Geology Reference
In-Depth Information
sected. The bisector is found by plotting both beds (for example, 2 and
H
in Fig. 5.18),
finding their line of intersection (
I
), and the great circle perpendicular to the
intersection (
dotted line
). The obtuse angle between the beds is bisected by the double
arrows. The great circle plane of the axial surface (shaded) goes through the bisection
point and the line of intersection of the two beds. If bed thickness is not constant across
the hinge, the partial interlimb angle is found from Eq. 5.12 and the appropriate partial
angle is marked off along the great circle perpendicular to the line of intersection of
the bedding dips.
Another method to determine the axial surface orientation is to find the plane through
two lines that lie on the axial surface, for example, the trace of a hinge line on a map or
cross section, and either the fold axis or the crest line (Rowland and Duebendorfer
1994). In the fold in Fig. 5.10, for example, hinge 3 plunges south and the surface trace
of the hinge is north-south and horizontal and so the axial surface is vertical and strikes
Fig. 5.18.
Determination of the axial
surface orientation using a
stereogram. Angle between
two constant thickness do-
mains, one horizontal (bed H)
and one 30, 110 (bed 2). The
axial surface (
shaded great
circle
) bisects the angle (
double
arrows
) between bed 2 and the
horizontal bed. The axial sur-
face attitude is 75, 290. Lower-
hemisphere, equal-area pro-
jection
Fig. 5.19.
Plunging dip-domain fold.
The axial surface intersection
lines (
α
) plunge in a direction
opposite to that of the hinge
line
