Geology Reference
In-Depth Information
Fig. 5.20.
The intersection line between
axial surfaces. Axial surface 1-2
bisects bed 1 (30,250) and bed 2
(30, 110). Axial surface 2-H
bisects bed 2 and bed H (0, 000).
The line of intersection of axial
surfaces (50, 000) is the
α
line.
Lower-hemisphere, equal-area
stereogram
north-south. In the general case, the axial trace and fold axis are plotted as points on
the stereogram and the great circle that passes through both points is the axial surface.
If the axial trace and the hinge line coincide, as is the case for hinges 1 and 2 in Fig. 5.10,
this procedure is not applicable. It is then necessary to have additional information,
either the trace of the axial surface on another horizon or the relative bed thickness
change across the axial surface.
The intersection of two axial surfaces (Fig. 5.19) is an important line for defining
the complete dip-domain geometry, and is here designated as the
α
line (
α
for axial
surface). An
line has a direction and, unlike a fold axis, has a specific position in
space. Angular folds may have multiple
α
line is
coincident with a hinge line (Fig. 5.14a), but in a plunging fold (Fig. 5.19) the hinge
lines and the
α
lines. In a non-plunging fold an
α
lines are neither parallel nor coincident.
The orientation of the
α
line is found by first finding the axial surfaces, and then
their line of intersection. The stereogram solution is to draw the great circles for both
axial surfaces and locate the point where they cross (Fig. 5.20), which is the orienta-
tion of the line of intersection. The analytical solution for the
α
line is that for the
line of intersection between two planes (Eqs. 5.5-5.8), in which the two planes are
axial surfaces.
α
5.4.3
Location in 3-D
For a 3-D interpretation the axial surfaces and the
lines must be located in space as
well as in orientation. If hinge lines are known from more than one horizon they can
be combined to create a map (Fig. 5.21). The axial surface is the structure contour
map of equal elevations on the hinge lines.
α
