Partitioning of thrust and strike/slip faulting in oblique subduction
Students download and study high-resolution multi-beam bathymetry data from an oblique subduction zone and identify fault traces, scarps, canyons, etc. From these they attempt to identify the fault type and fault movement from cutoffs, offset canyons and other features. They should estimate amount of strike/slip and thrust motion for each fault. The overall obliquity should be calculated from the amounts measured and compared to plate motion solutions and earthquake solutions for that subduction zone. Similarly, thrust zones on the back side of the island arc could also be analyzed for any strike/slip motion. Students learn to work with non-terrestrial data sets and interpret strain from geomorphic and traditional methods. (Puerto Rico is used in this example, other locations such as the Aleutians and Marianas are possible).
Content: Students should be able to understand the data from swath-mapping and side-scan sonar systems and how it can be visualized. They should understand that trenches and subduction zones are not necessarily perpendicular to the direction of relative plate motions. They should be able to identify features on the ocean floor such as faults, slumps, eruptive features and erosional features. They should understand that these features provide evidence for a variety of tectonic processes such as thrust belt development.
The main skills involved are data analysis and the determination of what features in a non-traditional subaqueous dataset can be used to provide evidence for a structural hypothesis. This progresses to the synthesis of ideas and models for the behavior of crust under strain. It provides a setting for modern lab techniques that apply to modern field work.
Context for Use
I envision this used in a 2nd-3rd year geology class (perhaps structural geology) in either lab, or regular class of up to 30 students that uses active learning methods. Students need access to computers, software to display and manipulate the image/data and perhaps print out some of it. Image software should be able to zoom in and out multiple times. Built in measurement/scaling tools would be advantageous but not absolutely necessary. It will probably take at least an hour, two hours would probably be better. Students should be familiar with more than just introductory plate tectonics and with types of faults, orogens and learning about field methods.
Description and Teaching Materials
Some initial discussion of swath mapping and sonar methods would be appropriate.
With this should be mention of the resolution of various mapping methods and the resolution (approx. =<10m) that is needed for this exercise to give a reasonable chance of answering the hypothesis of this exercise: being able to identify and measure faults.
A good overview of a cruise that collected appropriate data is:
Regional image for the area is here:
Example instructions for students to complete this exercise are here: Student Instructions for Subduction Strain Exercise (Microsoft Word 24kB May28 09). They are instructed to download the data set image(s), identify features, especially faults, and start some interpretation of the the fault and feature patterns seen.
The high-resolution image of the Puerto Rico forearc area is here: ( placeholder for image)
A second image is here for the deformation belt south (back-arc) of Puerto Rico:(placeholder)
Either place these on a server that students can access, or give them to the students somehow.
Other potential data sources would be the Gloria survey of the EEZ 200 mile coastal waters of the USA and the Aleutians. This and other potential data sets (perhaps newer than this writeup) can be found using the GeoMapApp http://www.geomapapp.org.
Teaching Notes and Tips
The bathymetry and/or side-scan data should have a discussion or practice interpreting the "picture". Examples of side-scan and bathymetric maps showing very well-defined objects/features might be useful before doing this exercise. http://www.punaridge.org/doc/factoids/DigitalData/Default.htm.
Students might find out that oblique subduction results in distinct sets of faults with either just thrust or mainly strike/slip motion and that back-arc thrust belts show little oblique strain compared to forearc areas. The number of faults found and the offsets are dependent on the resolution of the images/data used. Measurements give minimum offset/displacement - perhaps grossly underestimates and should be discussed under the topic of assumptions used.
Students should be able to create a report that discusses the data, the evidence they collect from the data and the resultant conclusions. I expect to get a report from the student that they were able to identify a certain number of faults of certain types and estimated an amount and direction of displacement for each and cumulative over the faults. Criteria for these estimates should be explicitly stated. There should be interpretative conclusions reached on the directions of convergence for the local area and the system as a whole and how they relate. For this exercise, reasonable conclusions might be that in an oblique subduction zone the strain is partitioned mostly into distinct strike/slip and thrust faults in the forearc belt and no strike/slip motion is transferred to the back arc thrust belt.
In addition, a scoring rubric such as the Washington Critical Thinking Rubric can be used as this exercise is meant to display the critical thinking skills of the students.
References and Resources
http://oceanexplorer.noaa.gov/explorations/03trench/trench/trench.html - short info on the Puerto Rico trench and hazards.