Subduction%20Zones

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Subduction Zones

• By Robert J. Stern
• Reviews of Geophysics, 40 (4), 2002

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Outline

• Introduction
• Subduction components
• Incoming plate
• Downgoing plate
• Mantle wedge
• Arc-trench complex
• Remaining questions

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Introduction

• Purposes summarize our present understanding of
  the subduction zone.
• Significances of subduction zone
• Subduction slabs provide most of the force to
  drive the plates.
• Oceanic lithosphere, sediments, and water are
  recycled through subduction.
• Slab melting and arc volcanism creates
  continental crust.
• Large earthquakes associated with the subduction
  zone pose great hazard.

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Subduction Zone Components
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Seismic Images
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Seismic Images
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Depth Dimension
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Incoming plate the input

• Mantle lithosphere
• Ages from 0 to 170 Ma.
• Negatively buoyant when older than 10-30 Ma.
• Two end-types.
• Crust
• Thick (gt15 km) crust causes subduction failure.
• Carry water, CO2, and incompatible trace elements.

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Downgoing plate

• Thermally cold initially.
• Progressively heated and squeezed.
• Metamorphism and melting for basaltic oceanic
  crust strongly depend on temperature and water
  contend.

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Metamorphism of the slab

• Olivine to wadsleyite at lt 410 km because of
  positive Clapeyron slope.
• Ringwoodite to pvmw at gt 660 km.

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Dehydration of subducted slab

• Breakdown of serpentinite to olivine, opx, and
  water occurs down to 250 km depth.

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Mantle wedge

• Induced convection by the subducting slab.
• Convecting asthenosphere interacts with
  slab-derived fluids and melts to generate arc
  magmas.

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Melting in the mantle wedge and slab

• The addition of aqueous fluids to normal
  asthenosphere leads to melting.
• Melting of subducted crust has been found
  (adakites).
• Melting of subducted sediments has also been
  suggested.

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Arc-trench complex

• Product of the subduction system.
• Can be studied directly.
• Andean-type arcs and intra-oceanic arcs.

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Remaining questions

• How does subduction begin?
• How much water is carried into subduction zones
  by serpentinized mantle?
• What is the thermal structure of the subduction
  zones?
• How do subducted sediments, crust and mantle in
  equilibrium with evolving fluids and melts change
  as they sink?
• What happens to subducted carbonates?

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Remaining questions

• How do fluids from the subducting slab move into
  and through the mantle wedge?
• How does mantle above subduction zones move?
• Does lithospheric mantle beneath the forearc
  interact with the mantle wedge?
• How and where are melts generated?
• What happens to melts when they arrive at the
  base of the crust and as they pass through it?