Fold Geometry - 3-D Structural Geology - page 109

Fig. 5.3. Cylindrical folds showing trend and plunge of the crest line. a Non-plunging. b Plunging.
c Tangent diagram of bed attitudes in a non-plunging fold. d Tangent diagram of bed attitudes in a
plunging fold. (After Bengtson 1980)
the fold is cylindrical. The straight line through the dip vectors goes through the origin
for a non-plunging fold (Fig. 5.3a,c) and is a straight line offset from the origin for a
plunging fold (Fig. 5.3b,d). A vector from the origin to the line of dip vector points,
perpendicular to the dip-vector line, that gives the bearing and plunge of the fold axis.
5.2.2
Conical Folds
A conical fold is defined by the movement of a generatrix line that is fixed at the apex of
a cone (Fig. 5.4a); the fold shape is a portion of a cone. A conical fold terminates along its
trend. On a stereogram the bedding poles fall on a small circle, the center of which is the
cone axis and the radius of which is 90° minus the semi-apical angle (Fig. 5.4b). It is usu-
ally difficult to differentiate between cylindrical and slightly conical folds on a stereo-
gram (Cruden and Charlesworth 1972; Stockmal and Spang 1982), yet this is an impor-
tant distinction because a conical fold terminates along trend whereas a cylindrical fold
does not. The tangent diagram is particularly good for making this distinction.