry in two dimensions at a given transversal section of a folded surface. (a) The hinge point is the point of
maximum curvature and the inflection point of minimum curvature. (b) Semicircular folds have constant curvatures and the hinge is defined in
the middle point of the arch and inflection points where there is a change in the bend polarity. (c) A general case where there is a hinge zone,
defined on the fold segment with a higher curvature than the reference point as shown in (d). (e) Folds with two hinge points and closure.
(f) An example of folded surfaces showing different geometric elements in 2D.

can be also related to bending of a cover of ductile rocks
over some rigid basement that is fractured or to the drag
effect of shear movements along a fault.

reference circle tangent to the inflection points at both
sides of the fold. Tracing perpendicular lines from the
inflection points will mark the center of the circle
(Figs 4.114-4.115d). The hinge point is defined in a 2D
transverse section as the location in a folded surface show-
ing the maximum curvature. In an individual section folds
can have one or several hinge points (multiple hinged
folds). The point of minimum curvature between two
adjacent hinge points of the same fold is called closure
(Fig. 4.115e). In three dimensions, joining all hinge points
along the surface defines the hinge line or hinge
(Fig. 4.116). Low curvature areas between the hinge lines
are the fold limbs or flanks . The inflection lines can be
defined in three dimensions joining all inflection points

4.16.1 Geometric description of folds

Folds can be described by their geometric characteristic,
both in two or three dimensions. The most basic geomet-
ric elements are described in a single folded surface in two
dimensions. Additional descriptions involve the 3D exten-
sion of the folded surface, and also the relation between
several superimposed folded layers. Curvature of a fold
may remain constant or can change. It can be defined by a