GEOS 304, Lab 7: Laramide Principal Stress/Strain Directions

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 through all of the slickenlines (both the right- and left-handed groups) and bisecting the angle between the right- and left-handed slickenline groups. 4) 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. 5) 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. 6) 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. 7) 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. Finding Stress/Strain Axes

Left-handed slickenlines

Sigma 1/ S3: Lies on great circle, halfway between right-handed and left-handed clusters

Sigm a 2/ S2: Pole to great circle Right-handed slikcenlines

Sigma 3/ S3: Lies on great circle 90 degrees from sigma 1

Best-fit great circle through all slickenlines Figure 1