This is perhaps the most common line problem in geology. There are not many purely linear structures that persist over very long distances. Fold axes and boreholes are the most common. On the other hand, there are many cases of intersecting planes: beds and faults, beds and dikes or sills, layers in a fold and joints or foliations, and so on. One very common application of this technique is locating the edge of a planar structure (bed, dike, sill) truncated by a fault.
To solve this problem, we need to recall these facts:
To solve the problem, we construct structure contours on the two intersecting planes. Where two contours of the same elevation cross, we have a point on the intersection line. Two or more such points determine the line. When we plot the line by connecting all the intersections, we automatically show the map trace and elevation points on the line.
Students very often commit these two errors. WATCH OUT!
There's nothing wrong with using different intervals; it may be useful and even necessary. But then, in locating the intersections, the student fails to take account of the different interval and pairs the contours like this: 100-100, 150-200, 200-300, 250-400, and so on.
The general principle in avoiding these pitfalls is this: always be sure the intersecting contours have the same elevation.
This problems combines several simpler operations, and the individual steps for those operations are omitted. Refer to the index for information on performing other operations.
+-----------------------------+ +-----------------------------+
|1 | |2 / / / / |
| / Elev. 597 m | | / / / / /|
| /\ | | / / / / / |
| / 43 | | / / / / / |
| | | / / / / / |
| | |/ / / / / |
| | | / / / / |
| | | / / / / /|
| | | / / / / / |
| Elev. 322 m | | / / / / / |
| ----- | | / / / / / |
| |38 | |/ / / / / |
| | | / / / / |
| | | / / / / /|
| 0 100 200 M | | / / / / / |
| |----I----| SCALE | | 400 300 200 100 0 |
+-----------------------------+ +-----------------------------+
1. Given the bed and the fault 2. Plot structure contours for
shown, find their one plane
intersection
+-----------------------------+ +-----------------------------+
|3 / / / / | |4 / / / / |
| / / / / /| | / X / / / /|
| / / / / / | | / / / / / |
|--/-----/-----/-----/-500-/--| |--/-----X-----/-----/-500-/--|
| / / / / / | | / / / / / |
|/ / / / / | |/ / X / / / |
| / / / / | | / / / / |
|----/-----/-----/-----/-400-/| |----/-----X-----/-----/-400-/|
| / / / / / | | / / / / / |
| / / / / / | | / / X / / / |
| / / / / / | | / / / / / |
|/-----/-----/-----/-300-/----| |/-----/-----X-----/-300-/----|
| / / / / | | / / / / |
| / / / / /| | / / X / / /|
| / / / / / | | / / / / / |
| 400 300 200 100 0 | | 400 300 200 100 0 |
+-----------------------------+ +-----------------------------+
3. Plot the structure contours 4. Draw a line through the
of the second plane. intersections of equal
contours. Find trend and
plunge of the line.
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Created 5 January 1999, Last Update 5 January 1999
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