Geology Reference
In-Depth Information
The symmetry of a fold is determined by the angle between the plane bisecting the
interlimb angle and the median surface (Fig. 1.17a; Ramsay 1967). The angle is close
to 90° in a symmetrical fold (Fig. 1.17a,c) and noticeably different from 90° in an asym-
metrical fold (Fig. 1.17b,d). An essential property of an asymmetrical fold is that the
limbs are unequal in length. Fold asymmetry is not related to the relative dips of the
limbs. The folds in Fig. 1.17b,c have overturned steep limbs and right-way-up gentle
limbs, but only the folds in Fig. 1.17b are asymmetric. This is a point of possible con-
fusion, because in casual usage a fold with unequal limb dips (Fig. 1.17b,c) may be
referred to as being asymmetrical. Folds may occur as regular periodic waveforms as
shown (Fig. 1.17) or may be non-periodic with wavelengths that change along the
median surface.
The vergence of an asymmetrical fold is the rotation direction that would rotate the
axial surface of an antiform from an original position perpendicular to the median
surface to its observed position at a lower angle to the median surface. The vergence
of the folds in Fig. 1.17b,d is to the right.
1.5.2
Three-Dimensional Geometry
A cylindrical fold is defined by the locus of points generated by a straight line,
called the fold axis, that is moved parallel to itself in space (Fig. 1.18a). In other
words, a cylindrical fold has the shape of a portion of a cylinder. In a cylindrical
fold every straight line on the folded surface is parallel to the axis. The geometry
of a cylindrical fold persists unchanged along the axis as long as the axis re-
mains straight. A conical fold is generated by a straight line rotated through a fixed
point called the vertex (Fig. 1.18b). The cone axis is not parallel to any line on the
cone itself. A conical fold changes geometry and terminates along the trend of the
cone axis.
The crest line is the trace of the line which joins the highest points on successive
cross sections through a folded surface (Figs. 1.18, 1.19a; Dennis 1967). A trough line
is the trace of the lowest elevation on cross sections through a horizon. The plunge of
a cylindrical fold is parallel to the orientation of its axis or a hinge line (Fig. 1.19b). The
most useful measure of the plunge of a conical fold is the orientation of its crest line
or trough line (Bengtson 1980).
Fig. 1.18.
Three-dimensional fold types.
a
Cylindrical. All
straight lines
on the cylinder surface are par-
allel to the fold axis and to the crestal line.
b
Conical.
V
vertex of the cone.
Straight lines
on the cone
surface are not parallel to the cone axis
