Problem Set 2: Interpretation of Faults 40

Structural Geology- Spring 2010 Geol 345

Problem Set 2: Interpretation of Faults [40]

Due: Thursday, February 25th

Consider the photo below showing a cross section view of faults in the Triassic Moenkopi Formation at the entrance to Arches National Park, Utah. There are a number of stratigraphic offset markers that allow the faults to be identified in this outcrop. The outcrop is 10 m high.

Answer the following questions related to this outcrop of faults:

1. Using the larger grayscale version of this photo (attached), use tracing paper or a graphics software package to create a detailed fault map of this outcrop. Show major bedding planes on your map and use arrows to show the sense of relative motion across each fault. (10)

2. Is the faulting in this outcrop due to brittle or ductile fault zone processes? Give your reasoning. (2)

3. Use a circle to identify at least one location on the cross section where the fault zone architecture can be described as showing clay smear and draw a sketch enlargement to illustrate the nature of the fault where it exhibits this architecture. (4)

4. Assuming that the cross section here is exactly perpendicular to the strike of the faults, what is the dip of the RIGHT fault in the closely spaced fault pair at the center of the image? (2)

5. On your cross section, use double-headed arrows to precisely label at least one example of each of the following quantities (use a different fault in each case if you wish):

(i) apparent slip (or offset) (2) (ii) stratigraphic separation (2) (iii) throw (2) (iv) heave (2) Structural Geology- Spring 2010 Geol 345

6. Use a vertical line to show at least one location where a drill hole cored into the rock would experience the problem of missing section. (2)

7. An examination of the fault pair from question 4 indicates that the slickenlines have a rake of 45° (measured away from the strike line in a clockwise sense, in the plane of the fault). Given this information, what type of faulting has occurred here? Include a sketch to illustrate your reasoning. (4)

8. If the orientation of the cross section is 060° and the azimuth (or trend) of the slickenlines is 000° (i.e., due north) on the LEFT fault in question 4, what is the total slip along this fault?

Hint: to calculate this, you will first need to determine the apparent dip of the fault in the direction of the slickenlines (using the trigonometric method in Ch. 1 of the lab manual, equation 1.3), as well as the fault throw. Assume that the outcrop is 10 m high in the grayscale image and that the true dip is the same as your answer to question 4. Make sure you carefully determine which of your measurements refer to a, b, and d in the equation. (8) Structural Geology- Spring 2010 Geol 345