Oceanic crust recycling centers - subduction zones and arc complexes.

Contents:


Introduction

Subduction zones are convergent plate boundaries where oceanic lithosphere gets recycled deep down into the mantle to depths of at least 670 km (the upper-lower mantle boundary). What is the significance of this particular depth? Why are volcanic arcs associated with this process?

Presently active subduction arc-complexes:

Island arcs vs. continental arcs:


Components of subduction-arc systems

These are described in order from the oceanic plate being subducted across the trench and into the overriding plate. Click on the figure to the right for a schematic cross section of many of the features.

Image from USGS site http://earthquake.usgs.gov/learning/glossary.php?term=accretionary%20wedge that summarizes many of the subduction-arc complex components.


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.

The descending slab

How do we know what is happening at greater depths?

Wadati- Benioff zone:


Thermal structure of the descending slab, and why it subducts

What are important input parameters in a model for the thermal structure of a subducting plate?

Phase changes in the slab and slab pull:

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.

What would be the consequence of elevated olivine to spinel transformation in slab due to the lower geothermal gradient within the slab?

Image of gravity anomalies associated with part of the Aleutian subduction zone. That is pertinent here is the broad backarc gravity low. The long wavelength of the low suggests a deeper origin, and one interpretation is that it is due to the elevated olivine to spinel transformation that occurs in the slab. Image from and more information at: http://pubs.usgs.gov/of/2000/ofr-00-0365/report.htm .


Miscellaneous remaining questions

 

Why do subduction zones produce the biggest earthquakes?

Why is the trace of the arc and trench arcuate in form (dimple tectonics)?

What is the eventual fate of the slab material? 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?

Volcanic arc and rear arc activity and tectonism: covered next week.

Expanding earth hypothesis: for those who accept seafloor spreading, but not subduction.


References:

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.