GEOS 304 Lab 8: Laramide Principal Stress/Strain Directions as inferred
from structural analysis of Sus Canyon
Fall 2002
Name:________________
Overview:
The Basin and Range and mid-Miocene extensional events that so prominently carve up southern
Arizona overprint an older episode of compressional deformation. That episode, the so-called Laramide
Orogeny began with a major shift in plate motions at around 80 Ma. It ended with another plate
reorganization at about 40 Ma, perhaps related to the initiation of the India-Asia collision. In southern
Arizona, the Laramide is the primary contractional deformation. Further to the north, the Laramide built
on the deformation of the earlier Sevier Orogeny. The Colorado Rockies and the broad uplifts of eastern
Utah are products of this era. From a subduction zone off the West Coast, Laramide deformation
extended halfway to St. Louis.
In the Tuscon Mountains, the Amole Granite was emplaced in the Late Cretaceous, near the
beginning of Laramide contraction. Because no other compression has affected southern Arizona since
emplacement, it is reasonable to believe that any compressional structures in the granite are products of
the Laramide. By carefully mapping a series of small strike-slip faults in the granite, we will attempt to
interpret the principal strain directions, and by inversion, the principal stress directions.
Area of Study: Suss Canyon, Saguaro National Park, Tucson Mountains
Rock Unit: Late Cretaceous Amole Granite
Purposes:
1) To interpret principal stress and strain directions for the Laramide Orogeny in southern Arizona.
2) To practice measuring linear features using the “rake” technique.
3) To use a stereonet to interpret strain directions from field measurements.
4) To leave with enough time to stop at Dairy Queen on the way home.
What to Do:
The whole lab will take place in the sections of the wash that appear on your map. Work individually,
comparing notes with others only after you have decided on your location and measured your own strike
and dip. Remember that neatness counts (for 10 points, to be exact). Your map should be clear and easily
legible.
In the Field (50 pts)
1) Find, measure, and plot on your map as many small faults as possible. In practice this will probably
mean 10-15 but more is better. Be careful to record strike, dip, rake, sense and amount of offset (look
for offset markers such as small dikes).
In the Office (50 pts)
2) Plot your data on a stereonet, being careful to distinguish between left and right-handed
faults/slickenlines (use a different color or symbol). All that you really want are the slickenline
orientations, so you don’t have to plot the faults.
3) If we have true conjugate faults, the bisector of the acute angle between the average orientations of
left- and right-handed slickenlines will be S3. Find it on a stereonet by drawing a best-fit great circle
4)
5)
6)
7)
through all of the slickenlines (both the right- and left-handed groups) and bisecting the angle between
the right- and left-handed slickenline groups.
Assuming that we are dealing with plane strain (i.e. 2-D strain rather than 3-D), the best-fit great
circle that you drew above is the S1-S3 plane. Find S3 by measuring 90 degrees along that circle from
S1.
The final strain direction, S2 is perpendicular to the first two. Find it by plotting the pole to the circle
containing the other two principal strain directions.
Assuming that strain is coaxial (i.e. pure shear) we can invert the strain directions to get principal
stresses (1=S3, 2=S2, and 3=S1). Label the principal stress directions on your stereonet. For help
in steps 2-6, examine Figure 1 below.
We have made two fundamental assumptions in our analysis. First, we assumed that we are dealing
with only a two-dimensional strain field and second, we assumed that deformation was strictly pure
shear. Were these valid assumptions? Explain your answer in a half-page essay.
***Hints*** Regarding the first assumption, check out Davis and Reynolds p. 314-315. Compare
the diagrams to what you saw in the field and on your stereonets. As for the second assumption,
consider whether there was a dominant set of faults (either the right-hand or the left-hand group) that
would suggest overall simple shear. Refer to the data you collected and see if either set had greater
numbers of faults or, ideally, greater total offset.
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