Geology Reference
In-Depth Information
spacing of structure contours along to the trend of the crest or trough gives the plunge
of the crest or trough line:
φ
=arctan(
I
/
H
) ,
(5.4)
where
= plunge,
I
= contour interval, and
H
= map distance between the structure
contours along the plunge.
φ
5.3
Dip Domain Fold Geometry
A dip domain is a region of relatively uniform dip, separated from other domains by
hinge lines or faults (Groshong and Usdansky 1988). Dip-domain style folding (Fig. 5.10)
is rather common, especially in multilayer folds. A hinge line is a line of locally sharp
curvature. On a structure contour map, hinge lines are lines of rapid changes in the
strike of the contours. In a fixed-hinge model of fold development, the hinges repre-
sent the axes about which the folded layers have rotated.
The orientations of the hinge lines can be found using a stereogram by finding the
intersection between the great circles that represent the orientations of the two adja-
cent dip domains (Fig. 5.11). If the intersection line is horizontal, as for hinge lines 1
and 2 in Fig. 5.10, the hinge line is not plunging. Hinge line 3 between the two dipping
domains plunges 11° to the south (Fig. 5.11). The fold geometry between any two ad-
jacent dip domains is cylindrical with the trend and plunge being equal to that of the
hinge line. If the bed attitudes from multiple domains intersect at the same point on
the stereogram, the fold is cylindrical, and the line is the fold axis, called the
β
-axis
when determined this way (Ramsay 1967).
The tangent diagram (Fig. 5.12) shows the dip-domain fold from Fig. 5.10 to be coni-
cal with zero plunge. The concave-to-the-south curvature of the line through the dip
vectors means that the fold terminates to the south, as observed (Fig. 5.10). If only the
plunging nose of the fold is considered (Fig. 5.12), the hinge line is perpendicular to
Fig. 5.10.
Dip-domain fold (modeled after Faill 1973a). Hinge lines separate domains of constant dip.
a
3-D oblique view to NW.
b
Bed attitudes and hinge lines (numbered), plan view
