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Subduction Zones

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					             Subduction Zones




•   http://www.geo.utep.edu/class_notes/PT99/Lectures/subduction.html
            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.
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.
              Physiography
•   Outer Swell
•   Outer Trench Wall
•   Trench
•   Forearc (Arc-Trench Gap)
•   Volcanic Arc
•   Back-Arc
             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
• 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.)
  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.
               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.
                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.
                     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.
    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
• 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
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.
• 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.
  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.
                   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.
          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.
        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.
        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.
        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.
                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
                          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).
                        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.
                 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.
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.
  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.
• 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.
• 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).
  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.
     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
   o Spreading caused by forceable injection of a diapir
     rising from the downgoing slab.
   o Spreading induced in the overriding plate by the
     viscous drag in the mantle wedge caused by the motion
     of the downgoing plate (comer flow).
   o 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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