Accretionary vs. erosive subduction margins

• 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

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).

Prior to the exercises, students can be given an introductory lecture about forearc basins and accretionary prisms.

Helpful Slides (PowerPoint 14.5MB Sep28 15)
Exercise Download (Microsoft Word 8.9MB Feb16 15)
Suggested solutions:

Solutions Download -- private instructor-only file

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Exercise Outline:

Part I. Accretionary wedges and sediment subduction

1. 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
2. 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.)
3. Identify major structures associated with accretionary wedge
4. 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

1. Identify mass wasting and forearc basin subsidence in margin wedges that have been interpreted as subduction erosion
2. Speculate on mechanisms of subduction erosion (e.g., von Huene and Ranero, 2003)

• 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

• Overview: Seismogenic Zone Experiment Mini-Lessons
• GeoMapApp
• 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.

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