By Chelsea Scott and Ramon Arrowsmith
School of Earth and Space Exploration, Arizona State University
We designed an exercise for structural geology classes where students learn about the classical Painted Canyon site (Sylvester and Smith, 1976; Sylvester, 1988) near the Southern San Andreas Fault, California, by analyzing a point cloud of a fold located near the fault. We developed this exercise for a graduate-level structural geology class at Arizona State University and expect that it would also fit well into undergraduate-level classes. Given the recent necessity for remote learning and the associated challenges with taking students to the field, we anticipate that this exercise could serve as an alternative to typical field activities with a structural geology emphasis.
In this exercise, students are given a point cloud of a fold in Painted Canyon. It was produced by the structure from motion (SfM) approach from photographs collected with a small Uncrewed Aerial Vehicle (sUAV) or drone (See this link for more educational material). Students use the Compass tool in CloudCompare to measure fold limb orientation. They plot their measurements in Stereonet (or could do so by hand), solve for the axial plane orientation, and calculate the average strain rate recorded in the folded unit Students are asked to then relate their measurements to the San Andreas Fault. Is the orientation consistent with the expected strain field given a right-lateral fault zone?
Here are learning goals for this activity:
1. Students will collect and plot structural measurements using CloudCompare and Stereonet. Even with the use of a computer, students must still consider how to make quality structural measurements. Students will likely collect their largest structural dataset yet, and must consider how to work with the larger variability in their data.
2. Students will interpret their structural measurements of the fold limbs. They will determine the axial plane orientation, fold hinge line, and appropriate nomenclature given the fold’s axial surface and hinge line.
3. Students will use principles learned in class to relate their measurements to the kinematics of the San Andreas Fault.
The materials include (1) an assignment sheet , (2) a document that illustrates how to make orientation measurements using CloudCompare, (3) the point cloud available at OpenTopography . (4) The answer key is available by emailing Chelsea Scott at firstname.lastname@example.org.
Acknowledgements: Scott was supported by the US National Science Foundation Postdoctoral Fellowship 1625221 and by the School of Earth and Space Exploration at Arizona State University. Scott thanks Sarah Titus for first introducing her to Painted Canyon during a Carleton College field trip.
Bergh, S.G., Sylvester, A.G., Damte, A., and Indrevær, K., 2019, Polyphase kinematic history of transpression along the Mecca Hills segment of the San Andreas fault, southern California: Geosphere, v. 15, p. 901–934, https://doi.org/10.1130/GES02027.1.
Sylvester, A.G., and Smith, R.R., 1976, Tectonic Transpression and Basement-Controlled Deformation in San Andreas Fault Zone, Salton Trough, California: AAPG Bulletin, v. 60, https://doi.org/10.1306/C1EA3A73-16C9-11D7-8645000102C1865D.
Sylvester, A.G., and Smith, R.R., 1986, Structure section in Painted Canyon, Mecca Hills, southern California: Centennial Field Guide Volume 1 - Cordilleran Section: Geological Society of America, v. Chapter 24, p. 103–108.
CloudCompare Compass Tool:
Thiele, S.T., Grose, L., Samsu, A., Micklethwaite, S., Vollgger, S.A., and Cruden, A.R., 2017, Rapid, semi-automatic fracture and contact mapping for point clouds, images and geophysical data: Solid Earth, v. 8, p. 1241–1253, https://doi.org/10.5194/se-8-1241-2017.