Geology Reference
In-Depth Information
(Stockwell 1950; Stauffer 1973). For the purpose of describing the fold geometry, the
virtual axial surface in a rounded fold is defined here as the surface which contains
the virtual hinge lines. In relatively open folds, virtual hinge lines can be constructed
as the intersection lines of planes extrapolated from the adjacent fold limbs (Fig. 5.15a).
In a tight fold the extrapolated hinges are too far from the layers to be of practical use.
In a tight fold (Fig. 5.15b) the virtual hinges can be defined as the centers of the circles
(or ellipses) that provide the best fit to the hinge shapes, after the method of Stauffer
(1973). Defined in this fashion, the axial surface divides the hinge into two limbs, but
does not necessarily divide the fold symmetrically nor is it necessarily the surface
where the sense of shear reverses. The axial surface, whether defined by actual or
virtual fold hinges, does not necessarily coincide with the crest surface (Fig. 5.16) or
the trough surface.
Fig. 5.15.
Virtual hinges and virtual axial surfaces in cross sections of round-hinge folds.
a
Virtual hinges
in an open fold found by extrapolating fold limbs to their intersection points.
b
Virtual hinges in a tight
fold found as centers of circles tangent to the surfaces in the hinge. (After Stauffer 1973)
Fig. 5.16.
A fold in which the crest sur-
face does not coincide with
the axial surface
