Geology Reference
In-Depth Information
Fig. 5.21.
Structure contour map of an
axial surface. Dashed lines are
hinge lines on three different
horizons, A , B , and C . Solid
lines are contours on the axial
surface. Contours are in meters
Fig. 5.22.
3-D axial surface geometry for
the fold in Fig. 5.19. The axial
surfaces bounding the left
limb and the domain-inter-
section surface (bounded by
α lines) are shaded , those
bounding the right limb are
indicated by their structure
contours. The heavy lines rep-
resent a vertical slice through
the structure: ast: axial surface
trace; bt: bedding trace
Axial traces can be determined from cross sections, and then mapped to form
a surface (Fig. 5.22). The cross sections may cross the structure in any direction
and need not be normal to the axial surfaces. In 3-D software, the axial traces can
be extracted from successive sections and these lines triangulated to form the
axial surface. Having multiple beds on the cross sections makes choosing the trace
straightforward. If only one horizon has been mapped, axial traces can still be
inferred but then the section must be normal to the hinge line and the effect of
thickness changes must be considered.
5.5
Using the Trend in Mapping
It may be necessary to know the fold trend before a reasonable map of the surface can
be constructed. Elevations of the upper contact of a sandstone bed are shown in
Fig. 5.23a. Preliminary structure contours, found by connecting points of equal eleva-
tion, suggest a northwest-southeast trend to the strike. There is nothing in the data in
Fig. 5.23a to definitively argue against this interpretation. Given the knowledge that
the fold trends north-south with zero plunge, the map of Fig. 5.23b can be constructed.
It can now be seen that the original map connected points on opposite sides of the
anticline. The correct structure of the map area could not be determined without
knowing the fold trend.
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