Text reading. p. 160-173

Paired metamorphic belts.

• last week we left off looking at the thermal structure of a subduction-arc complex.
• the following pattern first worked out for Japan by Miyashiro.
• the accretionary wedge is associated with a lower geothermal gradient, and distinctive blueschist and eclogite metamorphic facies.
• what is eclogite??
• recent paper in Geology - when the earth became cool enough to form eclogites, plate tectonics started.
• in contrast the arc-plutonic complex is associated with high heat flow a high geothermal gradient. The metamorphic facies is known as the greenschist and amphibolite facies.
• the two belts of contrasting metamorphism give rise to the term paired metamorphic belt.

• range in composition: (3 common series)
  • low-K tholeiitic series (mostly basalt).
  • calc-alkaline series (mostly andesite).
  • alkali series (alkaline basalts and shoshonites).
• partial melt mechanism(s):
  • shear heating (minimal due to low friction).
  • model of conditions at and above slab under volcanic arc: 500-700 degrees C (Peacock); and significance of low heat flow in arc-trench gap areas.
  • fluid release from prograde metamorphic reactions:
    • depths 80- 100 km: amphibolite -> eclogite + water. Water + pyrolite at these PT -> melt of asthenosphere.
    • depths >100 km serpentine dehydrates and partial melting of quartz eclogite crust produces acidic magmas, which interact with the mantle pyrolite and fractionate to produce calc-alkaline series. (Serpentine associated more with fracture zones (?) could set up some predictions for this model).
• evidence for slab contributions:
  • isotopic signature of continental derived sediments in volcanics.
  • Peacock suggests will have slab contributions only for anomalously warm subducted slab, i.e. when subducting a spreading ridge. Refers to models with a significant slab contribution as the minority view.
  • introduces idea of temporal evolution of magmas. May also be episodic.
• evidence for mantle contributions.
  • the presences of basalts demands a mantle soure.
• corner flow model and replenishment of the source material.

Growth of arc root: Beneath an arc can have an 18-20 km root (see diagram above). What are mechanisms of production?

• tectonic processes.
• magmatic underplating (mafic lower crust).
• batholiths as columns versus blisters.

Spatial patterns of volcanic and plutonic composition within arcs.

• distance from trench -> increase in slab depth.
• shallower dip yield wider arc and better expressed zonation.
• H2O soluble elements (B, Cs, As, and Sb) decrease away from trench - thought to reflect dewatering of slab (Ryan & others).
• K2O increases away from trench.
• very complex story that involves, free fluids, fluid expelled in dehydration reactions, potentially two source materials (slab and wedge), and contamination and fractionation.

Along strike spacing of volcanoes.

see map of volcanos associated with Marianas trench.

U.S. west coast volcanics.

Fitch faults and oblique subduction, Sunda style tectonics.

some major examples:

• Japan Sea
• Bering Sea
• Lau basin in back of Tonga arc.
• Mariana trough assoc. with Maraianas trench.
• East Scotia Sea backarc (extreme S Atlantic)

Characteristic traits of these basins:

• strong seismic wave attenuation in underlying mantle above the slab.
• trench axes migrating seaward in hotspot reference frame for arcs that show this.
• underlain by oceanic crust, sometimes with coherent magnetic anomalies, often not.
• a distinctive magmatic-volcanic suite is associated with the arc-rfiting event. MORB like flow.
• often thick sediment fill.
• sites of abundant hydrothermal activity.

Mechanical model for generation: sinking subducted plate pulls overriding plate with it (trench suck) producing tensile stresses in overriding plate which fails at weakest point, eventually causing lithospheric thinning and generation of basaltic crust. If it continues long enough a seafloor spreading center organizes.

Alternate model for back-arc basin - capture by outboard trench jump. They may be polygenetic.

Rear-arc fold-thrust belts.

• Good example the Andes (see exercise map) and the Laramides.
• Trench axes not migrating in hotspot reference framework.
• More common on continental margins.
• Crustal thickening definitely has major tectonic component.

Hamilton, W., 1979, Tectonics of the Indonesian region; Geological Survey Professional Paper 1078, 345 p. This has a wealth of informaton and one incredibly beautiful map.

Peacock, S., 1996, Thermal and Petrologic Structure of Subduction Zones: in Bebout et al. (eds.) Subduction Top to Bottom, American Geophysical Union Geophysical Monograph 96, p. 119-133. This is a nice recent compilation looking at the basic question of how arc volcanics are generated.

Ryan, J., Morris, J., Bebout, G. & leeman, B., 1996, Describing Chemical Fluxes in Subduction Zones: Insighs from "Depth-Profiling" Studies of Arc and forearc Rocks; in Bebout et al. (eds.) Subduction Top to Bottom, American Geophysical Union Geophysical Monograph 96, p. 119-133.

Taylor, B., 1995, Backarc Basins; Plenum Press, N. Y. 524 p. This has a succinct evolutionary history of backarc basins in the preface, and then articles with loads of details.

Worrall, D. M., 1991, Tectonic History of the Bering Sea and the Evolution of tertiary Strike-Slip Basins of the Bering Shelf; GSA ASpecial Paper 257, 120 p and many plates. A very detailed and richly supported history for the Bering Sea.

Course materials for Plate Tectonics, GEOL 3700, University of Nebraska at Omaha. Instructor: H. D. Maher Jr., copyright. This material may be used for non-profit educational purposes with appropriate attribution of authorship. Otherwise please contact author.