Geology Reference
In-Depth Information
Fig. 5.29.
Expansion index for a fold. Units
1
-
4
are growth units and units
5
-
6
are pre-growth units.
a
Cross section in the dip direction.
b
Expansion index diagram
and Groshong (1993). Using the same equation for both folds and faults facilitates the
comparison of growth histories of both types of structures where they occur together.
The magnitude of the expansion index is plotted against the stratigraphic unit to
give the expansion index diagram (Fig. 5.29b). The diagram illustrates the growth
history of the fold. An expansion index of 1 means no growth and an index greater
than 1 indicates upward growth of the anticlinal crest during deposition. The fold in
Fig. 5.29a is a compressional detachment fold (Fig. 11.37) in which tectonic thickening
in the pre-growth interval causes the expansion index to be less than 1. The growth
intervals show an irregular upward increase in the growth rate. An expansion index
diagram is particularly helpful in revealing subtle variations in the growth history of
a fold and in comparing the growth histories of different structures. The expansion
index is most appropriate for the sequences that are completely depositional. If ero-
sion has occurred across the crest of the fold the expansion index will be misleading.
5.8
Exercises
5.8.1
Geometry of the Sequatchie Anticline
Plot the attitude data from Table 5.1 on a stereogram and on a tangent diagram to find
the fold style and plunge. How do the two methods compare?
5.8.2
Geometry of the Greasy Cove Anticline
The bedding attitudes below (Table 5.2) come from the Greasy Cove anticline, a com-
pressional structure in the southern Appalachian fold-thrust belt. What is the
π
axis
of the fold? Use a tangent diagram to find the axis and the style of the fold.
