International Efforts for Understanding Convergent Margins, Island Arcs, Backarc Basins and Subduction Zones
Convergent margins are one of three kinds of plate tectonic margins, the other two being divergent and transform margins. Subduction zones are where oceanic lithosphere sinks back into the mantle and are associated with earthquakes, magmatic arcs, and explosive volcanism (see YouTube video for information about subduction zones and magmatic arcs). There are many convergent margins in the world, the total length of convergent plate margins is about 65,000 km <fig-1>. The IBM (Izu-Bonin-Mariana) Arc System is an excellent example of a convergent plate margin. It is 2500 km long convergent margin that extends south from Tokyo, Japan, to Guam, U.S.A. <fig-2, top>. This system is a natural laboratory for understanding convergent plate margin processes and products, for the following reasons:
- It is an endmember arc system, the type example of a decoupled arc, characterized by a deep, steep subduction zone due to subduction of the world’s oldest seafloor.
- The tectonic and magmatic history, from initiation of subduction ~52 million years ago onward, is better known than that of any other convergent margin <fig-3>. It is entirely intra-oceanic, so that anatexis of continental crust is precluded. This allows a much more confident assessment of mantle-to-crust fluxes, from the subduction zone up through arc magma.
- A high-quality Ocean Bottom Seismometer refraction profile provides the best image of crustal structure available for any intra-oceanic convergent margin <fig-2, bottom>. Because it is far removed from continents, sediment fluxes to the forearc and the subducting plate are low and the arc infrastructure is largely exposed, and there is no accretionary prism. This simplifies studying the forearc lithosphere, sampling of slab-derived fluids venting in the forearc, and estimating sediment input fluxes.
- The sedimentary sequence on the downgoing Pacific plate changes systematically parallel to the trench, principally involving changes in the relative proportions of pelagic sediments and intraplate lavas and volcaniclastic sediments.
- Convergence rates vary from 3 cm/year in the south to 7 cm/year in the north, providing an opportunity to examine relationships between arc products and processes vs. convergence rate.
- The distribution of strain and tectonic style varies simply along strike of the arc system, with back-arc spreading in the southern third, intra-arc rifting in the northern third, and an unrifted arc in the middle.
- Rifting episodes that preserve fossil arc crust in the Palau-Kyushu Ridge and West Mariana, along with preservation of tephra in pelagic sequences provide excellent opportunities to temporal variability of IBM arc volcanic activity and composition.
- In addition to being the simplest and best-known intra-oceanic arc system on the planet and a logical setting in which to focus studies of convergent margin processes, favorable logistical and political considerations exist and are likely to continue. In particular, efforts by the U.S. and Japanese scientific communities to carry out thoughtful, cooperative studies have occurred in the past and future efforts are likely to be favorably received.
More information on these and other aspects of the IBM convergent margin can be found in Stern et al. (2003).
GMTL geoscientists and students have worked on many aspects of the IBM arc system, especially in the Mariana arc. A map of the Mariana arc system is shown in <fig-4>; a simplified cross-section is shown in <fig-5>. The Mariana convergent margin has been the focus of GMTL research for almost 40 years. An annotated map showing the focus of many GMTL published studies of the Mariana convergent margin features is shown in <fig-6> and the references are provided in <gmtl-mariana-studies>.
Further Reading and Citations:
Leng, W., and Gurnis, M. 2015. Subduction initiation at relic arcs. Geophysical Research Letters 43, 7014-7021.
Stern, R. J., Bloomer, S. H., 1992. Subduction zone infancy: Examples from the Eocene Izu-Bonin-Mariana and Jurassic California Arcs. Geol. Soc. Am. Bull. 104, 1621-1636.
Stern, R.J., Fouch, M.J., and Klemperer, S., 2003. An Overview of the Izu-Bonin-Mariana Subduction Factory in J. Eiler and M. Hirschmann (eds.) Inside the Subduction Factory, Geophysical Monograph 138, American Geophysical Union, 175-222.
The translation to Czech language can be found at http://www.bildelarexpert.se/blogg/2016/11/20/izu-bonin-mariana-arc-system-mezinarodnimu-usili-pro-pochopeni-konvergentni-okraje/.