Accretionary vs. erosive subduction margins
This is one component of the Seismogenic zone Experiment Mini Lessons
Overview: The old paradigm for the fate of ocean basin sediments at subduction zones is that they are entirely off-scraped from their oceanic plate substrate during initial contact with the overriding plate, contributing to the continued growth of an accretionary prism. Recent research, however, demonstrates that many plate margins lack the accretionary prisms predicted from the global distribution of sediments, prompting investigation into the possibility of sediment subduction and the role that hydrous sediment entering the subduction interface could potentially play in influencing the distribution of large subduction zone earthquakes.
Lesson Summary: This module is designed to first inform students on the nature and variability of subduction margins, challenging them to critique the validity of the standard "accretionary prism" cartoon at subduction zones. The exercise then evaluates bathymetric and seismic images of modern forearcs, which yields insight into the size and structure of accretionary wedges and the sedimentation history of their adjacent forearc basins. Armed with these forearc geophysical records, the exercise then explores sediment accretion, subduction, or tectonic underthrusting of forearc material to deeper crustal levels at the subduction interface. This activity includes an overview of subduction erosion processes and causes as well as detailed instructions for exercises that students will draw upon in demonstrating a more accurate understanding of subduction zones.
- Students will learn about the general architecture of forearcs and accretionary prisms using both geological and geophysical datasets
- Using a mass balance approach, students will evaluate the fate of sediment at subduction margins and be able to distinguish erosive from accretionary forearcs
- Students will learn that the morphology of subducting lithosphere affects upper plate margin processes
- Students will recognize that subduction zones are diverse and there is much room for additional research
Context for Use
Prerequisite knowledge: Students should have a basic background in plate tectonics, particularly in the generalized architecture of subduction zones.
Time to complete within course: The current exercise is designed for completion in one lab session or as a homework assignment (2-3 hours).
Description and Teaching Materials
Prior to the exercises, students can be given an introductory lecture about forearc basins and accretionary prisms.
Part I. Accretionary wedges and sediment subduction
- Explore sediment distribution in Earth's ocean basins using Divins (2006) dataset in GeoMapApp (discuss the roles of climate and tectonic setting in redistributing this sediment) or NOAA ocean sediment thickness map
- Introduce accretionary wedge paradigm, terminology, and factors controlling sediment delivery/accumulation at trench (e.g., thickness of sediment on oceanic plate, orthogonal convergence rates, etc.)
- Identify major structures associated with accretionary wedge
- Using maps and cross sections of "margin wedges" in Japan and Costa Rica, calculate approximate cross-sectional areas of preserved sediment in wedges. Compare this to those predicted by observed sediment thicknesses and convergence rates.
Part II. Subduction erosion and forearc subsidence
- Identify mass wasting and forearc basin subsidence in margin wedges that have been interpreted as subduction erosion
- Speculate on mechanisms of subduction erosion (e.g., von Huene and Ranero, 2003)
Teaching Notes and Tips
- A particularly exciting aspect of this mini-lesson is that SEIZE research documenting sediment subduction and subduction erosion is highly multi-disciplinary and utilizes observations at many different scales
- This exercise is not intended to explore geophysical methods in detail but instead should be directed at recognizing their value in constraining subsurface processes.
- Research at active ocean-continent subduction margins is still discovering important processes that shape the Earth's surface and affect the global balance between the destruction and creation of continental crust. Recognition of ongoing exciting research can help motivate students and encourage creativity.
- Although a definitive relationship among sediment subduction, subduction erosion, and generation of large earthquakes has not been discovered, there are intriguing observations that warrant additional research.
- If conducted as a take-home exercise, the sequencing of activities creates dependence of results on prior results, so practice exercises should be conducted in class, ideally concurrently with the lecture.
- The mini-lesson will implement the following assessment strategies:
- Short answer interpretation of results from Margins research
- Identification and mark-up of structural features on bathymetric maps
- Basic calculations concerning mass balance and rates
- Short answer interpretation of the implications of the results in terms of tectonic processes
- Short answer thought exercise to encourage creativity and multiple working hypotheses
- One possibility is that following the lab exercise, students would work in groups to investigate case studies of other ocean-continent subduction margins. Students will then make oral presentations to the class
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References and Resources
- Overview: Seismogenic Zone Experiment Mini-Lessons
- Basemaps>Global Grids>Seafloor Sediment Thickness (Divins 2006) In Layer Manager, click the "i" for more info. Change the color bar from continuous to discrete and enter 1000 to change the contour interval. Select the Basemaps>Seafloor Depths, Ages, Sediments, and Spreading>Seafloor Sediment Thickness Focus Sites>NSF Margins>Costa Rica multi-beam bathymetry
- Screaton mini-lesson: Burial, compaction, and porosities in a subduction zone
- NOAA ocean sediment thickness map
- Nankai, Japan
- Nicoya, Costa Rica
- Expedition 344 Scientists, 2013, Costa Rica Seismogenesis Project, Program A Stage 2 (CRISP-A2): sampling and quantifying lithologic inputs and fluid inputs and outputs of the seismogenic zone: Integrated Ocean Drilling Program.
- Moore, G.F., Park, J.O., Bangs, N.L., Gulick, S.P., Tobin, H.J., Nakamura, Y., Saito, S., Tsuji, T., Yoro, T., Tanaka, H., Uraki, S., Kido, Y., Sanada, Y., Kuramoto, S., et al., 2009, Structural and seismic stratigraphic framework of the NanTroSEIZE Stage 1 transect, in Proceedings of the IODP, Proceedings of the IODP, Integrated Ocean Drilling Program.
- Ranero, C.R., Huene, R., Flueh, E., Duarte, M., Baca, D., and McIntosh, K., 2000, A cross section of the convergent Pacific margin of Nicaragua: Tectonics, v. 19, p. 335–357.
- Sak, P.B., Fisher, D.M., Gardner, T.W., Marshall, J.S., and LaFemina, P.C., 2009, Rough crust subduction, forearc kinematics, and Quaternary uplift rates, Costa Rican segment of the Middle American Trench: Geological Society of America Bulletin, v. 121, p. 992–1012, doi: 10.1130/B26237.1.
- Scholl, D.W., and Huene, von, R., 2007, Crustal recycling at modern subduction zones applied to the past—Issues of growth and preservation of continental basement crust, mantle geochemistry, and supercontinent reconstruction, in Geological Society of America Memoirs, Geological Society of America Memoirs, Geological Society of America, p. 9–32.
- Vannucchi, P., Sak, P.B., Morgan, J.P., Ohkushi, K., Ujiie, K., the IODP Expedition 334 Shipboard Scientists, 2013, Rapid pulses of uplift, subsidence, and subduction erosion offshore Central America: Implications for building the rock record of convergent margins: Geology, v. 41, p. 995–998, doi: 10.1130/G34355.1.
- Wells, R.E., Blakely, R.J., Sugiyama, Y., Scholl, D.W., and Dinterman, P.A., 2003, Basin-centered asperities in great subduction zone earthquakes: A link between slip, subsidence, and subduction erosion?: Journal of Geophysical Research-Solid Earth, v. 108. Expedition 344 Scientists, 2013, Costa Rica Seismogenesis Project, Program A Stage 2 (CRISP-A2): sampling and quantifying lithologic inputs and fluid inputs and outputs of the seismogenic zone: Integrated Ocean Drilling Program.
- Whittaker, J., Goncharov, A., Williams, S., Müller, R.D., and Leitchenkov, G., 2013, Global sediment thickness dataset updated for the Australian-Antarctic Southern Ocean, Geochemistry, Geophysics, Geosystems, v. 14, no. 8, p. 3297-3305.