Subduction Zones

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Subduction Zones

• http//www.geo.utep.edu/class_notes/PT99/Lectures/
  subduction.html

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Subduction zones

• also termed convergent or consuming plate margins
• occur where adjacent plates move toward each
  other and relative motion is accommodated by one
  plate over-riding the other.
• These zones are classified as either oceanic or
  subcontinental, depending on the overriding
  plate.
• If the "subducting" plate is continental,
  subduction will cease and a mountain belt will
  form within a collision zone.

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Where do subduction zones occur?

• along the "Ring of Fire" around the Pacific
  Ocean.
• Two short subduction zones occur at the Lesser
  Antilles, at the eastern side of the Carribean
  plate and the South Sandwich Island plate.
• Three short segments of the Alpine Himalayan
  system involve subduction of oceanic lithosphere.
• the Calabrian and Aegean boundaries in the
  Mediterranean Sea
• Makran boundary along the SW boundary of the Iran
  plate.

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Physiography

• Outer Swell
• Outer Trench Wall
• Trench
• Forearc (Arc-Trench Gap)
• Volcanic Arc
• Back-Arc

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Physiography 2

• Outer swell
• Low topographic bulge (a few hundred meters of
  relief)
• develops just outboard of where subducting plate
  bends down into the mantle.
• Outer Trench Wall
• Slope on ocean floor between the outer swell and
  the trench floor.
• Slope dip is typically -5 degrees

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• Trench
• Deep valley that develops at the plate boundary.
• Continuous for 1000s of km
• typically 10 - 15 km deep (5 - 10 km below
  surrounding ocean floor.)

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Forearc (Arc-Trench Gap)

• Consists of region between trench and the arc.
• steep inner trench wall (lower trench slope)
• dips of - 10 deg
• flattens into a gentle slope termed the forearc
  basin (upper trench slope).
• The inner trench wall is usually separated from
  the forearc by the outer ridge.
• The accretionary prism underlies the inner trench
  wall, the outer ridge and part of the forearc
  basin.

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Volcanic Arc

• Active arc built on a topographically high region
  of older rocks, the arc basement
• may be a shallow marine platform or an emergent
  region of older rocks.
• In continental arcs, the basement is continental
  crust standing a few kms above sea level.
• Volcanoes in island arcs are usually 1 - 2 km
  above sea level. Volcano elevation in continental
  arcs is strongly influenced by continental crust
  thickness.

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Back-Arc

• Area behind the volcanic arc.
• In island arcs this region consists of basins
  with oceanic crustal structure and abyssal water
  depths.
• Sometimes remnant arcs are preserved behind the
  island arcs.
• On continents this region is the continental
  platform which may be subaerially exposed, or the
  site of a shallow marine basin.

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Gravity

• Typically, similar free-air gravity profiles
• 50 mGal gravity high associated with the outer
  bulge
• 200 mGal low associated with the trench and
  accretionary prism
• 200 mGal high associated with the arc.
• Isostatic anomalies have the same polarity as the
  free-air gravity
• Suggests that the gravity anomalies are caused by
  the dynamic equilibrium imposed by the system by
  compression.
• Compressional forces cause the trench to be
  deeper and the arc to have less of a root than
  they would be if only isostatic forces were at
  work.

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Structure from Earthquakes

• Subduction zones are characterized by dipping
  seismic zones termed Benioff zones or
  Wadati-Benioff zones
• Result from deformation of the downgoing
  lithospheric slab. The zones have dips ranging
  from 40 to 60 deg

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• Because, the slab is colder and more dense than
  surrounding asthenosphere, it's position can be
  mapped seismically as high velocity anomalies and
  as high "Q" (little attenuation of seismic waves)
  zones in the mantle. High Q, and high velocity
  are thought to correspond to relatively high
  density, cold material

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earthquake hypocenters related to their position
within the slab

• Shallow depths
• predominantly thrust faults within the upper part
  of the downgoing plate or in the adjacent
  overriding plate.
• Down to depths of 400 km, down-dip extension.
• In some slabs, down-dip extension is found in the
  upper part of the slab, accompanied by down-dip
  compression at the base of the slab. The
  extension probably results from the lithosphere
  being pulled into the mantle by the weight of the
  downgoing portion.

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• Deep slabs usually show down-dip compression
• may result from increased viscous resistance at
  depth.
• deeper part of the slab will feel a push from the
  weight of the shallower portion of the slab.
• Between 70 - 300 km, faulting may occur due to
  dehydration of serpentinite.
• From 300 - 700 krn may also be due to the sudden
  phase change of olivine to spinel which may be
  accommodated by rapid shearing of the crystal
  lattice along planes on which minute spinel
  crystals have grown.

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Structural Geology- Trenches

• Trenches normally contain flat-lying turbidites
  deposited by currents flowing down into the
  trench from the overriding plate or along the
  axis of the trench. The outer swell is probably
  caused by elastic bending of the subducting
  plate.

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Forearc

• may be underlain either by the accretionary
  prism or arc basement rocks covered by a thin
  veneer of sediments or both.
• Where there is little sediment accumulation on
  the subducting plate, island arc or continental
  basement may extend all the way to the lower
  trench slope and little or no accretionary prism
  may occur.
• Forearc basement may draped by a thin veneer of
  sediment, and is commonly cut by normal faults
  toward the trench.

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Accretionary Prism

• wedge of deformed sedimentary rocks
• the main locus of crustal deformation
• Rocks are typically cut by numerous imbricate
  thrust faults that dip in the same direction as
  the subduction zone.
• As more material is added to the toe of the
  wedge, the thrusts are moved upwards and rotate
  arcwards.
• Rocks within the accretionary prism are derived
  from the downgoing and/or overriding plates.

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Accretionary Prism

• At the toe of the wedge, sediments are added thru
  offscraping
• propagation of the basal thrust into undeformed
  sediments on the subducting plate.
• This process results in progressive widening of
  the wedge, and eventually a decrease in dip on
  the subduction zone.

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Accretionary Prism

• When sediments on the downgoing plate are
  subducted without being disturbed they can still
  be added to the prism thru underplating
• propagation of the basal thrust into the
  downgoing undeformed sediments to form a duplex
  beneath the main part of the prism.

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Subduction Erosion

• erosion and subsequent subduction of rocks from
  the toe of the prism.
• Sediment on the subducting plate that is not
  added to the overriding plate thru these
  processes may descend into the mantle and
  contribute to the generation of arc magmas.

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Forearc Basin

• Wide sedimentary basin
• develops above irregular basement on the upper
  part of the arc-trench gap.
• Sediments from the active arc or arc basement
  rocks
• deposited by turbidity currents traveling along
  the basin axis or perpendicular to the arc.
• asymmetric basin
• inner part of the upper slope basin subsides
• outer edges rises due to accretion at the toe of
  the wedge.
• high-P, low-T metamorphism
• increases in grade toward the inner forearc
  region
• in the direction of subduction

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Arc

• Arc basement
• older more deformed and metamorphosed rocks in
  platform on which the modem arc is built.
• oceanic rocks
• On the continents, complex continental basement.
• Volcanic arc
• a chain of largely andesitic stratovolcanoes
  spaced at fairly regular intervals of 70 km.
• The structural environment of these arcs is
  commonly extensional (numerous normal faults)
• volcanoes in grabens termed volcanic depressions.
• underlain by large plutonic bodies (e. g. the
  Sierra Nevada).

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Arcs

• Metamorphism
• common and suggest a high geothermal gradient.
• Much of the lower crust may be at the melting
  temperature of granite.
• Sediments
• debris from active volcanoes.
• deposited as turbidites.
• In tropics, settings these volcanogenic sediments
  may interfinger with carbonate reefs.
• In continental arcs, sediments are often
  deposited subaerially.

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Back-arc

• extensional tectonics and subsidence.
• In oceans arc-derived sediments are deposited in
  an ocean basin behind the arc termed the back-arc
  basin.
• In continents, sediments are deposited into
  basins on the continental platform and are termed
  foreland basins or retro-arc basins.

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Foreland Fold and Thrust Belts

• Relation between foreland fold and thrust belts
  and subduction not understood
• not all continental arcs display these features.
• Possible explanations if there is a relation
• Thrust belt caused by compression at margin of
  overriding plate due to subduction of hot,
  buoyant lithosphere.
• Thrust belt associated with shallow dip of a
  downgoing slab.
• Thrust belt associated with subduction of an
  aseismic ridge.

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Models of thermal processes in subduction zones

• Rate of Subduction
• The faster the descent of the slab, the less time
  it has to absorb heat from the mantle.
• Slab Thickness
• The thicker the descending slab, the more time it
  takes to come into equilibrium with the
  surrounding lithosphere.
• Frictional Heating
• occurs at top of slab due to friction as slab
  descends and is resisted by the lithosphere.

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• Conduction
• heat into slab from the asthenosphere
• Adiabatic Heating
• associated with compression of slab with
  increased pressure at depth.
• Heat of Radioactive Decay
• decay of radioactive minerals in the oceanic
  crust (minor)
• Latent Heat of Mineral Phase Transitions
• olivine-spinel transition at 400 km is
  exothermic. Spinel-oxide transition at 670 km
  could be either exothermic or endothermic.

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• All thermal models show that the downgoing slab
  maintains its thermal identity to great depths
  (e. g. contrasts of 700 deg C can still exist at
  700 krn depth).

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If the slab is so cold, how do we get enough
heating to cause arc magmatism?

• Melting of Slab in Presence of Water
• Partial melting may take place at lower
  temperatures due to presence of water as slab
  dehydrates. Water is released by transition of
  amphibolite to ecologite, and dehydration of
  serpentinite at depths of - 100 km.
• Corner Flow and Melting of Mantle
• Downgoing slab may cause flow of hot mantle into
  the comer of the overriding mantle where it
  impinges on the downgoing slab. This may provide
  enough heat to cause melting.

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Origins of back-arc basins

• Entrapment of previous oceanic crust
• Change of plate motion may lead to abandonment of
  a fragment of oceanic crust behind the arc.
  (e.g., Aleutian Basin and West Philippines Basin
  )
• Formation of new crust - behind the arc. 3 models
• Spreading caused by forceable injection of a
  diapir rising from the downgoing slab.
• Spreading induced in the overriding plate by the
  viscous drag in the mantle wedge caused by the
  motion of the downgoing plate (comer flow).
• Spreading induced by the relative drift of the
  overriding plate away from the downgoing slab
  (slab fixed with respect to mantle). This is also
  termed roll-back.

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