A tour of the Mariana Subduction System
Summary
This is an interactive lecture based around a geophysical (primarily bathmetry and seismic reflection data) tour of the Mariana convergent margin. Through a powerpoint presentation and optional GeoMapApp and GeoWall visualizations Students explore the principal provinces of the Mariana convergent margin from the subducting plate through the backarc.
The Mariana convergent margin is a classic trench-volcanic arc-back arc system in the western Pacific. The deepest point on the Earth's surface, the Challenger Deep, is found at its southern end, just to the south of the U.S. territory of Guam. To the north the Mariana convergent margin transitions into the Izu-Bonin arc, which eventually intersects Japan. Subduction initially began at about 50 Ma. A volcanic island arc had formed by the Late Middle Eocene (~44 Ma) and then rifted in the Late Eocene to Early Oligocene. Subsequent spreading, from 29 Ma to 15 Ma, propagated north and south creating the Parece Vela back-arc basin. At ~9 Ma the arc again rifted, and seafloor spreading in the Mariana Trough back-arc basin, beginning at ~5 Ma, rafted away the remnant arc. Rifting and spreading propagated north, increasing the curvature of the Mariana island arc system. Explosive volcanism is ongoing along the modern Mariana arc. From 16–20.5°N most of the active arc volcanoes are subaerial; south of 16°N they are all submarine.
The modern Mariana Trough is characterized by prominent abyssal hill fabric. Oceanic crust and abyssal hill fabric in the east are buried under thick volcaniclastic sediments derived from the active arc. The extent of accreted back-arc basin crust on the eastern margin of the basin, proximal to the active volcanic arc, and the locations of paleospreading axes are unknown. In general, the spreading center is 40–50 km closer to the Mariana Arc than the remnant arc, suggesting asymmetrical crustal accretion or alternatively, that the active arc is constructed primarily on accreted lithosphere. Any asymmetry of the Mariana Trough may be caused by repetitive, small-scale ridge jumps to the east, towards the subduction zone. In this model, seafloor accretion along individual spreading segments would primarily be symmetric, but the ridge jumps would result in an overall asymmetric basin.
Arc rifting and back-arc basin formation are commonly thought to be caused by extension induced by the seaward migration of the trench, termed "trench rollback". However, in the central Mariana system where the near vertical subducting plate acts as a "sea anchor" resisting lateral motion, this may not be the case. Back-arc basin spreading along this region of the Mariana system may be caused by the combined effects of the sea anchor force and convergence of the Philippine Sea and Eurasian Plates. In contrast, the increase in spreading rate of the southern Mariana Trough may be the result of trench rollback along the Challenger Deep segment of the Mariana Trench
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Learning Goals
At the completion of this lesson students will be able to:
1) Identify the principal geological provinces of an active convergent margin
2) Identifty the principal geological hazards associated with a active convergent margin
3) Use GeoMapApp to explore the geological setting of other convergent margins
4) Use Google Earth to explore topography and bathymetry
Context for Use
This mini-lesson is intended for use as a highly interactive segment in a large introductory geology lecture class. It is anticipated that the students have been introduced to plate tectonics. The lesson is intended to take ~60-75 minutes. A computer with an internet connection, powerpoint, Google Earth, and a web-browser is required. This lesson is currently designed to be used in parallel with a GeoWall lab in which 3-dimensional visualizations of the Mariana system are presented (supplemental files). If a large computer lab is available, or students have their own laptops, it is beneficial to allow students to explore the data in GeoMapApp (supplemental instructions included). This is however not necessary.
Description and Teaching Materials
This activity is presented using a self contained powerpoint file powerpoint presentation (PowerPoint 39.6MB May29 09). Questions are embedded in the powerpoint and can be adapted to a specific interactive response system as needed. Hyperlinks to Google Earth are currently setup for Mac OS X. Edit the hyperlinks to reflect the location of the Google Earth executable if you are using Windows. If you have a GeoWall available a 3-D visualization of the Mariana system that uses ArcScene is provided ArcScene GeoWall visualization of Mariana 3-D topography and earthquake distribution (zip archive) ( bytes May28 09). Instructions for using GeoMapApp with this exercise are included here GeoMappApp instructions (Microsoft Word 26kB May28 09).
Teaching Notes and Tips
Familiarity with the Mariana system is important – the summary at the start of this exercise provides an introduction to the geology of the area. Users are referred to the reference list at the end of this exercise for more information. The powerpoint contains speaker notes that are intended to guide the discussion and suggest questions to pose to students. The instructor is encouraged to make use of think-pair-share type activities rather than simply posing a question. Extended small group discussions are also a useful tool. Both methods are designed to emphasize active and collaborative learning. The activity is currently used with a classroom response system (clicker) to gauge student learning in real time.
The included GeoWall visualization requires a GeoWall and ArcScene. When uncompressed the instructor will find an ArcScene file name Mariana in the Mariana folder.
Assessment
Basic questions posed through an interactive response system (or simply by a show of hands if such a system is not available) and "eavesdropping" on student discussions are the primary methods of assessment. See questions embedded in the powerpoint for details of the questions.
References and Resources
The Mariana system is discussed at length on the Margins web page. Much of the data presented in the lesson was collected during cruise EW0202 and EW0203 on the R/V Maurice Ewing. A web page has been set up to present the results of this cruise (http://www.geo.ua.edu/MARIANA/).
A good overall reference for the Mariana convergent margin is:
Stern, R., M. J. Fouch, and S. L. Klemperer (2003), An overview of the Izu-Bonin-Mariana Subduction Factory, in Inside the Subduction Factory, Geophys. Monogr. Ser., vol. 138, edited by J. Eiler and J. Hirshman, pp. 175-222, AGU, Washington, D. C.