Maps, Polyphase (Refolded) Folds, Stereonets, and Complex Fabric Analysis
John Weber and Jeffrey Amato,
Grand Valley State University and New Mexico State University
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This analysis involves analyzing meso- and macro- structures shown on beautiful, rich, detailed, and well-prepared outcrop bedrock geology of a real region. Students construct a number of stereonets step-by-step, and the map-scale complex (polyphase) fabrics come to life.
We use this exercise in our required undergraduate structural geology courses. We first did the exercise as a graduate students (JW at Northwestern University, where E. Tim Whitten, the map's creator taught; JA at Stanford University).
Skills and concepts that students must have mastered
Students must come into this exercise knowing how to plot lines, planes, and poles to planes on stereonets, with some very basic knowledge about how to describe folds, mastery of simple fabrics map/stereonet problems (see http://serc.carleton.edu/NAGTWorkshops/structure/activities/15585.html), and having been introduced to polyphase folds and complex fabrics (see supporting documents).
How the activity is situated in the course
I (JW) use the exercise as a culminating lab project near the end of the semester.
Content/concepts goals for this activity
From doing the exercise, students: 1) realize that well-prepared field maps contain a wealth of data, 2) realize that small- and large-scale structures "live" together in the Earth in a very systematic way, and 3) that polyphase folds and complex fabrics can be logically and successfully broken down into small simple parts that are easily studied and understood.
Higher order thinking skills goals for this activity
From doing the exercise students: 1) gain high-level stereonet analysis skills, and 2) relate data from maps to stereonets, 3) relate small-scale to large-scale structures, 4) learn how to construct the geologic history of a polyphase, complexly deformed region, and 5) gain a practical, working knowledge of how to recognize, map, and analyze refolded folds.
Other skills goals for this activity
Description of the activity/assignment
Students use patterns observed on an outcrop geologic map and stereoplots to study the structures in this real, polyphase deformed and metamorphosed belt in the United Kingdom. There are at lest two, and perhaps three episodes of deformation recorded on this map. Students learn how to systematically analyze polyphase folds and complex mesoscopic fabrics by dividing the complexly folded map area into small simple fabrics domains. They also learn how to extract information from a map and stereonets and piece together the geologic history of a region that has experienced polyphase deformation.
Determining whether students have met the goals
I (JW) encourage students to ask a lot questions as they work through this lab, try to prevent them from making major errors and getting stuck (without giving "the answers" away), and have a pretty good idea of the quality of their work and their comprehension level by the time they turn the labs in. So, the lab is not difficult to grade (I give set points for each answer, stereonets, and geologic history write-up).More information about assessment tools and techniques.
Teaching materials and tips
- Activity Description/Assignment (Microsoft Word 31kB Sep19 08)
- Brief description of complex fabrics, how to deal with them, and how they compare to simple fabrics (Microsoft Word 22kB Sep19 08)
- Whitten Map #3 ( 9.5MB Sep19 08)