Convergent zone where at least on one side oceanic lithosphere is involved, and gets recycled deep down into the mantle.

Presently active subduction arc-complexes:

• circum-Pacific.
• Malay archipeligo.
• Carribean
• Mediteranean
• inbetween South America and Antarctica

Island arcs vs. continental arcs:

• definition based on type of lithosphere/crust the overlying plate is composed of.
• does affect the character of geologic activity at the plate setting; e.g. the composition of arc volcanism.
• Andes as example of continental arc.
• Aleutians as example of continental arc that becomes an island arc with a trapped basin behind (a rear-arc basin.
• Tonga Kermadec system as an example of an island arc.
• Japan as a hybrid. Other possible hybrids?
• can accrete an island arc to a continental margin or to each other.

Components of subduction-arc systems:

• forebulge in descending plate.
• trench:
  • outer trench wall
  • inner trench wall
  • trench sediments
    • character of.
    • deformation of.
• accretionary wedge, arc-trench gap:
  • seismic images of, drill holes, local exposures, heat flow.
  • 
  • Above image from USGS site http://walrus.wr.usgs.gov/research/sopac.html. It shows a seismic image of the wedge. Note the typical well developed reflection off the top of the subducting slab. An interesting question is - what determines the taper angle of the wedge?
  • character of accretionary melange.
    • how big are the chunks?
    • Fransiscan melange.
    • tectonic vs. sedimentary melange.
    • image to the right is of melange from Washington state (http://geomaps.wr.usgs.gov/parks/noca/t3bellpass.html). Note the multiple 'surfaces' in the rock and their greenish coloration (hence the general descriptive term greenstone. This rock has been very highly sheared during low grade metamorphism.
  • high pressure metamorphic facies: blue schist and eclogite.
  • internal deformation (mixing) processes:
    • thrusting and folding, thin-skinned very common, with foreland propagation pattern.
    • mud and serpentine diapirism, and importance of high fluid pressures.
    • cleavage formation, penetrative deformation and mass solute transport.
    • slumping at a shallow level.
  • evolution of wedge with time:
    • accretionary growth through underplating at toe.
    • addition of arc sediments.
    • accretionary tectonic erosion.
    • stability and wedge dynamics.
  • internal basins: transient, catch turbidite flows.
  • Image from USGS site http://earthquake.usgs.gov/learning/glossary.php?term=accretionary%20wedge that summarizes many of the subduction-arc complex components.

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  In class exercise - evaluation of seismic images of accretionary complexes. In the xerox copies of portions of seismic reflection profiles of accretionary complexes identify as many geologic features as possible and relate them to the subduction process. Report on the three or four most significant features to the class.
• Brief tips on seismic interpretation:
  • Be aware of surface and other multiples.
  • Remember that as you go down seismic resolution decreases - the seismic 'waters' get muddier.
  • The vertical axis is travel time, not distance. Because velocities change, usually increase with depth, distortions can result. Structures will be more compressed at depth.
  • Fault truncations and other point reflectors produce a diffraction pattern that looks a bit like a upwards pointing bullet profile. This type of seismic noise is hard to get rid of.
  • In general, sedimentary rocks have a layered signature (with their bedding reflectors), while basement igneous or metamorphic rocks are structureless, or at least a lot less structured.
  • Important features that can often be well imaged: basement-cover contact, fold structures, fault structures, deltaic patterns.

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• descending slab:
  • Wadati- Benioff zone: defined by earthquakes (see diagram above).
  • dip of.
  • 3-D geometry of.
  • first motions studies of and stresses within.
• volcanic arc: covered next week.
• rear arc activity and tectonism: covered next week.

Models for the thermal structure of the descending slab.

What are important input parameters in such a model?

Phase changes in the slab and slab pull:

• mantle metamorphism: olivine -> spinel -> perovskite.
• 670-780 km discontinuity:
  • division between upper and lower mantle.
  • lower bound of deep earthquakes.
  • seismic velocity increase.
  • P-T consistent with position of phase changes.
  • viscosity boundary. How does it effect convection patterns in mantle?
• P-T maps of phase changes. Where do these maps come from?

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Consider the simple and schematic pressure-temperature maps above of where mineral A vs. mineral B is stable for a given composition. To the left is a temperature sensitive phase transition. Next in line is a pressure sensitive phase transition. Third and of interest to us is a positive slope for the phase change. Note that for a different geothermal gradients or P-T histories, the phase change will occur at different depths. Will the reaction occur at a deeper or shallower depth in a subducting slab in comparison to the surrounding mantle rock. The olivine-spinel phase transition map basically looks like this.

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What is the eventual fate of the slab?

It is likely different for the different parts of the slab, crust vs. lithospheric mantle. There is no reason why some of the later when warmed up doesn't join the mantle convection dance.

How does subduction initiate?

How does it usually conclude?

Expanding earth hypothesis.

• Most interesting as a case study of how a minority paradigm can persist.
• Remember earlier paradigm that earth had shrunk due to cooling.
• Link to overview of hypothesis.
• Abundant evidence idea does not work for the earth, but for other planets?

Caveat: these are some references I happen to know about. This list is not comprehensive, and a wealth of literature exists out there, including more recent stuff. Harmon D. Maher, Jr., 1995

Dickinson, W.R., 1973, Widths of Modern Arc-Trench Gaps Proportional to Past Duration of igneous activity in associated magmatic arcs: Journal of Geophysical Research, v. 78, p. 3395-3417.
After a concise description of major petro-tectonic elements, it describes how accretionary wedges may grow with time, noting exceptions suggesting loss. This mental framework initiated a mini-paradigm and much fruitful research.

Kerr, R., 1986, Sinking slabs puncture layered mantle model; Science, v. 231., p. 548-49.
This captures some of the debate as to how far subducted slabs descend into the mantle and argues against 'conventional wisdom' of a floor at 650-670 km. This has Implications for understanding large scale recycling and convective behavior. Lots more recent thought on this.

Kimura, G. & Ludden, J., 1995, Peeling oceanic crust in subduction zones; Geology, v. 23, p. 217-220.
describes ideas on how material is transferred from ocean plate to the accretionary wedge. Discusses implications for recycling of volatiles in arc-trench system.

Moore, J.C. & Silver, E.A., 1987, Continental margin tectonics: Submarine accretionary prisms: Review of Geophysics, v. 25, p. 1305-1312.

Platt, J. P., 1986, Dynamics of orogenic wedges and the upllift of high-pressure metamorphic rocks; GSA Bulletin, v. 97, p. 1037-1053.
one of a series of articles discussing how alternating periods of contraction and thickenting vs. extension and thinning could occur within accretionary wedges and produce uplift of deep cold rocks. This article is particularly well illustrated.

Schweller, W., Kulm, L., Prince, R., 1981, Tectonics, Structure, & Sedimentary Framework of the Peru-Chile Trench: GSA Memoir 154, p. 323-350.
Excellent detailed look at trench and trench slope processes for this subduction zone.

Silver, E.A., Moore, C.J., 1978, The Molucca Sea Collision Zone, Indonesia; Journal of Geophysical Research, v. 83., 1681-91.
Describes an incipient arc-arc collision due to two facing subduction zones.

Toksoz, M.N., 1975, The Subduction of the Lithosphere; Scientiric American, Nov. issue.
This is a nice summary article but with good depth of information, a good starting point in reading on subduction zones. It emphasizes deep mantle processes.

Von Huene, R. & Scholl, D.W., 1991, Observations at convergent margions concerning sediment subduction, subduction erosion, and the growth of continental crust: Reviews of Geophysics, v. 29, p. 279-316.

Course materials for Plate Tectonics, GEOL 3700, University of Nebraska at Omaha. Instructor: Harmon D. Maher Jr.. Material without attributions may be used for non-profit educational purposes with appropriate attribution of authorship. Otherwise please contact author.