Evolution of Normal Fault Systems During Progressive Deformation
H. Robert Burger
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This activity is based on QuickTime movies and color digital photographs derived from sandbox experiments that produce normal faults in a variety of boundary conditions following experiments developed by Ken McClay. Students view specially edited movies to gain awareness of the evolution of normal fault systems. They then investigate the formation and evolution of a fault system for a particular structural setting by tracing and labeling individual faults on a set of photographs taken at regular intervals during an experiment. This activity helps students develop an awareness of fault initiation, propagation, rotation, and inactivation during progressive deformation.
Undergraduate required course in structural geology
Skills and concepts that students must have mastered
General knowledge of general fault geometries and rock fracture.
How the activity is situated in the course
This hands-on, in-class exercise takes place about midway through my structural geology course. We already have discussed general fault geometries and rock fracture and are beginning discussion of normal, thrust, and strike-slip fault systems.
Content/concepts goals for this activity
The main goal of this activity is to free students from visualizing faulting as typically viewed in photographs or outcrops—often a single surface that establishes a somewhat misleading idea of the faulting process due to the small, static snapshot of a larger, more complex system in time and space.
Higher order thinking skills goals for this activity
A secondary goal is to encourage detailed observation of geologic features and help students arrive at hypotheses to explain the phenomena they are viewing.
Other skills goals for this activity
Finally, the students must explain what they have observed in writing in a concise and informative manner, which often is difficult for many students at this level. The students also learn how to place text and draw lines on a digital image using Adobe Illustrator
Description of the activity/assignment
Many papers by Ken McClay and colleagues have established the value of scale-model experiments in understanding the evolution and geometries of extensional fault systems (e.g. T. Dooley and K.R. McClay, 1997, Analogue modeling of pull-apart basins: American Association of Petroleum Geologists Bulletin, v. 81, no.11, p. 1804—1826).
There are few resources available that help students visualize the dynamic nature of faulting, especially the complex interplay of faults during growth and evolution of a fault system. Such an understanding is critical, however, if students are to think meaningfully about fault geometries and what they imply.
Conducting scale-model experiments in a class setting is useful, but very time-consuming, difficult for all students to see well, and very temporary, except for the end product. Accordingly, taking a cue from a movie produced by Ken McClay, I constructed a deformation apparatus, conducted and filmed several experiments conducted by McClay, and then produced QuickTime movies of the experiments. This approach makes it possible for students to observe an experiment in a minute or two that took 30-45 minutes to produce and to view the experiment repeatedly, so as to become very familiar with all that is taking place.
Individual frames from the movie provide a template on which students can identify the sequence of fault development, rotation of features, and cessation of motion on some faults as they become inactive. Requiring students to document their observations, establish a chronological sequence of events, and explain in writing what happens during the experiment results in an increased awareness of the faulting process.
Determining whether students have met the goals
Students hand in an image of the final fault system which the chronological sequence of fault development indicated by numbers. Letters may be placed on the image for reference purposes to support material in their writing assignment. Both the image and writing assignment are graded. We also discuss during a subsequent class period what students have observed and their explanations for what is taking place.
More information about assessment tools and techniques.
Download teaching materials and tips
T. Dooley and K.R. McClay, 1997, Analogue modeling of pull-apart basins: American Association of Petroleum Geologists Bulletin, v. 81, no.11, p. 1804—1826