Geology Reference
In-Depth Information
Fig. 6.43.
Projection along plunge onto
a vertical cross section.
Num-
bers
on the map (
square box
)
are the topographic elevations
of the points to be projected
The ratio of the vertical projection length AB to the cross-section projection length
CB is constant for all points, CB : CA = CF : CE = sin
. Projection by hand is very rapid
if a proportional divider drafting tool is used. Set the divider to the ratio CB / AB; then
as the projection length AC is set, the divider gives the required length CB.
The method can be modified for other cross-section orientations by changing the
orientation of the line of section on the plunge projection (Fig. 6.43). The orientation
of the plane of section on the plunge projection is CB. In Fig. 6.43, CB is at 90° to AC,
making the section plane vertical. The ratio CB : CA is constant for all points. Follow
steps 1 to 5 above, changing the orientation of the line CB.
The plunge of a fold typically changes along the axis. Cylindrical fold axes may be
curved along the plunge and cylindrical folds will change into conical folds at their
terminations. Projection along straight plunge lines should be done only within do-
mains for which the geometry of the structure is constant. Variable plunge can be
recognized from undulations of the crest line on a structure-contour map or as exces-
sive dispersion of the bedding dips around the best-fit curves on a stereogram or tan-
gent diagram. If the plunge is variable, then the geographic size of the region being
utilized should be reduced until the plunge is constant and all the bedding points fit
the appropriate line on the stereogram or tangent diagram. If the sequence of plunge
angle changes along the fold axis direction can be determined, then the straight line AB
(Figs. 6.42, 6.43) could be replaced by a curved plunge line.
φ
6.6.2
Projection by Structure Contouring
Structure contours represent the position of a marker horizon or a fault between the control
points. Contouring provides a very general method for projecting data and can be used
where the plunge cannot be defined from attitude measurements. This is a convenient
method in three-dimensional interpretation. The projection technique is to map the marker
surfaces between control points and draw cross illustrative sections through the maps.
