Jackson School of Geosciences at The University of Texas at Austin corners
Jackson School of Geosciences
Jackson School of Geosciences
Department of Geological SciencesBureau of Economic GeologyInstitute for Geophysics

Earth Surface and Hydrologic Processes Seminar
Spring, 2009

In the Earth Surface and Hydrologic Processes Seminar, a weekly graduate-level forum offered each semester at the Institute for Geophysics (UTIG), research scientists and students from the Jackson School of Geosciences, investigate fundamental geologic problems. This spring's seminar explored accretionary wedge formation. Accretionary wedges are along subduction zones as oceanic tectonic plates slide beneath an overriding plate at deep marine trenches. Marine sediments scraped off the downgoing oceanic plate pile up in the accretionary wedge landward of the trench through a complex system of thrust faulting.

The forum focused the entire class on a single paper, Mechanics of Fold-and-Thrust Belts and Accretionary Wedges (Davis, Suppe, and Dahlen, 1983). This classic paper describes how accretionary wedge systems grow similarly to the way snow or soil piles up in front of an advancing plow. A key contribution of Davis, et al., is the quantitative relationship between wedge tapers and the strength of wedge sediments and underlying (non-subducting tectonic plate) materials.  The forum combined an examination of the numerous detailed mathematical derivations in this paper with discussions of the geological impacts of variations in critical parameters.

To gain a hands-on understanding of this relationship, the group simulated three accretionary wedges in a mechanized sandbox. Each wedge, made of sand, rode over a different basal material: A) sand, B) glass beads, and C) silicon polymer (approximates the mechanical properties of salt). [For details, watch Critical Wedge: The Movie] Following the experiment, students measured critical parameters defined mathematically in the Davis et al. paper, such as wedge taper, and then solved for the internal friction angle of each of the three materials using the theoretical equations of Davis et al. Their findings supported those reported by Davis, et al., which is that the stronger the material comprising the wedge (as indicated by the calculated coefficient of friction), the steeper the accretionary wedge slope.

Figure showing composite image of three accretionary wedges in a mechanized sandbox. In each section, the accretionary sediments are sand. In Section A, the basal material is also sand. In Section B, the underlying material is glass beads. In Section C, the underlying material is salt. Green box in upper left shows plan view.

At the end of the class, the group presented their work to Dr. John Suppe, a co-author on the 1983 paper and an international leader in the study of accretionary wedge, when he visited JSG (coincidentally) this spring. The experience built a strong bridge between DGS, UTIG, and BEG, researchers and students and provided a great atmosphere in which to share different perspectives on a key geologic issue.       

Faculty: Peter Flemings, Mike Hudec, Sean Gulick, Nathan Bangs
Students: Mike Braunscheidel, Yao You, Derek Sawyer, Lindsay Worthington, Kylara Martin, Hilary Strong

Experiment conducted by Tim Dooley, Applied Geodynamics Laboratory