Thin-Skinned Models for Undergraduate Teaching Labs – "Flour Structures
Summary
The lab described here a simple, inexpensive way for an undergraduate class to create model thin-skinned wedges A simple model using flour and marker chalk, pushed across the base of a box using a piece of wood, can be used to demonstrate many aspects of the growth of thin-skinned mountain belts. Depending upon the strength of the décollement, the cross-sectional taper of the deforming wedge will be thin or broad, internal deformation will be mild or intense, and the structures either close to symmetric or strongly forward-vergent, just as in the analogous natural thrust belts.
Key words:
Key words:
- analog model
- structural geology
- thin-skinned tectonics
Collapse
Context
Audience
undergraduate course in structural geology
Skills and concepts that students must have mastered
- simple trigonometry
- fundamental structural geology terminology: thrust faults, thin-skinned thrust belts, shortening & strain
How the activity is situated in the course
It is best suited to an upper-division course in structural geology, after the students have had an initial introduction to the concepts of thrust faulting and thin-skinned mountain belts. If the course includes a field trip to sites in a thin-skinned fold-and-thrust belt, the concepts explored in this lab can serve as a useful prelude to the field trip.
Goals
Content/concepts goals for this activity
- shortening and strain
- thin-skinned tectonics and critical wedge mechanics
- thrust fault geometry
- scaling relationships
Higher order thinking skills goals for this activity
The student handout asks the students to make observations, measurements, and interpretations of their own experiment – and to compare/contrast it with two other experiments. This requires a high-level understanding of the underlying physics. Did they calculate tapers correctly and interpret them logically in terms of the basal boundary conditions? Did they measure and interpret strain appropriately? Did they understand the differences in tectonic style, fault vergence, and strain distribution between the models they compared? Did they understand the model scaling and how it may or may not be appropriate?
Skills goals for this activity
The students must work in a group to create and run an analog model. In addition to answering specific questions (regarding their calculated wedge tapers, thrust vergence and surface strains) the lab is also intended to have them write an open-ended comparative analysis of their model with two others. Students can find such a task a bit unnerving, but it provides an excellent opportunity to give them guidance on how to approach more realistic scientific problems than they typically encounter in lab assignments.
Description and Teaching Materials
This hands-on lab will allow students to create model thin-skinned wedges and to:
- Understand the relationship between boundary conditions and deformation in thrust belts.
- How basal friction controls wedge taper.
- How basal friction affects the balance between forward- and backward vergent thrusts.
- How thrust belts can gain along-strike complexity.
- Predict boundary conditions based upon surface observations
- Directly measure strain and understand its relation to displacement.
- Exhume their model mountain belts and observe their internal structure:
- Observe the relationship between thrusts and their surface expression.
- Appreciate repetition of strata as indicative of thrusting.
- Map out forward and lateral ramps.
- Description of Thin-Skinned Flour Structures Lab (Acrobat (PDF) 7.6MB Jun11 15)
- Student Lab Handout for Thin-Skinned Flour Structures Lab (Acrobat (PDF) 66kB Jun11 15)
Teaching Notes and Tips
This lab requires a full 3-hour lab period. It cannot be interrupted in the middle if the lab room is to be used in the interim by other classes. It is best suited to an upper-division course in structural geology, after the students have had an initial introduction to the concepts of thrust faulting and thin-skinned mountain belts. If the course includes a field trip to sites in a thin-skinned fold-and-thrust belt, the concepts explored in this lab can serve as a useful prelude to the field trip.
The options in this lab are nearly limitless. Assign each group one model to construct and arrange them in rough order of increasing complexity, to be run in sequence during the last hour of the lab. By doing this you allow the students to grow in comprehension through the lab. Typically, they become able to integrate what they learn about the physics of thin-skinned tectonics during the early parts of the lab and use those insights to make insightful interpretations of what they see in later parts of the lab.
The options in this lab are nearly limitless. Assign each group one model to construct and arrange them in rough order of increasing complexity, to be run in sequence during the last hour of the lab. By doing this you allow the students to grow in comprehension through the lab. Typically, they become able to integrate what they learn about the physics of thin-skinned tectonics during the early parts of the lab and use those insights to make insightful interpretations of what they see in later parts of the lab.
Share your modifications and improvements to this activity through the Community Contribution Tool »
Assessment
The student handout asks the students to make observations, measurements, and interpretations of their own experiment – and to compare/contrast it with two other experiments. This requires a high-level understanding of the underlying physics. Did they calculate tapers correctly and interpret them logically in terms of the basal boundary conditions? Did they measure and interpret strain appropriately? Did they understand the differences in tectonic style, fault vergence, and strain distribution between the models they compared? Did they understand the model scaling and how it may or may not be appropriate?