13. Subduction Zones William Wilcock

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13. Subduction ZonesWilliam Wilcock
OCEAN/ESS 410
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Lecture/Lab Learning Goals

• Be able to sketch the different kinds of
  convergent plate margins and label key processes
• Understand the processes in the subduction zone
  factory
• Understand the Wilson cycle
• Understand the different forces that drive
  subduction and that control the angle of the
  subducting slab.
• Know the different kinds of earthquakes that
  occur in subduction zones
• Be able to interpret focal mechanisms from
  subduction zone settings (LAB)

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3 Types of Convergent Margin (Plate Boundary)
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Subduction Zone Processes

• Subduction zones are important because they are
  the downwelling branches in Earths mantle
  convection.
• Subduction zones are responsible for some of the
  primary geologic processes on earth
• Convergence leads to the growth of continents by
• volcanism
• accretion of terrains
• and loss of the continents by
• Tectonic erosion
• Sediment subduction
• Subduction zone processes dominate the
  development of active geologic structures on the
  continents

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Subduction Factory
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Subduction Factory
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Peridotite SolidusWater lowers the melting
temperature of mantle peridotiteSketch Not
to scale
Mantle Geotherm Old Plate
Wet Solidus gtgt1 water
1 water
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Wilson Cycle - Cyclical growth and loss of ocean
basins leads to continental growth
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Passive Margin
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Convergent (Active) Margin
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Stable Continent Bigger than Stage A
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Tectonic Erosion
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Sediment Trapped Subduction
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Forces acting on a subducting slab
The plate sinks under gravity (red arrow)
according to its weight, thus how cold and dense
it is. The slab also drags along adjacent mantle
(black arrows). This mantle is pushed up against
the subducting slab on the left hand side
generating a high pressure region. The mantle is
dragged down with the slab on the right hand side
generating a low pressure. This pressure
differential tends to lift the slab.
Velocity
Age
Velocity
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Balance of Gravitational and Pressure Forces
Influences Slab Dip

• Old (Cold) Plate Slow Subduction
• Large gravitational force, small pressure force.
  Steep subduction angle
• Young (Warm) Plate Fast Subduction
• Small gravitational force, large pressure force.
  Shallow subduction angle

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Earthquake Maximum Magnitude
Subduction Rate, cm/yr
Plate Age
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Slab Dips
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Island Arcs - Back Arc Spreading
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Back-Arc Spreading

• Two Ideas
• Subducting slab falls away
• Mantle flow in wedge creates extension

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Thermal Structure
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Deep Forces Resulting From Phase Changes
Enhances Subduction

• Opposes Subduction

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Sometimes but not always mantle slabs do not
penetrate 670 km discontinuity
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Subduction Zone Earthquakes
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Earthquakes

• Shallow Earthquakes
• Plate Boundary - Megathrust
• Surrounding Plates
• Deep Earthquakes
• Mineral phase changes as pressure increases and
  loss of water bearing minerals - incompletely
  understood

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Cascadia Locked Zone
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Cascadia Subduction ZoneLast earthquake 1700.
Recurrence interval 200-1000 years (average
500 years)
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• Block diagram/cross section of tectonics

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Cascadia
Warm slab implies dewatering at shallow depths
and weak arc volcanism
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Deformation in Subduction Zones
Accretionary Prism forms when sediments are
present and scraped off subducting slab
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Oblique Subduction Leads to Shearing

• This shearing also affects the forearc causing
  rotation of the strong Oregon block.
• Compression of Puget Sound