Geology Reference
In-Depth Information
Fig. 2.17.
Tangent diagram. Arrow rep-
resents a plane dipping 40°
to the west (40, 270). (After
Bengtson 1980)
tion of the crest lines of cylindrical and conical folds (Chap. 5). The concentric circles
on the diagram represent the dip magnitude and their spacing is proportional to the
tangent of the dip, hence the name tangent diagram. The center of the diagram is
zero dip. The azimuth is marked around the margin of the outer circle. The attitude
of a plane is represented by a vector from the origin in the direction of the azimuth
and having a length equal to the dip amount (Fig. 2.17). The attitude can be shown
with the complete vector or as a point plotted at the location of the tip of the vector.
See Sect. 2.8 for how to plot dip vector points on a tangent diagram using a spread-
sheet. A major convenience of the tangent diagram is that no overlay is required and
that certain problems are solved very quickly and without the rotations that are re-
quired with the stereogram. The drawback of the tangent diagram is that very steep
dips require a very large diagram. The diagram in Fig. 2.17 extends to a dip of 65°. A
calibrated scale that can be used to plot dips from 65° to 80° is given at the bottom of
this figure. To use it, plot the vector along the appropriate radius and use the auxil-
iary scale to find the added length of the vector beyond the outer circle of the dia-
gram. The diagram is not practical if a significant percentage of the dips are over 70°,
for which a stereogram is more suitable. The tangent diagram can be used as a circu-
lar histogram, even for steep dips, by plotting the steep dips with their correct azi-
muths along the outer circle.
A tangent diagram is a convenient tool for finding the true or apparent dip. The
apparent dip in a given direction (Fig. 2.18a) is the vector in the appropriate direction.
Search WWH ::




Custom Search