Geology Reference
In-Depth Information
Fig. 5.4.
Conical fold geometry.
a
Shape of a conically folded bed (after Stockmal and Spang 1982).
b
Lower-hemisphere stereogram projection of a conical fold (after Becker 1995)
On a tangent diagram, the curve through the dip vectors is a hyperbola (Bengtson
1980), concave toward the vertex of the cone. Type I plunge is defined by a hyperbola
concave toward the origin. The fold spreads out and flattens down plunge (Fig. 5.5a,c).
Type II plunge is defined by a hyperbola convex toward the origin on the tangent dia-
gram (Fig. 5.5b,d) and the fold comes to a point down plunge. In the strict sense, a bed
in a conical fold does not have an axis and therefore does not have a plunge like a
cylindrical fold. The orientation of the crest line in an anticline (Fig. 5.5) or trough line
in a syncline provides the line that best describes the orientation of a conically folded
bed, in a manner analogous to the axis of a cylindrical fold. The plunge of a conical
fold as defined on a tangent diagram is the plunge of the crest or trough line, not the
plunge of the cone axis. If the crest line is horizontal, the fold is non-plunging but
nevertheless terminates along the crest or trough. In general, each horizon in a fold
will have its vertex in the same direction but at a different elevation from the vertices
of the other horizons in the same fold.
The trend and plunge of the crest or trough of a conical fold is given by the magnitude
the vector from the origin in the direction normal to the curve through the bedding dips
(Figs. 5.6, 5.7). For points on the fold that are not on the crest or trough, the plunge direc-
tion is different. To find the plunge line, draw a line tangent to the curve at the dip rep-
resenting the point to be projected. A line drawn from the origin, perpendicular to the
tangent line gives the plunge amount and direction (Bengtson 1980). In a type I fold
(Fig. 5.6), the minimum plunge angle is that of the crest line and all other plunge lines
have greater plunge angles. In a type II fold (Fig. 5.7) the plunge line at some limb dip has
a plunge of zero. The trend of this plunge line is normal to the tangent line (Fig. 5.7), the
same as for all the other plunge lines. At greater limb dips in a type II fold, the down-
plunge direction is away from the vertex of the cone (Fig. 5.7).
Because the tangent of 90° is infinity, a practical consideration in using a tangent
diagram is that dips over 80° require an unreasonably large piece of paper. The non-
linearity of the scale also exaggerates the dispersion at steep dips. A practical solution
for folds defined mainly by dips under 80° is to plot dips over 80° on the 80° ring
(Bengtson 1981b). This will have no effect on the determination of the axis and plunge
of the folds (Bengtson 1981a,b) and will have the desirable effect of reducing the dis-
