Contractional Strain

At the end of this exercise, students will:

• Be able to describe and calculate the bulk strain within a foreland fold-and-thrust belt, if the undeformed geometry is known.
• Be able to use gesture to illustrate the deformation within a contractional tectonic setting.
• Recognize that horizontal shortening within a lithological layer can be accommodated by multiple mechanisms, including vertical thickening.

This exercise accompanies a lecture on contractional fault systems, convergent tectonics, and strain, in an undergraduate Structural Geology course. It is helpful, but not necessary, for students to know something about folds and reverse faults prior to this exercise.

If students complete the extra credit exercise at the end of this problem set, they will compare these contractional deformation experiments to extensional deformation experiments shown here: Evolution of Normal Fault Systems During Progressive Deformation. In that case, students should be able to analyze these two types of deformation for similarities and differences. This makes an excellent introduction to extensional deformation.

In this exercise, students use gesture to describe the bulk deformation and local deformation apparent in images of a contractional analog experiment. This uses embodied learning to help students conceptualize contractional deformation. It also helps break up the lecture and gives the instructor immediate visual feedback about whether students understand these basic concepts. Students then calculate bulk shortening and bulk thickening for the experiment and describe the structures accommodating that strain.

Contractional Strain exercise (Microsoft Word 2007 (.docx) 423kB May19 15)

We walk around the room, watching and talking with the students as they work through the exercise. It's a quick and easy way to see whether students understand bulk contractional deformation and if not, where they are struggling. At the end of the exercise, students turn in their calculations and answers.

Dixon, John M. and Liu, Shumin, Centrifuge modeling of the propagation of thrust faults, in Thrust Tectonics (Ken R. McClay, ed.), 1992.

Goldin-Meadow, Susan (2011). Learning Through Gesture. Wiley Interdisciplinary Reviews: Cognitive Science, v. 2, n. 6, pp. 595–607.

Using Gesture to Support Spatial Thinking highlights the value of gesture in communicating spatial information. It consists of two short exercises, and can be used in preparation for any other exercise in which students will be asked to use gesture to communicate spatial information.